Plant Water-Stress Response

Mechanisms

Surajit Bhattacharjee and Ajay Krishna Saha

Abstract
Water, the central molecule of life, plays a profound role in a number of
plant life processes ranging from photosynthesis to macromolecular inter-
action through hydrophobic bond. Due to imbalances in natural status of
the different physiological, environmental conditions and during natural
calamities, plants are exposed to either deficit of water (i.e. drought) or
excess of water (i.e. flooding). Both of these conditions lead to water
stress on plants which in turn results in disruption of agriculture and food
supply in different parts of the world. In this chapter, a brief idea on the
causes, indicators, responses and adaptation processes to the water stress
in plants and the associated molecular mechanisms has been presented. In
this chapter, the stresses related to water are expressed as “drought”. The
cellular and molecular responses of plants to water stress have been
studied intensively throughout the world. Understanding the mechanisms
by which plants perceive water stress and transmit the subsequent signals
to cellular machinery and modulate expression of genes and their products
to activate adaptive responses is of fundamental importance to plant
biology. Knowledge about water-stress signal transduction is therefore
vital for continued development of rational breeding and transgenic
strategies to improve stress tolerance in crops. Factors controlling water-
stress conditions alter the normal equilibrium and lead to a series of
morphological, physiological, biochemical and molecular changes in
plants which adversely affect their growth and productivity. However,
plants also have developed innate adaptations to water-stress conditions

S. Bhattacharjee
Department of Molecular Biology and Bioinformatics,
Tripura University (A Central University),
Suryamaninagar, TR 799022, India
e-mail: sbhattacharjee@gmail.com
A.K. Saha (*)
Department of Botany, Tripura University (A Central
University), Suryamaninagar, TR 799022, India
e-mail: aksaha.58@gmail.com

R.K. Gaur and P. Sharma (eds.), Approaches to Plant Stress and their Management, 149
DOI 10.1007/978-81-322-1620-9_8, # Springer India 2014
150 S. Bhattacharjee and A.K. Saha

with an array of biochemical and physiological interventions that involve

the function of many stress-associated genes. Water-stress-associated
hormones like ABA are found to play a central role in orchestrating the
molecular and physiological responses leading protective responses in
plants. Overall, this chapter provides a systemic glimpse of integrated
cellular and whole plant responses to water stress.

Keywords
Water stress
Drought
Signal transduction
Water-stress resistance

Stress tolerance

MAPK Mitogen-activated protein kinase

Abbreviations MYC Myelocytomatosis oncogene
NAADP Nicotinic acid adenine dinucle-
ABA Abscisic acid otide phosphate
ABREs Abscisic acid responsive NAD Nicotinamide adenine
elements dinucleotide
APX Ascorbate peroxidase NADP Nicotinamide adenine dinucle-
AtHD6 Histone deacetylase 6 otide phosphate
ATP Adenosine triphosphate NFYA5 Nuclear factor Y, subunit A5
BapA Boiling staple protein PL Phospholipids
bZIP Basic leucine zipper domain PLC Phospholipase C
CBL Calcineurin B-like protein PS Photosystem
CDSP Chloroplast drought-stress RAB Responsive to ABA
protein RC Reaction centre
CIPKs CBL-interacting protein kinases RD Responsive to dehydration
CO2 Carbon dioxide RNA Ribonucleic acid
COR Cold regulated ROS Reactive oxygen species
CPKs/CDPKs Calcium-dependent protein RuBisCO Ribulose-1,5-bisphosphate
kinases carboxylase oxygenase
DAG Diacylglycerol RWC Relative water content
DREs Dehydration-responsive SOD Super oxide dismutase
elements WD Water deficit
ERD Early response to dehydration
ET Electron transport
ETR Electron transport rate
Fs Steady state of chlorophyll Introduction
fluorescence
GL Glycolipids Plants in nature are constantly exposed to various
GRase Glutathione reductase abiotic stresses resulting from unfavourable
HSP Heat-shock protein environmental conditions which adversely affect
IP3 Inositol 1,4,5-trisphosphate their growth and development (Atkinson and
KIN Cold inducible Urwin 2012). Water stress is one of the main
LEA Late embryogenesis abundant abiotic stresses to which crops are exposed in
LEAPs Late embryogenesis abundant India. Plant water stress, often caused by
proteins drought, can have major impact on plant growth
Plant Water-Stress Response Mechanisms 151

and development (Jaleel et al. 2009). When short-term responses which are primarily linked to
drought occurs, then it can be the cause of stomatal regulation appeared. Short-term
lower yields and possible crop failure. The responses lead to reduction in water loss by tran-
effects of plant water stress vary between the spiration and maximising CO2 intake. Optimum
plant species. Early recognition of water-stress efficiency of these initial responses is found to be
symptoms can be critical to maintain the growth responsible for maintenance of constant ratio of
of a crop. The most common symptom of plant transpiration to photosynthesis (Kozlowski et al.
water stress is wilt. As the plant undergoes water 1991). Midterm responses also known as acclima-
stress, the water pressure inside the leaves tion comprise of the fine-tuning of the osmotic
decreases and the plant wilts. Drying to a condi- potential by accumulation of solute, modifications
tion of wilt will reduce the growth of any plant in cell wall elasticity and morphological
(Kaur and Gupta 2005). variations. Long-term adaptation to drought is
From an irrigator’s perspective, managing characterised by variation in gene responses,
water to minimise stress means knowing plant anatomical modifications of specific organs and
water availability, recognising symptoms of acquisition of modified physiological mechanisms
water stress and planning ahead. This chapter with an aim to reduce the overall growth to balance
outlines how water stress impacts plant growth resource utilisation (Chapin 1980, 1991) (Fig. 1).
and development and how to anticipate plant Under field conditions, these responses can be
water stress to minimise negative consequences. synergistically or antagonistically modified by
Drought (water stress) is one of the main abiotic the superimposition of other stresses.
stress factors that affect all organisms’ lives. The most severe form of water deficit is des-
Drought occurs when soil moisture level and rela- iccation – when most of the protoplasmic water
tive humidity in air are low, while temperature is is lost and only a very small amount of tightly
also high. Almost every plant process is affected bound water remains in the cell. It is reported that
directly or indirectly by water supply (Akıncı water stress encompasses both destructive and
1997; Lobell et al. 2013). Plants, as one of the constructive elements and acts as a determining
basic food sources, either in nature or in factor as well as a driving force for improving
cultivations, in their growing period, require resistance and adaptive evolution (Larcher
water or at least moisture for germination. It is 1987). Plant resistance to water stress which
obvious that most land plants are exposed to leads to adaptation results from either tolerance
short- or long-term water stresses at some times or a mechanism that supports avoidance. Whole
in their life cycle and tend to develop some adap- plant can contribute to the avoidance of water
tive mechanisms for adapting to changing environ- deficit through an array of mechanisms during
mental conditions. The extent and duration of the the plant’s life cycle, and evasion to water stress
water deprivation determines the magnitude of can also occur at the cellular level. The important
stress response (Pugnaire et al. 1999). Some plants determinants of these adaptive responses include
may adapt more easily than others giving them an the species and genotype, the extent and severity
advantage over competitors. Water stress may of water loss, the age and phase of development,
range from moderate, and of short duration, to the organ and cell type and the subcellular com-
extremely severe and prolonged summer drought partment. An example of avoidance at the cellu-
(Pereira and Chaves 1993, 1995; Bottner et al. lar level is the process of osmotic adjustment
1995). At the whole plant level, the effect of where the osmotic potential of the cell is lowered
water stress is usually perceived as a decrease in in order for the water potential gradient to
photosynthesis and growth and is associated with favour water uptake and maintenance of turgor
alteration in carbon and nitrogen metabolism (Bray 1997) (Fig. 1).
(Cornic and Massacci 1996; Mwanamwenge In response to water stress, a plethora of mod-
et al. 1999). It is observed that within a ification occurred in the intracellular milieu of
few seconds following the onset of water stress, the plant cells. The changes include the
152 S. Bhattacharjee and A.K. Saha

