MINISTRY OF Ministry of Higher

AGRICULTURE, Education and

HYDRAULIC Scientific Research.
RESOURCES and
fishing.
University of
Institution of Agricultural Carthage
Research and Higher
Education

The Higher School of agriculture Of Mateur

Bibliographic research

Engineering cycle

Theme:
Specificity of root symbionts

Elaborated by: Ourari Sahar

Supported the: 24 /02 /2021

In front of the jury:

Superviser : M me Sana Dhane Fitouri

President : M me Sana Dhane Fitouri

Examiner : M me Rassaa

academic year 2020-2021

 Table of contents:

Introduction……………………………………......……………………….....……………1

A. Legume rhizobia symbiosis:…………..…………………………….……..2

I. Definition:……………………………………………..………………………………..2
II. Symbiosis partners:……………………………………...…………………….……….2
1) Plant partner: legumes ………………………………...……….………………….2
2) Bacterial partner: Rhizobia………………………………...……………………...2
3) Interests of legume-rhizobia symbiosis………………………...………………….3

III. Biological nitrogen fixation…………………………………………...…….………….3

1) Discovery....………………………………………………………………………….3
2) Nitrogen role………………………………………………………...…………...….4
3) Plant available nitrogen in soil…………………………………….……………….4
4) Nitrogen fixation andnodulation………………………………………….………..5
a. Nodule types……………………………………………………………………..6
b. Stages of implementation……………………………………………………….6

IV. Host specificity…………………………………………………………………………..7

1) Examples………………………………………………………………….………….7
a. Papilionoideae-Rhizobia Symbioses……………………...…….…...……….....7
b. Caesalpinioideae-Rhizobia Symbioses……………………….………………..11
c. Mimosoideae-Rhizobia Symbioses………….…………………………………11
2) Nodulation genetics in bacteria……………………………………..……………..13
a. Nod genes………………………………………………………………………..13
b. Nif genes…………………………………………………..……………………..14
c. Fix genes……………………………………………………..…………………..15
3) Plant immunity and host restriction…………………………………………….....15

B. Mycorrhizal symbiosis………………………………………………………16
1) Definition…………………………………………………………………………….16
2) The different types of mycorrhizal symbiosis……………………………………..16
3) The vegetal partner…………………………………………………………..……..16
4) The fungal partner………………………………………………………………….17
I
 5) Host specificity…………………………………………….……...…………………17
6) Implementation of symbiotic interaction……………….…………………………17
7) Ecological impact of mycorrhizal symbiosis……..………………………………..18

C. Interests of root symbioses…………………………………....…………….18

D. Use of symbionts in biofertilization………………………...……………….19

Conclusion…………………………………………………………………………………….20

Bibliographic references……………………………………………………………………..21

List of tables:

Table1: Some key dates of history of research on nitrogen, biological nitrogen fixation (BNF),
and biofertilizers………………………………………………………………………………..4

Table 2: Legume-rhizobia symbioses of the legume sub-family Papilionoideae with

indeterminate nodules…………………………………………………………………………..7
Table 3: Legume-rhizobia symbioses of species in the sub-family Papilionoideae with
indeterminate nodules…………………………………………………………………………..8
Table 4: Legume-rhizobia symbioses of species in the sub-family Papilionoideae with
determinate nodules……………………………………………………………………………..9
Table 5: Legume-rhizobia symbioses in the legume sub-family Mimosoideae. All species have
indeterminate nodules………………………………………………………………………….11

II
III
List of figures:

Figure1: Classification of rhizobia forming nodules in legumes……………………………....3

IV
List of abreviations:

AMF: arbuscular mycorrhizal

ATP: adenosine triphosphate

BNF: biological nitrogen fixation

GS: glutamine synthetase

HSN: host specific nodulation

IRLC: inverted repeat lacking clade

IT: infection thread

LCO: lipo-chitooligosaccharides

N: nitrogen

P: phosphorus

PGPM: plant growth-promoting microorganisms

PGPR: plant growth promoting rhizobacteria

V
Introduction:

