PHYSIOLOGY OF HORMONE ACTION

• PLANT HORMONE – organic compound

synthesized in one part of a plant and
translocated to another part

• Very low concentrations – physiological

response

• Site of formation -----site of action of plant

hormones (no obligating divisions)
 PHYSIOLOGY OF HORMONE ACTION
• If required, they are able to act on the same
cells or tissues in which they were formed

• Hormones – are endogenous or naturally

occurring compounds

• Hormones do not act alone (act in conjunction

or opposition to each other)
 PHYSIOLOGY OF HORMONE ACTION
• Gibberellins can counteract the abscisic acid-
induced bud or seed dormancy (induce or
break dormancy)

• Physiological efficiency of a hormone –

interaction between concentration of the
effective hormone and sensitivity of cells
reacting to the hormone
 PHYSIOLOGY OF HORMONE ACTION

• Certain concentration of ethylene –trigger

senescence in a slightly yellowing leaf
(sensitive)—not on mature green leaf

• Gibberellins – stimulate cell elongation on

dwarf rice (sensitive) but ineffective if used in
non-dwarf cultivar of rice
 PHYSIOLOGY OF HORMONE ACTION

• The same concentration of auxin promotes

cell elongation in shoot tissues but inhibits
root elongation
 HORMONE BREAKDOWN

• Rapid breakdown or secretion –avoiding

hormone accumulation at the site of action
 HORMONE PHYSIOLOGY

• Hormones have multiple effects

• The same hormone can cause different

physiological reactions to different cells
 HORMONES

• Hormones can be in free (zeatin) or bound

form (zeatin riboside)
 FIVE PRINCIPAL CLASSES OF PLANT
HORMONES

• AUXINS
• GIBBERELLINS
• CYTOKININS
• ETHYLENE
• ABSCISIC ACID
 AUXINS
• Indoleacetic acid (IAA) – the principal auxin in
higher plant
• Synthesized from the amino acid tryptophan
• Highest concentration of free auxins – apical
meristem of shoots and young leaves
• Auxin levels – controlled by its synthesis and
degradation and by its deactivation as it forms
bound IAA (IAA-glucose)
 AUXINS
• Promotes cell elongation in stems and
coleoptiles—yet same concentration may
inhibit root elongation
• Promotes cell division in stems but inhibits
growth in lateral buds (apical dominance)
• Mediates the effects of light and gravity on
growth, tropism, which is response to the
direction of the stimuli
 AUXINS

• Promotes formation of lateral roots and

adventitious roots
• Regulates initiation of DNA replication (G1
phase)
• Delays the onset of leaf abscission
 AUXINS

• Regulates fruit development (production of

seedless fruits or parthenocarpy)
• Regulates expression of specific genes
• Induces vascular differentiation
 ACID-GROWTH HYPOTHESIS
• Cell elongation as induced by auxin

➢ auxin causes responsive or competent cells to

extrude protons actively into the cell wall
region
➢ the resulting decrease in pH activates wall-
loosening enzymes that promote the breakage
of essential cell wall bonds
 ACID-GROWTH HYPOTHESIS
➢ this increases cell wall extensibility, resulting
to cell elongation

➢At the same time, auxin may also promote

synthesis of cell wall proteins needed for
growth to sustain elongation for a longer
period. This step involves auxin regulation of
gene expression
 AUXINS
• Phototropism and Gravitropism – tropical
growth responses mediated by auxins

• Cholodny-Went Theory of Lateral Distribution

of Auxin - differential growth responses result
from the unequal distribution of IAA as
induced by light or gravity
 GIBBERELLINS
• Isoprenoid compounds synthesized from
mevalonic acid in tissue and seeds
• Immature seeds – high level of GA
• Vegetative tissues (young leaves, buds and
upper stem) – low levels
• Roots also synthesize GA
• 90 known GA
• Passively transported along the xylem or
phloem
 GIBBERELLINS
• Regulates stem elongation in intact plants
• Enhances bolting (growth of long floral stalk)
and flowering in long day plants
• Stimulates α- amylase activity in germinating
cereal seeds and mobilizes food and minerals
in seed storage cells
• Promotes growth of dormant buds and
induces seed germination (photodormant
seeds)
 GIBBERELLINS

• Stimulates fruit setting and growth of some

fruits
• Induces maleness in flower
• Causes reversion from mature to juvenile state
in leaves
 GIBBERELLINS
• Regulates stem elongation by increasing cell
wall extensibility which results from the
prevention of reactions that lead to the
stiffening of the cell wall

• Promotes hydrolysis of starch, sucrose and

other sugars (helps create a more negative
water potential---water enters more rapidly----
causing cell expansion)
 GIBBERELLINS
• Increases cell division

• Enhances the transcription of α – amylase

RNA in the aleurone layer of seeds

• This enzyme hydrolyzes stored food and

makes it more usable as substrate for
respiration----initiating earlier germination
 CYTOKININS
• Adenine or amino purine derivatives found in
plant apical meristems and young organs

• Root apex – main site of CK production

• Transported to the shoot through the xylem as

CK nucleotides

• Zeatin – naturally occurring cytokinin in most

plants
 CYTOKININS
• Promotes cell division and organ formation
(together with auxin, regulates organogenesis
in cultured tissues)
• Delays senescence and increases nutrient sink
activity
• Regulates events on the cell cycle leading to
mitosis
• Promotes maturation of chloroplasts
 CYTOKININS