Fig. 1 Causes of water stress and variable responses shown by plants against drought

modification of different intracellular metabolic be involved in the generation of the plant

pathways, changes in the nutrient and ion uptake, hormones like abscisic acid (ABA), ethylene
synthesis of new proteins, modulation of free and salicylic acid (SA) which in turn initiate a
radical generation and all these changes found second round of signalling which may be respon-
to be preceding the induction of signal transduc- sible for the adaptive and tolerance responses
tion pathways. Water stress induces a multiple associated with water stress (Xiong et al. 2002).
signal transduction pathway which follows the Specificity in water-stress responses in plants
generation of second messengers (e.g. inositol is further determined by a complex regulatory
phosphates, lipid mediators and reactive oxygen network of molecular mechanisms which include
species). These second messengers in turn mod- the interaction between transcription factors,
ulate intracellular Ca2+ level and activate kinases kinase cascades, production of reactive oxygen
to initiate protein phosphorylation cascades. species as well as involvement of heat-shock
These events lead to activation of target proteins factors and small RNAs (Atkinson and
which are directly involved in cellular protection Urwin 2012).
or acting as transcription factors controlling One of the important adaptive features
genes explicitly involved in regulation of water acquired by the plants to water stress includes
stress. The activation of these genes is found to sun-type or shade-type chloroplast adaptation
Plant Water-Stress Response Mechanisms 153

which is also induced by many other stress conditions of low soil moisture, more energy is
factors including drought (Lichtenthaler et al. required by the plants to remove water from the
1981). Regions with adequate but non-uniform soil; thus, the matric potential is greater.
precipitation also experience water-limiting Symptoms of water stress have been experienced
environments. The general effects of drought on by plants when the soil is dry and the matric
plant growth are fairly well known. However, the potential is strong (Glyn Bengough et al. 2011).
primary effect of water deficit at the biochemical This condition is recognised as the matric effect.
and molecular levels is not considerably under- It has also been shown that heat is an indirect
stood yet, and such understanding is crucial. driver of reduced crop yield through increased
Knowledge of the biochemical and molecular plant evapotranspiration (Lobell et al. 2013)
responses to drought is essential for a holistic (Fig. 1).
perception of plant resistance mechanisms to
water-limited conditions in higher plants. The
response to abiotic stress results in a dramatic Measurement of Water
change of the whole plant transcriptome. It is Stress in Plants
reported that the transcriptomic response to
drought can vary with the time of day. These The extent of water stress experienced by plants
responses seem to interact with hormonal and in their habitat can be assessed by measuring
other stress pathways that naturally vary during the soil moisture and analyses of the distribu-
the course of the day (Wilkins et al. 2010; tion of precipitation. Measurement of water
Cramer et al. 2011). Sometimes a comparison potential (ψ) in plants is found to be the most
between cellular response and whole plant fundamental indicator of water stress. No water
response may reveal the level of organisation stress (small negative water potential values)
where the adaptation operates (Kar 2011). In was found in soils with high water-holding
this chapter, we provide an overview of the capacity. On the other hand, moderate to high
current understanding of plant responses to water stress was recorded at the end of the
drought. In addition, we will describe the cel- season in those sites with low water-holding
lular signalling mechanisms leading to protect capacity. A linear relationship between predawn
the plant from the deleterious effects of leaf water potential and stem water potential is
drought. also reported (McCutchan and Shackel 1992).
Another commonly used indicator of plant
water status is relative water content or RWC
which at one time had been less accurately
termed as relative turgidity. Tissue water con-
Origin of Plant Water Stress
tent (percent of fresh weight) and fresh weight
have also been used as indicators of water sta-
Water stress in plants results either from
tus. Unfortunately, water content or fresh
restricted water supply to their roots or due to
weight of tissue at full turgor is normally not
increased rate of transpiration. Plants growing
given as a reference. Water content can be very
under arid and semiarid environments frequently
misleading because of its superficial resem-
experienced the water stress associated with
blance to RWC (Hsiao 1973). In some studies,
drought. It is reported that high temperatures
visual wilting is considered as the sole indicator
act as an indirect driver of plant water stress
of water status. Although wilting is dependent
(Lobell et al. 2013). Roots are the primary site
on turgor pressure, it is also a function of the
of water intake in plants. The extent of force
mechanical properties of cell wall and tissue
required for a plant to absorb water from the
(Hsiao 1973; Joly 1985) (Fig. 2).
soil is known as the matric potential. In
154 S. Bhattacharjee and A.K. Saha

Fig. 2 Indicator of plant

water stress

interference with many other basic metabolic

Plant Responses to Water Stress processes (Kramer and Boyer 1995).
The inhibition of photosynthesis during water
Photosynthetic Responses to Drought stress could be explained as the cause of the
stomata closure and the internal CO2 concentra-
Reduced rate of photosynthesis is a usual effect tion decrease. Stomatal limitation is more severe
of water stress in plants. Water stress reduces when a plant is stressed than when it is not.
photosynthesis by decreasing both leaf area and Therefore, it is rather surprising that photosyn-
photosynthetic rate per unit leaf area (McCree thesis often decreases in parallel with or more
1986). Photosynthesis is severely inhibited and than stomatal conductance. Limitation of carbon
may cease altogether as water deficits increase in uptake during water stress might be associated
crop plants. Water deficiency in plants generates with stomatal control of water (Chaves 1991;
metabolic changes along with functional and Cornic and Massacci 1996). Stomata close in
structural rearrangements of photosynthesising response either to a decline in leaf turgor and/or
apparatus. The decrease in leaf growth or water potential or to a low-humidity atmosphere
increasing senescence of leaves under drought (Maroco et al. 1997).
conditions may also inhibit photosynthesis in The photosynthetic rate in higher plants
existing leaves (Boyer 1976). Decreasing water decreases more rapidly than respiration rate
content is accompanied by loss of turgor and with increased water stress. A nearly effect of
wilting, cessation of cell enlargement, closure water reduction in leaves is usually partial or
of stomata, alteration of photosynthesis and complete stomatal closures which markedly
Plant Water-Stress Response Mechanisms 155