The rhizosphere refers to the area of soil surrounding the roots and being under the
influence of the plant's metabolism. It is divided into three zones: the endorhizosphere (root
tissue), the rhizoplan (root surface) and the ectorhizosphere (root adhering soil or rhizospheric
soil). (Lynch & Whipps 1990; Nguyen 2003). Plants, being autotrophic organisms or primary
producers, are key organisms which introduce carbon into the ecosystem, through
photosynthesis. Part of this carbon is released as more or less complex molecules at the
rhizosphere level, thanks to the rhizodeposition process. These compounds act as signal
molecules and nutrients for soil microorganisms, which are mainly heterotrophic, in the so-
called rhizosphere effect. This process is costly for the plant but beneficial to the
microorganisms. In return, microorganisms contribute to plant nutrition and health, which is
costly but provides them with a beneficial source of nutrients. These trophic exchanges,
however, are based on a balance which depends on the biotic (Microbial biodiversity) and
abiotic (globalchanges) conditions that affect each partner. (Clémentine LEPINAY,2013) The
plant is therefore the main structuring force of microbial communities. It is estimated that
about 20,000 plant species (out of about 300,000) are entirely dependent on microbial
organisms for their growth and survival (van der Heidjen et al 2008). This dependence implies
the establishment of recognition and functioning systems between the plant and the
microorganisms that must be regulated so that the association takes place; this will therefore
lead to a specificity of the associations between the plant and the so-called symbiotic
microorganisms. Symbionts set up mechanisms for recognition of the appropriate host plant
that will provide them with the necessary nutrients (Brencic & Winans 2005). They are
actually to be mutualistic when they have a beneficial effect on the nutrition and/or health of
the plants with which they interact. For example, this is the case of plant-Rhizobium symbiosis
leading to the formation of root nodules in which rhizobia establish themselves in order to fix
atmospheric nitrogen. (Young et al 2003). The same applies to the supply of phosphorus to
mycorrhized plants. The arbuscular mycorrhizal fungi (AMF: Arbuscular Mycorrhizal Fungi)
will establish themselves in the roots and then capture the phosphorus from the soil, transport it
along their hyphae and then deliver it to the host plant. (Bever et al 2012)

1
 A. Legume rhizobia symbiosis:
I. Definition:

Symbiosis is a mutually beneficial association between two organizations, often established

between an autotrophic partner and a heterotrophic partner. The most well-known nitrogen-
fixing symbiosis involves the Legumes family with its bacterial symbionts, Rhizobia. For
mutual benefit, this association gives rise to multiple interactions between the two partners
during which a new organ: the nodule, is formed on the roots or more rarely on the stems from
dormant root primordia arranged in rows along it. It is within this protective and nutritive
organ that atmospheric nitrogen is fixed by bacteria.

II. Symbiosis partners:

1) Plant partner: legumes

The Leguminosae (Fabaceae, the legume family) comprises 19,300 species within 750 genera
that occur as herbs, shrubs, vines or trees in mainly terrestrial habitats and are components of
most of the world’s vegetation types , currently, the legume family is split into three sub-
families, the Caesalpinioideae, Mimosoideae and Papilionoideae.

Legumes subfamilies:

a) The Caesalpinoideae:
The Caesalpinoideae comprising about 150 genera and 2200 species, mainly gathers trees or
shrubs. Only a few members of this subfamily are capable of nodulation, 30% as against 90%
within the other two subfamilies.
b) The Mimosoideae:
The Mimosoideae has over sixty genera and about 2500 species with notably the genera
Acacia and Albizia.
c) The Papilionoideae:
The Papilionoideae represents the foremost diverse group with about 430 genera and over
12000 species. The plants of this subfamily are mainly herbs (annual plants), but also include
trees and shrubs.
Legumes include beans (Phaseolus vulgaris), soybeans (Glycine max), peas (Pisum sativum),
chickpeas (Cicer arietinum), broad beans (Vicia faba), cowpeas (Vigna unguiculata), lentils
(Lens esculenta), and peanuts (Arachis hypogea) etc.