• Regulates protein synthesis via polysome

formation
• Regulates calcium concentration in the cytosol
 CYTOKININS
• Delays senescence –stimulating the synthesis of
specific chloroplast proteins (encoded by nuclear
genes and synthesized by cytoplasmic ribosomes)
• Stabilizing specific mRNAs and by slowing their
degradation
• In detached leaves, CK delays senescence by
protecting the tonoplast membrane against
degradation…preventing leaking protease from
leaking out and destroying chloroplasts
 CYTOKININS

• Synthetic CK like benzyladenine –used to

slow down aging in cut flowers and fresh
vegetables

• Kinetin – common component in callus

culture and in most cultures of in vitro
experiments
 ETHYLENE
• Gas hormone

• Lighter than air (under physiological

conditions)

• It can be oxidized to produce ethylene oxide;

hydrolyzed to yield ethylene glycol; completely
oxidized to form carbon dioxide
 ETHYLENE
• In higher plants, methionine is the precursor
of ethylene

• Immediate precursor – 1-aminocyclopropane-

1-carboxylic acid (ACC)
• All plant parts produce ethylene
• Easily released from tissues through diffusion
 ETHYLENE

• Ethylene biosynthesis is induced by stressed

conditions such as drought, flooding, salinity
or wounding
• “Stress ethylene” – onset of stress responses
(abscission, senescence, and associated
physiological acclimation to the stress)
 ETHYLENE
• Promotes fruit ripening
• Causes abscission, shedding of leaves, flowers,
fruits and other organs
• Induces epinastic (downward) curvature of
leaves
• Controls hook opening in seedlings (co-action
with phytochrome)
• Induces root formation in leaves, stems and
flowers
 ETHYLENE

• Triggers foliar and flower senescence

• Control sex expression (cucurbits)
• Synchronizes flowering and fruiting in
pineapple and mango
 ETHYLENE

• Softening of cell walls---ripening---correlates

with increasing activity of cellulase and
polygalacturonase (catalyze the hydrolysis of
cellulose and pectin)

• Ethylene regulates the transcription of genes

encoding these cell wall degrading enzymes
ABSCISIC ACID
• Widely distributed in nature
• 15-C sesquiterpenoid synthesized from
mevalonic acid in chloroplasts and other
plastids
• Acts more as inhibitor rather than as promoter
• It is transported in both xylem and phloem
and in parenchyma cells outside vascular
bundles
 ABSCISIC ACID
• Maintenance of seed and bud dormancy
• Induction of stomatal closure under water
stress
• Stimulation of stress tolerance
• Inhibition of growth (prevents cell wall
acidification)
• Stimulation of abscission (in few species) and
senescence
 ABSCISIC ACID

• Stimulation of water and ion flux in roots

• Reduction in leaf area while increasing water-
absorbing area of roots in stressed plants
ABSCISIC ACID

• Affects proteins synthesis under certain

conditions
• In salt tolerance, ABA induces a gene to
encode specific proteins (osmotin) that confer
tolerance to salt
• ABA can also inactivate certain genes
PHYSIOLOGY OF PLANT MOVEMENTS

• TYPES OF MOVEMENTS:
• Tropistic
• Nastic
• Turgor
PHYSIOLOGY OF PLANT MOVEMENTS
• TROPISMS – growth responses in which the
direction of the stimulus determines the
direction of movement
• NASTIC MOVEMENTS – direction of growth or
movement is unrelated to the direction of
stimulus
• TURGOR MOVEMENTS – differential
reversible uptake of water
 NASTIC MOVEMENTS
• Reversible responses
• A. Epinasty/Hyponasty – leaves/leaflets—
caused by difference in growth of cells in the
pulvini at base of petiole

• Epinasty – occurs when cells on top of the

petiole or blade elongate irreversibly than
those on the bottom
• Epinasty is a perplexing behavior seen during
the flooding of roots of plants.
• Leaf cells on the top part of the leaf and
maybe especially the leaf stem outgrow the
bottoms ones and the leaf drops from a
horizontal to a more vertical position.
• Epinasty is caused by ethylene that is released
by the leaves when the roots are flooded.
• Hyponasty. An upward bending of leaves or
other plant parts, resulting from growth of the
lower side.
 NASTIC MOVEMENTS
• B. Nyctinasty – leaf movements from nearly
horizontal during the day and nearly vertical
at night; sleep movements as in legume
leaves; response to light
 NASTIC MOVEMENTS
• C. Thigmonasty/Seismonasty – movement
resulting from touch
• Touch, shaken, heated, rapidly cooled

• C. Thermonasty – temperature-induced
movement such as the opening and closing of
tulip flowers; high temp. – flower opens; low
temp. – flower closes
 TROPISMS
• Directional differential growth
• A. Phototropisms – coleoptile curvature
towards the lighted side
• Cells in the illuminated side – inhibited in their
growth
• Cells in the shaded side – grow
correspondingly faster
• Auxin
 TROPISMS
• B. Heliotropism or solar tracking – leaf
movement or orientation following the
direction of the sun’s rays

• C. Gravitropism - growth movements towards

or away from the earth’s gravitational pull
• Orthogravitropism - vertical growth
• Diagravitropism – horizontal growth
 TROPISMS

• Plageogravitropism - growth at any

determined angle to vertical
 TROPISMS
• D. Thigmotropism – tendrils bend toward the
point of contact wrapping around the support