decrease the movement of carbon dioxide into the key enzyme for carbon metabolism in leaves,
the assimilating leaves and reduce the photosyn- is reported to be strikingly decreased in conditions
thetic rate up to ten times depending on the of water stress. Inhibition of the RuBisCO activity
amount of water removal and the sensitivity of during water stress is found to be associated with
the plant (Ghannoum 2009; Akıncı and Lösel acidification of the chloroplast stroma. Further-
2012; Chaves et al. 2003). more, water-stress-associated suppression in
In C4 plants, stomatal closure is found to be a RuBisCO activity is also related to the alterations
major determinant in the inhibition of photosyn- of the chloroplast structure, conformational
thesis under water stress, while non-stomatal change of the RuBisCO, lack of the substrate
factors like metabolic impairments are also and reduction in the activity of the coupling factor
reported to play the major role in this inhibition. – ATPase – and sometimes due to damage, the
In both C3 and C4 plants, the rate of photosyn- plastids may lose RuBisCO. Activity of other
thesis decreases under the drought conditions. It photosynthetic enzymes like NAD-dependent
is evidenced that the rate of photosynthesis is malate dehydrogenase, phosphoenolpyruvate car-
more affected in C4 plants (like corn) than C3 boxylase, fructose-1,6-bisphosphatase and other
plants (such as wheat) in conditions of water related enzymes also is found to be inhibited to
deficits. This explains the fact that hot arid different extents (Ramachandra et al. 2004; Lisar
areas with prevalence of C4 plants are more sus- et al. 2012) (Fig. 3).
ceptible to frequent drought. A number of Water stress also disrupts the cyclic and non-
cofactors like (a) low CO2 uptake due to stomatal cyclic types of electron transport during the light
closure and resistance, (b) qualitative and quan- reaction of photosynthesis. The disruption is
titative changes in photosynthesising pigments clear in the oxygen-releasing complex and elec-
and (c) poor assimilation rates in photosynthetic tron transfer from protochlorophyllide to P700.
leaves are found to be affected under water stress Lower electron transport rate negatively affects
which in turn decreases the rate of photosynthe- photophosphorylation process and decreases
sis in plants. Water stress is also found to inhibit ATP synthesis as well as NADP+ reduction.
chlorophyll synthesis and subsequently decrease ATPase inhibition under water deficiency is
chlorophyll content of leaves. In severe stress, also responsible for the reduction in ATP levels
photosynthesis may be more controlled by the in chloroplasts. All these factors cumulatively
chloroplast’s capacity to fix CO2 than by the affect the intensity of photo-assimilation and
increased diffusive resistance (Faver et al. 1996; the stability of the photosynthetic apparatus
Herppich and Peckmann 1997). under the conditions of water stress. Both of
Unlike chlorophyll, other plant pigments like the PSs in chloroplasts are affected by water
xanthophyll are found to be less sensitive to deficiency; however, PS1 of some plants is
water stress. During water stress, the synthesis more severely damaged compared to PS2,
of xanthophyll pigment is shown to be though there is an opposite concept as well
upregulated which supports the finding that xan- (Ramachandra et al. 2004; Lisar et al. 2012).
thophyll pigments have a protective role in plants
under stress and also are found to play an inhibi-
tory role on reactive oxygen species (ROS) pro- Transpiration and Stomata
duction (Lisar et al. 2012). The photosynthetic
enzymes have been shown to be significantly Stomatal closure is commonly the principal
affected by water stress. In case of C4, it is mechanism responsible for restricting transpira-
difficult to draw a conclusion regarding the spe- tion rates in plants during exposure to water
cific pattern in the modulation of enzyme activity stress. Transpiration is directly proportional to
in response to drought stress, whereas in C3 cycle the gradient of water vapour concentration from
enzymes are found to be consistently inhibited in the internal evaporation surface to the bulk air
response to water stress. Activity of RuBisCO, outside the leaf and inversely proportional to the
156 S. Bhattacharjee and A.K. Saha

Fig. 3 Schematic presentation of photosynthetic control mechanisms under water stress. ETR electron transport rate,
RC reaction centre, Fs steady state of chlorophyll fluorescence

total resistance to water vapour transport of the significant reductions in transpiration as water
air boundary layer and of the leaf. In addition, stress develops. The “wall” resistance is small
increased stomatal resistance may not cause pro- in turgid leaves and tends to rise with moderate
portional decreases in transpiration rate because water deficits to a significant level which is nev-
diminished dissipation of heat by vaporisation ertheless still minor compared with the expected
and the consequent rise in leaf temperature stomatal resistance (Crafts 1968). Adaxial and
increase the water vapour concentration inside abaxial stomata have been observed to differ in
the leaf. In most situations, the rise in leaf tem- response to water stress in some cases but appar-
perature accompanying substantial reduction in ently not in others (Wang et al. 1998). The above
transpiration has been calculated or measured to results indicate that stomata are somewhat insen-
be only a few degrees (Hsiao 1973; Chaves et al sitive to mild water stress. However, this conclu-
2003). Therefore, it would be reasonable to sion probably cannot be generalised, since there
assume that elevation in leaf temperature does are direct or indirect indications that stomata of
not play a general role in water-stress effects. other species may be sensitive to small water
Some other non-stomatal factors in the leaf deficits. The stomatal response is found to
like “mesophyll” or “wall” resistance cause be dependent on threshold water status. It is
Plant Water-Stress Response Mechanisms 157