2) Bacterial partner: Rhizobia:

2
All the Rhizobia characterized are Gram-negative, rod-shaped bacteria in the free state, non-
sporing and generally mobile thanks to the presence of one or more flagella, present in the soil
and belonging to the subclasses α and β proteobacteria. (Mousavi, 2016)
Rhizobia have two lifestyles, a free-living state in soil and a symbiotic state in plant cells when
engaged in symbiosis. (Gibson and al., 2008) Rhizobia therefore have to adapt to changing
conditions outside and inside the host plants.
They are first defined by their ability to induce the formation of nitrogen-fixing nodules on the
roots or stems of legumes and to reduce atmospheric nitrogen into ammonium that can be
assimilated by the plant. This reaction is catalyzed by an enzymatic complex:
Nitrogenase/Hydrogenase.
Rhizobia taxonomy:
Over the past twenty-five years, DNA-based methods have become increasingly used to
characterize rhizobia.

Figure1: Classification of rhizobia forming nodules in legumes. (Pooja Suneja and al., 2017)

3) Interests of legume-rhizobia symbiosis:

This symbiosis has many advantages for legumes. Indeed, it allows them to have good growth
on soils deficient in nitrogen. As a result, this symbiosis is undoubtedly the major factor behind
the great success of the Legumes family among plants.
3
 In contrast, the plant supports the energetic needs of the bacteria during this symbiosis by
supplying carbon substances resulting from photosynthesis. It also offers a very special
microenvironment necessary for nitrogen fixation. Moreover, in addition to increasing the
population of host legume-specific rhizobia at ground level after cultivation, the symbiosis
would provide a beneficial breeding framework that would promote the evolution of bacterial
species.

III. Biological nitrogen fixation:

1) Discovery:

Biological nitrogen fixation was discovered by Hellriegel and Wilfarth (1886), who reported
that some legumes could use nitrogen gas (N2) from nodules on their roots. Two years later
(1888), the N-fixing bacteria strain Rhizobium leguminosarum was isolated for the first time
by Beijerinck. In 1893, Winogradsky reported the isolation of Clostridium pasteurianum while
the concept of inoculation of legumes with N-fixing rhizobia was introduced to australian
farmers by Guthrie in 1896. In 1901, the aerobic heterotroph Azotobacter has been isolated and
described by Beijerinck. The first field trials using a culture of rhizobium and field peas have
been carried out in 1905 in Australia (Hawkesbury agricultural college) and later in 1914,
farmers have been supplied with rhizobial inoculants. In 1979, Steward used the Nitrogen-15
(15N) tracing technique to demonstrate nitrogen fixation in cyanobacteria. For archaea, their
ability to fix nitrogen was only recently highlighted by independent discoveries of diazotrophic
growth in two different methanogenic archaea: Methanosarcina barkeri and Methanococcus
thermolithotrophicus. In this respect, in the late 90s many public and private organizations in
eastern and southern Africa have been involved in the production of N-fixing inoculants and in
a few years these discoveries led to the development of numerous commercial biofertilizers
across the world (ABDOULAYE SOUMARE, 2020)

Some major events in the history of research on nitrogen and BNF are summarized in Table 1.

Table1: Some key dates of history of research on nitrogen, biological nitrogen fixation (BNF),
and biofertilizers (ABDOULAYE SOUMARE, 2020)

Date Events
1836 Identification of nitrogen as a nutrient for plants
1886 Hellriegel and Wilfarth demonstrated the ability of legumes to convert N2
1888 First rhizobia were isolated from nodules
1893 Isolation of Clostridium pasteurianum (Free-living N fixers)
4
1895 First commercial inoculant (Nitragen)
1901 Isolation Azobacter
1913 Carl Bosch performed Haber’s ammonia synthesis on an industrial scale
1946 Second commercial inoculant (Azotogen)
1953 Identification of two nitrogen fixing bacteria: Beijerinckia fluminensis and
Azotobacter paspali
1969 Positive results for 15N2 uptake by cyanobacteria
1972 Isolation of Enterobacter cloacae from corn roots
1975 Isolation of Spirillum sp. and demonstration of their nitrgenase activity
1984 Nitrogen fixation in Methanogens (archaea)
2011 The European Nitrogen Assessment provides the first integrated and
comprehensive look at N use in Europe
2012 Database of all nifH sequences available in the Genbank nucleotide database

2) Nitrogen role:

Nitrogen is an essential component of urea and amino acids, nucleic acids and adenosine
triphosphate (ATP) in all living cells. IT is also a major component of chlorophyll, the most
important pigment required for photosynthesis, and plays a critical role for plant growth and
production. Dinitrogen (N2) is the most available gas and the major constituent of the
atmosphere. However, only the reactive types of nitrogen like oxidized (NOx) or reduced
(NH3 and amines) nitrogen types can be assimilated by plants. The conversion of N2 to the
biologically available form of nitrogen (NH3) could be performed either by the industrial
Haber-Bosch process or via biological nitrogen fixation by certain bacteria and archaea
(diazotrophic prokaryotes) (Kristina Lindstrom and Seyed Abdollah Mousavi, 2019)

3) Plant available nitrogen in soil

There are two major forms of nitrogen: inorganic, as mineral nitrogen (2%), and organic
(98%). Inorganic forms include ammonia (NH3), ammonium (NH4 +), nitrite (NO2 −), and
nitrate (NO3 −), while organic forms are found in living organic matter (soil biota and fresh
animal and plant debris) and non-living organic matter (humified and non-humified
compounds).

Mineral nitrogen is available to plants in two forms, either as ammonium nitrogen (NH4+) or
as nitrate-nitrogen (NO3-). Organic nitrogen is not directly available to plants and must be
5
converted through a slow process (mineralization) to ammonium or nitrate. Thus, when
available nitrogen is subject to strong competition between plants and microorganisms. In
addition to that, nitrogen is continually lost through soil erosion, denitrification, leaching,
chemical volatilization, and most importantly, removal of Nitrogen-containing crop residues
from the land. That’s why, nitrogen is often in short supply in many croplands causing limited
crop growth and productivity. Synthetic nitrogen fertilizers have been introduced to
compensate this deficiency in agricultural soil. The Haber–Bosch process, developed in 1913,
is still the main industrial procedure for the production of ammonia (NH3), by combining
atmospheric nitrogen (N2) with hydrogen (H2). The produced ammonia (NH3) can then be
used to make numerous other nitrogen compounds such as nitrate, ammonia, ammonium, and
urea. This process adds more reactive nitrogen to the global nitrogen cycle consisting of
nitrogen fixation, assimilation, mineralization, nitrification, and denitrification. Furthermore,
the production of nitrogenous fertilizers requires huge quantities of fossil fuels, which
represent 2% of the world’s energy consumption. Unfortunately almost 25% of the nitrogen
supplied as fertilizer is lost during various agricultural processes and due to leaching and other
factors. These cumulative effects are negatively affecting soil health and environment in
general causing waste and pollution. Biological nitrogen is an environmentally friendly source
of N for crop production and does not require fossil fuels and the fixed nitrogen is less
susceptible to denitrification, leaching, and volatilization because it is directly absorbed by
plants. Therefore, optimization of biological nitrogen fixation (BNF) is critical to sustain both
food production and environmental health.

4) Nitrogen Fixation and nodulation:

The process of nitrogen-fixing symbiosis begins with the formation of nodules in the roots or
stems of the host plant. The formation of these nodules occurs when rhizobia penetrates their
host in a coordinated and controlled manner. It has been discovered that the legume-rhizobia
symbiosis is initiated through mutual chemical communication. Many genes belonging to both
the bacteria and the host plant are involved in this process. The plant root releases flavonoids,
which induce rhizobia to produce specific lipo-chitooligosaccharides (LCOs), called
nodulation factors (Nod factors, or NFs) (Dénarié et al., 1996). These NFs are specifically
recognized by the host plant and trigger a sophisticated symbiotic signaling cascade in the host
root cells to coordinate rhizobial infection, nodule organogenesis and later on nitrogen fixation.
(Jianping Wang and al, 2018)