observed that the optimum water content for balance, specifically the balance between ABA
stomatal opening can be actually something less and cytokinins. It was reported that the rapidity
than the tissue water content at full turgor. Full and ready reversibility of the action of ABA on
turgor can cause some stomatal closure, presum- stomata would make it a good modulator of sto-
ably because of excessive back pressure from the matal behaviour. Although stomatal opening is
epidermal cells surrounding the guard cells. reduced during stress by a concerted effect of
Once the threshold water status for stomatal clo- depressed cytokinin level and rise in ABA but
sure is reached, leaf resistance increases sharply, kinetin, a member of cytokinin family, can pro-
rising 20- or 30-fold. Such large increases in leaf mote stomatal opening within a few hours of
resistance may be taken as indicative of almost application. Unfortunately, the stomatal response
complete stomatal closure. Aside from leaf water to kinetin is dependent on the duration of expo-
status, there is some evidence that water vapour sure and age of the plants. Stomata of many
content of the air may be very important in deter- species and apparently of younger leaves do not
mining stomatal opening. In case of maize leaves respond to kinetin. It is also observed that kinetin
it is reported that at the same water deficit the is unable to reverse the ABA-mediated inhibition
diffusive resistance is upto several times as great of stomata (Xiong and Zhu 2003; Hsiao 1973;
in dry air (nearly zero humidity) as in moist air. Yokota et al. 2006).
Light may also modify stomatal response to
water deficit. At higher light levels, more water
deficit seemed to be required to induce closure. It Respiration
has also been reported that stomatal response to
water stress was attenuated by oxygen-free air Water stress exerts a variable response on plant
(Hsiao 1973; Yokota et al. 2006). respiration which ranges from inhibition to stim-
Stomatal opening and closing result from tur- ulation under different water-stress conditions. In
gor differences between guard cells and the different plant organs like leaves, shoots, roots,
surrounding subsidiary or epidermal cells. Sto- flowers or whole plants, a decreased rate of res-
matal interactions with environmental factors piration in response to water stress has been
such as light and CO2 are complex and appear reported. In contradiction, some other reports
to be mediated by a net gain or loss of guard cell have shown that in water-stressed plants the
potassium and turgor with the consequent stoma- rate of respiration is almost unaffected or even
tal movement. As the opening of the stomata is increased. Leaf respiration shows a biphasic
turgor dependent, water deficits by reducing leaf response to relative water content (RWC),
turgor would directly reduce opening. It has also decreasing in the initial stages of water stress
been reported that mild water deficit is associated (RWC > 60 %) and increasing as RWC
with marked loss of solutes from guard cells decreases below 50 % (Flexas et al. 2005).
which is concurrent with stomatal closure. Under this hypothesis, the initial decrease in
Thus, a part of the water-stress effect on stomatal respiration would be related to the immediate
closure and associated decrease in the rate of inhibition of leaf growth and, consequently, the
transpiration may not be direct but is linked to growth of respiration component. The increase of
the regulation of osmotic solutes in guard cells. respiration at lower RWC would relate to an
Another important determinant in modulating the increasing metabolism as the plant triggers accli-
stomatal opening during water stress is found to mation mechanisms to resist water stress. These
be abscisic acid (ABA). It is reported that ABA mechanisms would increase the maintenance
rises markedly in leaves subjected to water stress component of respiration and, as such, the over-
and that exogenous ABA is a potent and fast- all respiration rate. In case of root, the changes in
acting inhibitor of stomatal opening; it is also rate of respiration in response to water stress are
being hypothesised that stress affects stomata found to be age dependent. Respiration in the
via its effect on ABA levels or on plant hormonal established root and rain root is shown to respond
158 S. Bhattacharjee and A.K. Saha

differentially in response to water stress. In adjustment, the process in which solutes accumu-
established root, the rate of respiration never late in growing cells as their water potential falls
reached zero in response to water stress and of osmotic potential arising from the net accumu-
rapidly recovers upon direct rewatering, whereas lation of solutes in response to maintain turgor in
it has been shown that in rain root the rate of tissues (Turner and Jones 1980; Morgan 1984).
respiration rapidly reached zero and did not Osmotic adjustment may allow growth to con-
recover upon rewatering (Graham and Nobel tinue at low water potential. Osmotic adjustment
1999). It is hypothesised that the differential usually depends mainly on photosynthesis to
rate of respiration in response to water stress supply compatible solute. Osmotic adjustment
occurs at a certain threshold of water-stress has been defined as “the lowering water deficits
intensity. It has been reported that dark respira- or salinity” (Turner and Jones 1980). With
tion is generally suppressed, more or less propor- continued water limitation, osmotic adjustment
tionately but not very markedly, by moderate to delays, but cannot completely prevent, dehydra-
severe water stress. Similar kind of response was tion (Kramer and Boyer 1995). Osmotic adjust-
observed in light respiration. It is observed that ment has been found in many species and has
the effects might be due to plasmolysis rather been implicated in the maintenance of stomatal
than water stress. The biphasic response of respi- conductance, photosynthesis, leaf water volume
ration in whole plants against water stress has and growth (Turner and Jones 1980; Morgan
also been observed. The initial tendency is for the 1984). In wheat and other cereals, osmotic adjust-
rate of respiration to decrease probably as a con- ment leads to rapid responses for decreasing the
sequence of decreased energy demand for effect of water stress (Richter and Wagner 1982).
growth. A second trend that appears at severe It is reported that water stress increases the
water stress is the increase of respiration rates, osmotic pressure of the cell sap, increasing the
possibly as a consequence of enhanced metabo- percentage of sugar in sugar cane and often in
lism (osmoregulation, water-stress-induced sugar beet, although the yield per acre may be
senescence processes). It has been reported that reduced (Russel 1976). Solutes known to accu-
the fast-growing plant species show a more pro- mulate with water stress and contribute to
nounced biphasic response than slow-growing osmotic adjustment in non-halophytes include
species (Flexas et al. 2005) (Fig. 5). inorganic cations, organic acids, carbohydrates
and free amino acids. In some plants, potassium
is the primary inorganic cation accumulating dur-
Osmotic Adjustment Mechanisms ing water stress, and it is often the most abundant
Under Water Stress solute in a leaf (Jones et al 1980; Ford and Wilson
1981). Osmotic adjustment is usually not perma-
Water is essential in the maintenance of the tur- nent, and plants often respond rapidly to
gor which is essential for cell enlargement and increased availability of water. Morgan and
growth and for maintaining the form of herba- Condon (1986) showed that such increase in sol-
ceous plants. Turgor is also important in the ute concentration gives tissues a temporary
opening of stomata and the movements of leaves, advantage, enabling turgor to be maintained at
flower petals and various specialised plant low water potentials by decreasing their osmotic
structures (Kramer and Boyer 1995). The turgor potentials (Morgan and Condon 1986).
measurements on the lamina have often appeared
to show declining rates of leaf growth with
decreasing turgor (Kramer and Boyer 1995; Cell Growth and Cell Division
Meyer and Boyer 1972; Michelena and Boyer
1982; Westgate and Boyer 1985). The turgor Because plant growth is the result of cell division
decrease may or may not occur during soil dry- and enlargement, water stress directly reduces
ing, and this is believed to be due to osmotic growth by decreasing CO2 assimilation and
Plant Water-Stress Response Mechanisms 159