6
The available data indicate that rhizobia enters the roots of most legume species via root hair
infection, hosts cell wall material and grows around the developing “infection”, forming an
infection thread, which grows through the cortex of the root, branching repeatedly.
Generally, rhizobia are released from the tips of these infection threads into membrane-bound
structures within host cells called symbiosomes where they differentiate into their N2-fixing
form known as bacteroids. (Mitchell Andrews and al, 2017)
Biological nitrogen fixation converts di-nitrogen (N2) into plant-usable form (NH4+
primarily). The process consists of combining N2 with the hydrogen ions from water. N2-
fixation is not only a biologically-mediated process because lightning or fire can also oxidize
N2 to nitrate (NO3 −). Actually all organisms (eukaryotes and prokaryote) naturally depend
directly or indirectly on biological nitrogen fixation for their nitrogen supply. But BNF is an
energetically expensive process because it needs 16 ATP molecules to break down an N2
molecule, 12 additional ATP molecules are required for NH4 + assimilation and transport,
totaling 28 ATP molecules. The nodulating plants must provide 12 g of glucose to their
bacterial partners to benefit 1 g N in part. Thus this process is still energetically less expensive
than the Haber–Bosch process developed in 1913 and which requires a temperature of 400–500
◦C and a pressure of 200–250 bars to produce the same amount of nitrogen. N2 fixation is
catalyzed by an enzyme called nitrogenase, it is quite similar in most of the nitrogen-fixing
bacteria. In addition to that, several regulatory proteins are involved in BNF and are encoded
by nif genes. Depending on the requirements of molybdenum, vanadium, or iron there are three
different forms of nitrogenase and each one contains an active site for the reduction of the
substrate. Nitrogenase is extremely O2 sensitive because oxygen inactivates, destroys
nitrogenase and has an inhibitory effect on nitrogen fixation and assimilation pathways. Some
nitrogen-fixing microorganisms have evolved various strategies to avoid the inhibitory or toxic
effect of oxygen. For instance, many diazotrophic bacteria fix N2 only under anaerobic or
microaerophilic conditions. The mechanisms in cyanobacteria (free-living photosynthetic) is to
separate the O2 they produce from their nitrogenase system. Thus, some groups of
cyanobacteria developed heterocyst as specialized cells for nitrogen fixation, they have thick
cell walls which protect the dinitrogenase enzyme complex against O2. In other non-heterocyst
cyanobacteria, there is a temporal separation between the N2 fixation and O2 production.
Nitrogen fixation is achieved during darkness in the absence of O2 production. Some bacteria
such as Azotobacter express a high respiratory rate to maintain a low oxygen concentration
inside the cell. At high O2 level, they can change the conformation of the nitrogenase in
protected inactivated state and prevents its irreversible damage. (Abdoulaye Soumare 2020)

7
 a. Nodule types:
The process of root hair infection can lead to the formation of either indeterminate or
desmodioid determinate nodules. A second mode of rhizobial infection occurs with species in
the Dalbergiae (aeschynemonoid nodules) where rhizobia enter roots at the sites of lateral root
emergence (“crack” entry), and infection threads are not involved in the infection
process .Thirdly, for at least some members of the Genisteae (Lupinus spp.) and Crotalariae
(Listia spp.), rhizobia enters the roots directly through the root epidermis at the junction
between epidermal cells, and again, infection threads are not involved in the infection process.
(Mitchell Andrews and al, 2017).

b. Stages of implementation:
 Rhizobia colonize the rhizosphere and attach themselves to the absorbent hairs.
 The bacteria induce the deformation of the absorbent hair (curling) and initiate an
infection thread (it) in the center of the curvature from the infectious center.
 The cord lengthens and reaches the base of the epidermal cell.
 The cord branches at the approach of the nodular primordia formed as a result of
cortical cell division.
 Bacteria are released into the nodule cells and form symbiosomes, where they
differentiate into fixative bacteroids .Poly beta-hydroxybutarate granules accumulate in
the bacteroids surrounded by a peribacterial membrane.
 Nitrogen fixation and ammonium transport in the symbiosome. Leghemoglobin
maintains a low oxygen concentration allowing the operation of the nitrogenase which
transforms nitrogen into ammonia in the bacterium. The ammonia diffuses into the
peribacteroid space where it is transformed into ammonium which is then exported into
the plant cytoplasm via a channel and assimilated into glutamine by the plant.
 Ammonia can also be assimilated at the bacteroid level in amino acids which are then
exported to the plant cytoplasm.
(PERRET and al., 2000; DAY and al., 2001)

IV. Host specificity:

8
Specificity can be defined by the taxonomic diversity of the partners with which the other
symbiote can associate. Specificity can be strict (a single species of bacterium for a single
genus of legume) or broad, covering several bacterial or legume taxa (PRIN et al., 1993). In
general, each legume can only be infected by a limited number of rhizobia strains.
Some examples are illustrated below:
 The legume Medicago truncatula can only be nodulated by Sinorhizobium meliloti and S.
medicae (VERNIE, 2015).
 Concerning rhizobia, while some have a very restricted specificity, others have a very
broad host specificity. Thus, the strain Sinorhizobium (Ensifer) sp. NGR 234 can nodulate
353 legume species belonging to 112 different genera and representing the three
subfamilies of legumes including a single non-legume Parasponia andersonii.(PUEPPKE
and BROUGHTON, 1999; NOEL, 2009).
 Plants of the genus Galega (subfamily Papilionidae) can only be nodulated by one Rhizobia
species, Rhizobium galegae.
 The legume Sesbania rostrata establishes a symbiosis only with Azorhizobium caulinodans.

These host specificities are generally dependent on the composition of root exudates, as well as
on the nature of molecular determinants secreted by Rhizobia called Nod factors.
1) Examples:
Legume species differ greatly in their specificity for rhizobial symbionts.

a) Papilionoideae-Rhizobia Symbioses

Table 2: Legume-rhizobia symbioses of the legume sub-family Papilionoideae

with indeterminate nodules. (Mitchell Andrews and AL.,2017)
Papilionoidieae Tribes Rhizobia-Field
and Genera

Cicereae
Cicer arietinum Mesorhizobium
Cicer canariense Mesorhizobium
Fabeae
Lathyrus aphaca, L. Rhizobium
nissolia, L. pratensis
Lathyrus japonicus Rhizobium
Lathyrus odoratus Rhizobium
Lens culinaris Rhizobium
9
Pisum sativum Rhizobium
Vicia faba Rhizobium
Vicia sativa Rhizobium
Galega
Galega officinalis Neorhizobium
Galega orientalis Neorhizobium
Galegeae
Astragalus melilotoides Ensifer, Mesorhizobium
Glycyrrhiza glabra Mesorhizobium, Rhizobium
Lessertia annulans, L. Mesorhizobium
capitata, L. diffusa, L.
excisa, L. frutescens, L.
herbacea, L. microphylla,
L. pauciflora

Lessertia sp. Ensifer

Oxytropis meinshausenii Rizobium
Oxytropis sp. Phyllobacterium
Hedysareae
Caragana microphylla Mesorhizobium
Hedysarum coronarium Rhizobium
Hedysarum coronarium Rhizobium
Onobrychis viciifolia Phyllobacterium
Trifolieae
Medicago lupulina Ensifer
Medicago polymorpha Ensifer, Neorhizobium
Medicago sativa Ensifer, Neorhizobium, Rhizobium
Medicago scutellata Ensifer
Medicago tuncatula Ensifer
Melilotus officinalis Ensifer
Trifolium
Trifolium fragiferum Bradyrhizobium, Mesorhizobium, Rhizobium
Trifolium pratense phyllobacterium
Trifolium repens Bradyrhizobium, Ensifer, Rhizobium
10
 Table 3: Legume-rhizobia symbioses of species in the sub-family Papilionoideae with
indeterminate nodules. (Mitchell Andrews and AL.,2017)

Papilionoideae Tribes (Genera) Rhizobia-Field

Abreae
Abrus precatorius Ensifer
Amorpheae
Amorpha fruticosa Bradyrhizobium, Mesorhizobium
Dalea purpurea Mesorhizobium, Rhizobium
Crotalarieae
Aspalathus callosa Burkholderia

Aspalathus linearis Bradyrhizobium, Burkholderia,

Mesorhizobium,Rhizobium
Listia angolensis Listia angolensis
Lotononis sp. Bradyrhizobium, Mesorhizobium
Rafnia sp. Burkholderia
Genisteae
Adenocarpus hispanicus Phyllobacterium
Argyrolobium uniflorum Ensifer
Cytisus balansae, C. multiflorus, C. Bradyrhizobium
striatus
Cytisus villosus Bradyrhizobium
Lupinus honoratus Ochrobactrum
Lupinus luteus Bradyrhizobium
Lupinus texensis Microvirga
Lupinus sp. Bradyrhizobium
Retama monosperma Bradyrhizobium
Hypocalypteae
Hypocalyptus coluteoides, H. Burkholderia
oxalidifolius, H. sophoroides
Indigofereae
11
Indigofera angustifolia Burkholderia
Indigofera astragalina, I. hirsuta, I. Bradyrhizobium
senegalensis, I. tinctoria
Loteae
Coronilla varia Mesorhizobium, Rhizobium
Ornithopus compressus, O. sativus Bradyrhizobium
Millettieae
Millettia pinnata Rhizobium
Tephrosia purpurea Bradyrhizobium, Ensifer, Rhizobium
Tephrosia wallichii Ensifer
Podalyrieae