reducing cell division and elongation. The effect leaves of various water-stressed plants is altered
of water stress is more evident on cell wall and may act as a metabolic signal in the response
expansion because cell enlargement involves to drought (Akıncı and Lösel 2009, 2010; Chaves
the extensibility of the cell wall under turgor et al. 2003; Koch 1996; Jang and Sheen 1997).
pressure. Therefore, any loss in turgor pressure Munns et al. (1979) and Quick et al. (1992)
as a consequence of the imbalance in the plant showed that sugars are major contributors to
water content could result in reduced growth and osmotic adjustment in expanding wheat leaves
even in the total absence of growth under dry (Munns et al. 1979; Quick et al. 1992). The
environmental conditions. increase of sugar in various plant tissue
Cell growth rate, Gr, can be expressed as a responses to water stress supports the idea of
function of turgor pressure, P, and the extensibil- contribution of solutes while the plants are
ity coefficient, Φ, by the equation Gr ¼ Φ exposed to different stress levels. The studies
(P  Y) where Y is the yield threshold pressure. have shown that soluble sugars accumulate in
The equation shows that growth rate decreases as leaves during water stress and have suggested
P decreases, but it could also be maintained if that these sugars might contribute to osmoregu-
either Φ increases or Y decreases. Therefore, lation, at least under moderate stress (Morgan
reduced growth rate may not rely only on 1984; Quick et al. 1992; Jones et al. 1980;
reduced turgor caused by desiccation. There is Munns and Weir 1981; Ackerson 1981; Kameli
some evidence of reduced growth without loss of and Losel 1993, 1996; Al-Suhaibani 1996).
turgor in plants subjected to desiccation stress, Increase in total carbohydrate is recorded in
but this reduction may be part of the osmotic cotton by Timpa et al. (1986) and Evans et al.
adjustment process. Some mechanism may con- (1992). Total soluble sugar is found to be
trol cell wall extensibility through the perception increased in wheat, alfalfa, lupins, bean and
of soil dryness, giving rise to smaller plants and cucumber (Kameli and Losel 1996, 1993;
hence lower water requirements and higher sur- Irigoyen et al. 1992; Quick et al. 1992; Al-
vival (Hsiao 1973). Suhaibani 1996; Akıncı and Lösel 2009). But
depletion of sucrose and starch content is also
recorded in soya bean, grapevine, lupins, bean
and cucumber by Westgate et al. (1989),
Plant Metabolic Response to Water
Rodriguez et al. (1993), Quick et al. (1992),
Stress
Steward (1971) and Akıncı and Lösel (2009).
Plant adaptations to dry environments can be
Plant Proteins: Responses to Water Stress
expressed at four levels: phenological or devel-
Many specified proteins synthesised under water
opmental, morphological, physiological and met-
scarcity have been isolated and characterised by
abolic. The metabolic response is least known
researches (Singh et al. 1987; Close 1997; Pelah
where the metabolic or biochemical adaptations
et al. 1997; Claes et al. 1990). The water-stress-
are involved (Hanson and Hitz 1982). Physiolog-
specific proteins (stress induced) have been
ical and biochemical changes including
described as dehydrins (polypeptide) and LEA
carbohydrates, proteins and lipids are observed
(late embryogenesis abundant), RAB (responsive
in many plant species under various water-stress
to ABA) and storage proteins (in vegetative
levels which may help in better understanding
tissues) (Artlip and Funkhouser 1995). Under
survival mechanisms in drought.
water-stress conditions, plants synthesise
alcohols, sugars, proline, glycine, betaine and
Carbohydrate Changes Under Water putrescine and accumulate that of those molecu-
Stress lar weights which are low (Chopra and Sinha
The available reports stated that the content of 1998; Galston and Sawhney 1990). Dehydrins
soluble sugars and other carbohydrates in the have been the most observed group among the
160 S. Bhattacharjee and A.K. Saha

accumulated proteins in response to loss of water proteins, Beta alanine amino peptidase A (BapA,
and increased in barley, maize, pea and 87 kDa proteins) and chloroplast proteins
Arabidopsis. Under water stress, LEA proteins (CDSP32 and CDSP 34) are recorded by many
play an important role as protection of plants. scientists. Protein content decrease has been
Osmotin is also an accumulated protein under recorded in Avena coleoptiles (Xu et al. 1996;
water stress in several plant species such as Artlip and Funkhouser 1995; Ramagopal 1993;
tobacco, triplex, tomato and maize (Ramagopal Sinha et al. 1996; Bray 1995; Naot et al. 1995;
1993). Pareek et al. 1997; Pelah et al. 1997; Mantyla
Heat-shock proteins (HSPs) and late embryo- et al. 1995; Pruvot et al. 1996).
genesis abundant (LEA)-type proteins are two Inhibition and/or decrease in protein synthesis
major types of stress-induced proteins during has been recorded in Avena coleoptiles (Dhindsa
different stresses including water stress. Protec- and Cleland 1975), in sugar beet (Shah and
tion of macromolecules such as enzymes, lipids Loomis 1965) and in Pisum sativum L. nodules
and mRNAs from dehydration is the well-known (Gogorcena et al. 1995). Water stress inhibits cell
function of these proteins. LEA proteins accumu- division and expansion, consequently leaf expan-
late mainly in the embryo. The exact functions sion, and also halts protein synthesis. The direct
and physiological roles of these proteins are significance of the inhibition of protein synthesis
unknown. HSPs act as molecular chaperones by stress to growth and leaf expansion is difficult
and are responsible for protein synthesis, to assess. Free proline accumulation in response
targeting, maturation and degradation in many to drought in many plant species tissues is well
cellular processes. They also have important documented (Andrade et al. 1995; Aspinall and
roles in stabilisation of proteins and membranes Paleg 1981; Chandrasekhar et al. 2000;
and in assisting protein refolding under stress Tholkappian et al. 2001; Nair et al. 2006). The
conditions. Expression of LEA-type genes functions of many of these proteins have not been
under osmotic stress is regulated by both ABA- established (Hughes et al. 1989). However, water
dependent and independent signalling pathways. stress may inhibit the synthesis of different
Genes encoding LEA-type proteins are diverse – proteins equally while inducing the synthesis of
RD (responsive to dehydration), ERD (early a specific stress protein (Dasgupta and Bewley
response to dehydration), KIN (cold inducible), 1984).
COR (cold regulated) and RAB (responsive to Treshow (1970) concluded that water stress
ABA) genes (Lisar et al. 2012; Wang et al. 2004; inhibits amino acid utilisation and protein syn-
Singh et al. 2005). thesis (Treshow 1970). Due to unutilisation of
Changes of amino acids and protein have been amino acids, they are accumulated, giving a
mentioned in many reports which have stated 10–100-fold accumulation of free asparagine,
that water stress caused different responses valine and glutamic acid, but alanine levels
depending on the level of stress and plant type. decreased. Barnett and Naylor (1966) found no
Water stress has a profound effect upon plant significant differences in the amino acid and pro-
metabolism and results in a reduction in protein tein metabolism of two varieties of Bermuda
synthesis. Several protein contents were reduced grass during water stress. They have also
by stress in maize mesocotyls (Bewley and reported that during water deficit, amino acids
Larsen 1982; Bewley et al. 1983). Dasgupta and were continually synthesised but protein synthe-
Bewley (1984) pointed out water stress reduced sis was inhibited followed by decrease in protein
protein synthesis in all regions of barley leaf. content.
Vartanian et al. (1987) mentioned the presence
of drought-specific proteins in taproot in Plant Lipid–Water-Stress Interactions
Brassica. Along with proteins, lipids are the most abundant
Various water-stress-induced proteins like component of membranes, and they play a role in
dehydrins, LEAs, RABs, osmotins, boiling staple the resistance of plant cells to environmental
Plant Water-Stress Response Mechanisms 161