Sesbania aculeata, S. grandiflora, S. Ensifer

pachycarpa, Sesbania sp.
Sesbania cannabina Ensifer Neorhizobium, Rhizobium
Sesbania herbacea Rhizobium
Sophora Phyllobacterium, Rhizobium
Sophora longicarinata, S. microphylla,
S. prostrata, S. tetraptera
Thermopsideae

Ammopiptanthus nanus, A. mongolicus Ensifer, Neorhizobium,

Pararhizobium,Rhizobium
Anagyris latifolia Mesorhizobium
Thermopsis lupinoides Mesorhizobium

Table 4: Legume-rhizobia symbioses of species in the sub-family Papilionoideae with

determinate nodules. (Mitchell Andrews and AL.,2017)
Papilionoideae Tribes and Genera Rhizobia-Field
Dalbergieae
Adesmia bicolor Rhizobium
Aeschynomene americana Bradyrhizobium
Aeschynomene rudis Bradyrhizobium
Arachis duranensis Bradyrhizobium

12
Pterocarpus officinalis Bradyrhizobium
Pterocarpus indicus Bradyrhizobium
Zornia glochidiata Bradyrhizobium
Desmodieae
Desmodium caudatum, D. fallax, D. Bradyrhizobium
triflorum
Desmodium elegans Bradyrhizobium
Desmodium gangeticum Bradyrhizobium
Desmodium heterocarpan Bradyrhizobium
Kummerowia stipulacea Bradyrhizobium, Rhizobium
Lespedeza capitata, L. cuneata, L. juncea, Bradyrhizobium
L. procumbens, L. stipulacea, L. striata
Lespedeza daurica Bradyrhizobium, Ensifer Mesorhizobium