stresses (Kuiper 1980; Suss and Yordanov 1986). phospholipid, glycolipid and linoleic acid
Strong water deficit leads to a disturbance of the contents and an increase in the triacylglycerol
association between membrane lipids and of leaf tissues exposed to long periods of water
proteins as well as to a decrease in the enzyme deficits. Enzyme activity and transport capacity
activity and transport capacity of the bilayer are affected by the composition and phase
(Caldwell and Whitman 1987). In plant cell, properties of the membrane lipids (Kuiper
polar acyl lipids are the main lipids associated 1985; Gronewald et al. 1982; Whitman and
with membranous structures (Harwood 1979; Travis 1985). Wilson et al. (1987) observed that
Bishop 1983). Glycolipids (GL) are found in water deficit caused a significant decline in the
chloroplast membranes (more than 60 %), and relative degree of acyl unsaturation (i.e. FA
phospholipids (PL) are thought to be the most unsaturation) in phospholipids and glycolipids
important mitochondrial and plasma membrane in two different drought-tolerant cotton plants
lipids (Harwood 1980). Many workers have (Wilson et al. 1987). Pham Thi et al. (1987)
investigated the effect of different levels of pointed out that changes in oleic and linoleic
water stress on lipid content and composition in acid during water stress resulted in desaturation
different parts of plants (Kameli 1990; Al- and water stress markedly inhibited the
Suhaibani 1996; Pham Thi et al. 1982, 1985, incorporation of the precursors into the leaf lipids
1987; Navari-Izzo et al. 1989, 1990, 1993; (Pham Thi et al. 1987).
Douglas and Paleg 1981; Liljenberg and Kates The study of Navari-Izzo et al. (1989)
1982). Fatty acid, phospholipid, total lipid, etc., revealed the responses of maize seedling to
are recorded to be increased in soya bean, cotton, field water deficits and found that the
wheat, alfalfa and maize by various workers diacylglycerol, free fatty acid and polar lipid
(Navari-Izzo et al. 1990; Pham Thi et al. 1982; contents decrease significantly with stress
Kameli 1990; Al-Suhaibani 1996; Douglas and (Navari-Izzo et al. 1989). The dry land
Paleg 1981; Quartacci et al. 1994; Poulson et al. conditions induced a decrease of more than
2002). It is observed that for Arabidopsis, poly- 50 % in phospholipid levels, and triacylglycerols
unsaturated trienoic fatty acids may be an impor- increased by about 30 % over the control. Pham
tant determinant of responses of photosynthesis Thi et al. (1982) have shown that the most
and stomatal conductance to environmental striking effects are a decrease of total fatty
stresses such as vapour pressure deficit. When acids especially trans-hexadecenoic acid. Water
Vigna unguiculata plants are submitted to deficits inhibit fatty acid desaturation resulting in
drought, the enzymatic degradation of galacto- a sharp decrease of linoleic and linolenic acid
and phospholipids increased. The stimulation of biosynthesis. Wilson et al. (1987) and Navari-
lipolytic activities is greater in the drought- Izzo et al. (1993) found that in plasma
sensitive than in drought-tolerant cvs (Sahsah membranes isolated from sunflower seedlings
et al. 1998). grown under water stress, there is a reduction of
Phospholipid and glycolipid decline is about 24 % and 31 % in total lipids and
recorded in cotton (Wilson et al. 1987; Ferrari- phospholipids, respectively, and also significant
Iliou et al. 1984; El-Hafid et al. 1989), wheat and decreases in glycolipids and diacylglycerols.
barley (Chetal et al. 1981), sunflower and maize
(Quartacci and Navari-Izzo 1992). Total lipid
content decrease is recorded in cucumber and Drought and Nutrient Uptake
squash by Akıncı (1997). Linoleic, linolenic The capacity of plant roots to absorb water and
acid, galactolipid, hexadecenoic acid and nutrients generally decreases in water-stressed
diacylglycerol are found to be decreased in cot- plants, presumably because of a decline in the
ton (Pham Thi et al. 1982, 1985) and in maize nutrient element demand (Alam 1999). It is well
(Navari-Izzo et al. 1989). Investigations on vari- documented that essential plant nutrients are
ous crop species record a general decrease in known to regulate plant metabolism even the
162 S. Bhattacharjee and A.K. Saha

plants exposed to drought by acting as cofactor or downstream protein kinases and phosphatases.
enzyme activators (Nicholas 1975). Drought-inducible genes display characteristic
Many reports stated that water stress mostly promoter cis-acting elements, the dehydration-
causes reduction in uptake of nutrients (Levitt responsive elements (DREs) which at least par-
1980), for instance, phosphorus, K+, Mg2+ and tially resemble those of the cold-induced genes
Ca2+ in some crops (Foy 1983; Abdalla and El- (Bray 1997). Abscisic acid triggers a major sig-
Khoshiban 2007; Bie et al. 2004); Ca2+, Fe3+, nalling pathway in drought-stress response. Acti-
Mg2+, nitrogen and phosphorus and potassium vation of the abscisic acid responsive elements
in Spartina alterniflora (Brown et al. 2006); (ABREs) by several transcription factors such as
Fe3+, Zn2+ and Cu2+ in sweet corn (Oktem the DRE-binding factors and bZIP proteins leads
2008); and Fe3+, K+ and Cu2+ in Dalbergia sissoo to the expression of drought-stress tolerance
leaves (Nambiar 1977). Gerakis et al. (1975) and effectors such as dehydrins or enzymes
Kidambi et al. (1990) stated that nutrient catalysing low molecular weight osmolytes.
elements increased in forage plant species and The signal transduction pathway of ABA
alfalfa. An increase in some specific elements involves cADP ribose, NAADP and Ca2+ as sec-
such as K+ and Ca2+ was reported in maize ond messenger (Quatrano et al. 1997). Calcium
(Tanguilig et al. 1987) and K+ in drought-tolerant appears as a prime candidate in drought-stress
wheat varieties (Sinha 1978). In leaves of signal transduction resulting in a metabolic or
Dalbergia sissoo, nitrogen, phosphorus, Ca2+, structural mitigation of the effect of the stressor.
Mg2+, Zn2+ and Mn2+ increased with increasing Therefore, proteins, which sense changes in the
water stress (Singh and Singh 2004). cytoplasmic calcium concentrations, are impor-
It is generally accepted that the uptake of phos- tant components of the signal transduction chain.
phorus by crop plants is reduced in dry soil Calcium-dependent protein kinases (CDPKs or,
conditions (Pinkerton and Simpson 1986; Simpson in Arabidopsis, CPKs) act as sensor responders
and Lipsett 1973). According to Singh and Singh by combining Ca2+-binding and kinase activity
(2004), availability of soil nutrients decreases with in the same polypeptide. CPK4 and CPK11 have
increasing soil drying, with K+, Ca2+, Mg2+, Zn2+, also been identified as positive transducers of
Fe3+ and Mn2+ decreasing by 24 %, 6 %, 12 %, Ca2+-dependent ABA signalling. Strong ABA
15 %, 25 % and 18 %, respectively. insensitivity in stomata closure and increased
drought sensitivity were reported in the cpk4
Drought Perception, Signal Transduction and cpk11 single and double mutants, with oppo-
and Response site phenotypes observed in CPK4 and CPK11
Plant response to water stress depends on their overexpression lines. Calcineurin B-like proteins
ability to sense the extent or severity of drought (CBLs) are sensor relay proteins that, upon Ca2+
they are exposed. It has been reported that water binding, interact with and modulate the activity
stress can be sensed by a membrane-bound two- of CBL-interacting protein kinases (CIPKs).
component histidine kinase which is activated by CBL1 an isoform of CBL was identified as a
high osmolarity. The increase of a cell osmolar- relay for ABA-mediated responses and can act
ity upon water loss during drought therefore as a positive regulator of drought signalling.
triggers the signal transduction in response to CBL1-overexpressing plants exhibit enhanced
drought. The active signal receptor activates drought tolerance and constitutive expression of
phospholipase C (PLC) which hydrolyses stress genes. Although not only CBL single
phosphatidylinositol 4,5-bisphosphate to yield mutant is ABA hypersensitive in guard cells but
the second messengers inositol 1,4,5- also the cbl1cbl9 double mutant was reported to
trisphosphat (IP3) and diacylglycerol (DAG) be more drought tolerant in wilting assays and
(Mahajan and Tuteja 2005). IP3 releases calcium the stomatal closure response in the double
from internal stores, and the Ca2+ sensor mutant was hypersensitive to ABA. It has been
(calcineurin B-like protein, CBL) activates shown that in the vasculature and in guard cells,
Plant Water-Stress Response Mechanisms 163

luciferase reporter expression under the control stress (Kaur and Gupta 2005; Xiong et al. 2002;
of ABA-responsive AtHD6 (histone deacetylase Raghavendra et al. 2010).
6) promoter was detected in response to drought,
suggesting a role for tissue autonomous ABA DNA Elements Controlling Gene
synthesis in addition to long-distance root-to- Expression During Water Deficit
shoot movement of ABA in response to water The most comprehensive information about the
stress. It has been observed that the transcription mechanism of regulation of gene expression in
factors like NFYA5 (nuclear factor Y, subunit response to water deficit has been obtained from
A5) in Arabidopsis and the maize NF-YB2 func- the investigation of DNA elements and
tion as positive regulators of drought-stress sequence-specific DNA-binding proteins. Pres-
responses, suggesting a possible role of the ently, two classes of DNA elements have been
CCAAT box element and its binding partner identified: the ABA-responsive element (ABRE)
NF-Y in ABA/drought-stress signalling. Besides and the dehydration-responsive element (DRE).
transcriptional induction by ABA, NFYA5 gene The ABRE has been shown to be sufficient for
expression is further enhanced by posttranscrip- ABA-regulated gene expression during water
tional control of NFYA5 mRNA stability. deficit, but in some genes it must be associated
NFYA5 transcripts contain a target site for the with a coupling element. The dehydration-
microRNA, miR169, which is downregulated by responsive element from the rd29A gene from
drought. Furthermore, overexpression of miR169 Arabidopsis, TACCGACAT, has been shown to
and a T-DNA insertion mutation in NFYA5 both be involved in the regulation of this gene by an
caused drought sensitivity (Raghavendra et al. ABA-independent pathway induced by water
2010; Xiong et al. 2002). deficit. It has been shown that these are insuffi-
Other intracellular hazards observed in plants cient for controlling the genes that are induced by
in response to drought stress are the generation of water deficit, and new additional DNA elements
reactive oxygen species (ROS), which is being and several of these elements are beginning to be
considered as the cause of cellular damage. How- defined. In the Arabidopsis gene rd22, which
ever, recently, a signalling role of such ROS in requires protein synthesis for expression, there
triggering the ROS scavenging system that may is a DNA element, CACATG, that is similar to
confer protection or tolerance against stress is the element bound by the transcription factor
emerging. Such scavenging system consists of MYC (Kaur and Gupta 2005; Xiong et al. 2002).
antioxidant enzymes like SOD, catalase and
peroxidases and antioxidant compounds like
ascorbate and reduced glutathione; a balance Mechanisms of Acclimation to Water
between ROS generation and scavenging ulti- Deficit and Stress Tolerance
mately determines the oxidative load. As Plants have developed multiple mechanisms in
revealed in case of defences against pathogen, order to protect PSA against different kinds of
signalling via ROS is initiated by NADPH stresses. At the cellular level, plants attempt to
oxidase-catalysed superoxide generation in the alleviate the damaging effects of stress by alter-
apoplastic space (cell wall) followed by conver- ing their metabolism to cope with the stress.
sion to hydrogen peroxide by the activity of cell Many plant systems can survive dehydration
wall-localised SOD. Wall peroxidase may also but to a different extent. According to Hoekstra
play role in ROS generation for signalling. et al. (2001) on the basis of the critical water
Hydrogen peroxide may use Ca2+ and MAPK level, two types of tolerance are distinguished:
pathway as downstream signalling cascade. 1. Drought tolerance can be considered as the
Plant hormones associated with stress responses tolerance of moderate dehydration, down to
like ABA and ethylene play their role possibly moisture content below which there is no bulk
via a crosstalk with ROS toward stress tolerance, cytoplasmic water present – about 0.3 g
thus projecting a dual role of ROS under drought H2O g–1 DW.
164 S. Bhattacharjee and A.K. Saha

of the thylakoid lumen that induces Zx and Ax

synthesis. It has been proposed that the
photoprotective process results in the diversion
of energy away from the reaction centres (Ruban
and Horton 1995; Medrano et al. 2002).
According to Tambussi et al. (2002), the non-
photochemical fluorescence quenching (qN),
as well as the content of zeaxanthin and
antheraxanthin after moderate WS, increased sig-
nificantly. However, at severe WS, a further rise
in these xanthophylls was not associated with
any increase in qN. In addition, the β-carotene
content rose significantly during severe WD,
suggesting an increase in antioxidant defence.
Besides the above-mentioned mechanisms of
energy dissipation, there are also other ways.
Fig. 4 Water stress induced the synthesis of accessory
For example, the energy dissipation in closed
photosynthetic pigments like zeaxanthin and
antheraxanthin stomata can occur via ATP and NADPH, which
are used for other metabolic processes, and they
are obviously important mechanisms of tolerance
2. Desiccation tolerance refers to the tolerance and protection against water stress and photooxi-
of further dehydration, when the hydration dative damage (Lichtenthaler 1996) (Fig. 4).
shell of the molecules is gradually lost. Desic- During dehydration, anhydrobiotes pass
cation tolerance includes also the ability of through hydration ranges that also necessitate
cells to rehydrate successfully. protection against drought. The desiccation tol-
According to Bohnert and Shen (1999), a erance programme can be switched on by dehy-
nearly universal reaction under stress conditions, dration and the plant hormone ABA (Ingram and
including WD, is the accumulation of “compati- Bartels 1996). Upon water loss, the cellular vol-
ble solutes”, many of which are osmolytes (i.e. ume decreases and cell content becomes increas-
metabolites whose high cellular concentration ingly viscous and the chance for molecular
increases the osmotic potential significantly) interactions rises. The danger of protein
considered to lead to osmotic adjustment. These denaturing and membrane fusion increases. But
observations indicate that “compatible solutes” a range of compatible solutes which do not inter-
may have other functions as well, namely, in the fere with cellular structure and function hinder
protection of enzyme and membrane structure this process. It is considered that at lower water
and in scavenging of radical oxygen species. contents, molecular oxidants (glutathione, ascor-
One of the principal mechanisms employed by bate, tocopherol) play a preponderant role in
plants to prevent or to alleviate damage to the elevating oxidative stress. Hoekstra et al. (1997)
PSA is non-photochemical chlorophyll fluores- showed that desiccation may increase the transfer
cence quenching (qN) (Ruban and Horton 1995). of these amphiphiles from the polar cytoplasm
In this mechanism, excess light energy is into the lipid phase of membranes. They thought
dissipated as heat in the light-harvesting antenna that this partitioning into membrane might be
of PS2. This dissipation is primarily controlled extremely effective in automatically inserting
by the trans-thylakoid pH gradient (pH) amphiphilic antioxidant into membranes upon
(Gounaris et al. 1984). dehydration.
When CO2 fixation and therefore ATP con- Reduction of metabolism coincides with sur-
sumption are decreased at low RWC, the func- vival of desiccation (Leprince et al. 1999). In
tioning electron flow gives rise to an acidification vegetative tissues, genes encoding enzymatic
Plant Water-Stress Response Mechanisms 165

Fig. 5 Schematic presentation showing integrated approach of acclimation to water stress

antioxidants such as APX, SOD and GRase are changes and loss of enzymatic function.
upregulated during drying or rehydration According to the water replacement hypothesis,
(Fig. 5). When the bulk water is removed sugars act as a water substitute by satisfying the
(below 0.3 g H2O g–1 DW), the mechanism hydrogen-bonding requirement of polar groups
keeping the macromolecules preferentially of the dried protein surface (Carpenter and
hydrated through amphiphiles fails to work, Growe 1988; Wolkers et al. 1998). At around
because there is no water left for preferential 0.3 g H2O g–1 DW, the cytoplasm vitrifies and
hydrations (Crowe et al. 1990). It has been exists in a so-called glassy state, an amorphous
established that during desiccation, soluble metastable state, retaining the disorder and phys-
sugars interact with the polar head groups and ical properties of the liquid state (Franks et al.
replace the water molecules. Phospholipid 1991). This state decreases the probability of
molecules largely retain the original spacing chemical reactions and is indispensable for sur-
between one another. When water dissipates viving the dry state. A very important role in this
from the water shell of macromolecules at mois- process is played by late embryogenesis abun-
ture contents below 0.3 g H2O g–1 DW, the dant proteins (LEAPs), especially their Group 1 –
hydrophobic effect responsible for structure and dehydrins, in stabilisation and protecting during
function is lost. After bulk water is lost, the desiccation. It was observed that their accumula-
hydrogen bonding and glass formation are the tion coincides with the acquisition of desiccation
mechanisms by which membranes and proteins tolerance (Bartels et al. 1988). Group 1 proteins
are structurally and functionally preserved. have very high potential for hydration – several
Sugars are special in that they allow the times greater than that for “normal” cellular
removal of the closely associated water from proteins (McCubbin et al. 1985). Because of
protein without this leading to conformational these special features, LEAPs potentially bind
166 S. Bhattacharjee and A.K. Saha

to intracellular macromolecules coating them reactive oxygen species are also induced. It is
with a cohesive water layer and preventing their difficult to ascertain whether the induction of
coagulation during desiccation (Close 1996). these genes is to repair damage caused directly
Upon removal of their own hydration shell, by reduced water content or if they accumulate to
these proteins would still be capable of playing ameliorate damage caused by a secondary stress or
a role in stabilising macromolecular structures. to restrict pathogen invasion. The characterisation
They could provide a layer of their own of genes induced by water deficit has greatly
hydroxylated residues to interact with surface improved our understanding of plant responses to
groups of other proteins, acting as “replacement the environment.
water” (Cuming 1999; Buitink et al. 2002).
Wolkers et al. (1999) suggested that LEAPs
embedded in the glassy matrix might confer sta-
bility on slowly dried carrot somatic embryos. Conclusion
Another class of proteins associated with des-
iccation tolerance are low molecular weight The multitude of different stressors, their spatial
HSPs. Coordinated expression of LEAPs and and temporal character, their variation in inten-
sHSP transcripts is observed during embryo sity and dose and their potential interaction yield
development in response to ABA, indicating the an abundance of scientific questions. One of the
existence of common regulatory elements of most interesting aspects of water-stress physiol-
LEAPs, sHSPs and desiccation tolerance ogy is how mild or moderate stress is transduced
(Wehmeyer et al. 1996). But so far, there is no into alterations in metabolism. The foregoing
direct experimental evidence for a specific role of considerations make it seem unlikely that mild
sHSPs in desiccation tolerance. Satoh et al. stress could, by any of the mechanisms men-
(2002) followed recovery of the photosynthetic tioned, damage biochemical components or
system during rewatering in a terrestrial, highly organelles of the cell; yet mild stress does have
drought-tolerant cyanobacterium Nostoc com- pronounced effects. It is more probable that
mune. With absorption of water, the weight of the changes in metabolism elicited by mild stress
Nostoc colony increased. Fluorescence intensities represent plant regulatory responses rather than
of phycobiliproteins and PS1 complexes recovered damage. This in turn implies that many of the
almost completely within 1 min, suggesting that changes in plant processes brought about by
their functional forms were restored very quickly. stress arise indirectly. Among all the changes,
PS1 activity and cyclic ET flow around PS1 recov- the most important aspect of water stress proven
ered within 2 min, while the PS2 activity recovered to be results reduced cell growth. Inhibition of
after a time lag of 5 min. Photosynthetic CO2 cell growth during water stress is found to cor-
fixation was restored almost in parallel with the roborate with inhibition of protein synthesis, cell
first recovery phase of PS2 reaction centre activity wall synthesis, membrane proliferation, etc. For
(Fig. 5). maintaining balance of metabolites, the plant has
There is need to search for valuable approaches probably evolved controls which slow down syn-
in order to identify those metabolic steps that are thesis of cell building blocks when low turgor
most sensitive to drought and to elucidate which prevents expansion. This may be a likely expla-
metabolites and gene products are of primary nation for the susceptibility of cell wall synthesis
importance for increasing drought tolerance of and polyribosomes (hence protein synthesis) in
plants. Many proteins are involved in damage lim- growing tissue to very mild water stress. It has also
itation or the removal of toxic compounds which been reported that water stress is associated with
are induced during water deficit. For example, impaired lipid synthesis in such tissue. These
ubiquitin, chaperones and proteases may all be explain how cell wall synthesis is impaired during
involved in the recovery of proteins or their build- water stress, and it may be coupled with suppres-
ing blocks. Genes encoding enzymes that detoxify sion of plant growth. Various other changes may
Plant Water-Stress Response Mechanisms 167

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