Lespedeza davidii Rhizobium

Lespedeza inschanica, L. tomentosa Ensifer

Phaseoleae
Amphicarpaea bracteata, A. edgeworthii Bradyrhizobium

Amphicarpaea trisperma Rhizobium

Cajanus cajan Cajanus cajan

Dipogon lignosus Burkholderia

Glycine max Bradyrhizobium, Ensifer,Rhizobium

Glycine soja Bradyrhizobium, Ensifer, Rhizobium

Pachyrhizus ferrugineus, P. tuberosus Bradyrhizobium

Phaseolus Bradyrhizobium, Burkholderia, Ensifer,

Pararhizobium, Rhizobium
Rhynchosia aurea Ensifer

Rhynchosia minima Bradyrhizobium

Vigna sinensis Bradyrhizobium

Vigna unguiculata Bradyrhizobium,Burkholderia,Microvirga,

Rhizobium
Psoraleae
Psoralea asarina Burkholderia

13
Psoralea corylifolia Ensifer

Psoralea pinnata Bradyrhizobium,Burkholderia,Mesorhizobium

Loteae
Lotus arabicus, L. arinagensis Ensifer

Lotus bertheloti, L. callis-viridis, L. Mesorhizobiu

campylocladus, L. pyranthus
Lotus corniculatus Mesorhizobium

Lotus creticus Ensifer, Mesorhizobium, Rhizobium

Lotus halophyllus Ensifer

Lotus tenuis Mesorhizobium,Rhizobium

Lotus uliginosus Bradyrhizobium

Legume species differ greatly in their specificity for rhizobial symbionts. Galega
officinalis (tribe Galegeae) and Hedysarum coronarium (tribe Hedysareae) have been
highlighted as being highly specific with respect to their rhizobial symbionts. Both of these
species are in the inverted repeat lacking clade (IRLC). The IRLC is marked by the loss of
one copy of the inverted region of the plastid genome. Almost all genera in the IRLC are
temperate; all have indeterminate nodules, and where examined, their bacteroids were
terminally differentiated and could not return to their bacterial form. The IRLC contains
several important temperate grain (e.g., Pisum sativum and Vicia faba) and forage legumes
(e.g., Trifolium spp. and Medicago spp.) .There is evidence that at least some of these crop
legumes have a high degree of rhizobial specificity. For example, an analysis of core and
symbiotic genes of rhizobia nodulating Vicia faba and Vicia sativa from different continents
showed that they belong to a phylogenetically-compact group indicating that these species
are restrictive hosts. In contrast, Macroptilium purpureum and the grain legumes Phaseolus
vulgaris and Vigna unguiculata in the tribe Phaseoleae are nodulated by rhizobia from
different genera across the α- and β-proteobacteria. The Phaseoleae are of
tropical/subtropical origin, have desmodioid determinate nodules with bacteroids, which are
not terminally differentiated.

b) Caesalpinioideae-Rhizobia Symbioses

Of the three legume sub-families, the Caesalpinioideae contains the smallest proportion of
nodulated genera with nodulation confirmed for Campsiandra, Chidlowia, Dimorphandra,
14
 Erythrophleum, Jacqueshuberia, Melanoxylon, Moldenhauwera and Tachigali in the tribe
Caesalpinieae and Chamaecrista in the tribe Cassieae. Only two studies have genotypically
characterised bacteria confirmed as rhizobia of Caesalpinioideae species.

Data are available for bacterial isolates from nodules of other Caesalpinioideae species, but
their ability to produce N2 fixing nodules on their legume host under axenic conditions
was not tesed.

c) Mimosoideae-Rhizobia Symbioses

Rhizobia have been characterized from 15 species across seven genera in the tribe
Ingeae and 120 species from 13 genera in the tribe Mimoseae within the sub-family
Mimosoideae. Bradyrhizobium, Ensifer, Mesorhizobium and Rhizobium were each reported
to nodulate species in the Ingeae and the Mimoseae. Furthermore, Ochrobactrum was
reported to nodulate Acacia mangium (Ingeae); Allorhizobium and Devosia were reported to
nodulate Neptunia natans (Mimoseae); and there are many reports that Cupriavidus and
Burkholderia nodulate Mimosa spp. and related species (Mimoseae). In addition, excepting
Acacia auriculiformis (Ingeae) and Mimosa pigra (Mimoseae), all species that were
examined in three or more separate studies, Acacia mangium, Acacia saligna, Calliandra
grandiflora and Senegalia senegal (Ingeae), Leucaena leucocephala, Mimosa diplotricha,
Mimosa pudica, Parapiptadenia rigida, Prosopis alba and Vachellia tortilis (Mimoseae),
were nodulated by at least three different rhizobial genera. Thus, a range of rhizobial
genera, including both α- and β-proteobacteria, can nodulate legume species across the two
Mimosoideae tribes, and generally, where tested over different studies, species within the
Ingeae and Mimoseae tribes were promiscuous with respect to their rhizobial symbionts.

Table 5: Legume-rhizobia symbioses in the legume sub-family Mimosoideae.

All species have indeterminate nodules. (Mitchell Andrews and AL.,2017)

Mimosoideae Tribes Rhizobia-Field

and Genera
Ingeae
Acacia auriculiformis Bradyrhizobium
Acacia mangium Bradyrhizobium, Ochrobactrum, Rhizobium

15
Acacia mangium X Bradyrhizobium
A.auriculiformis
Acacia mearnsii Ensifer
Acacia melanoxylon Bradyrhizobium
Acacia saligna Bradyrhizobium, Ensifer, Rhizobium
Acaciella angustissima Ensifer
Calliandra calothyrsis Ensifer, Rhizobium
Calliandra grandiflora Ensifer, Mesorhizobium, Rhizobium
Faidherbia albida Bradyrhizobium
Inga edulis Bradyrhizobium
Inga laurina Bradyrhizobium
Mariosousa acatlensis Ensifer
Senegalia laeta Ensifer
Senegalia macilenta Ensifer
Senegalia senegal Ensifer, Rhizobium Mesorhizobium
Mimoseae
Anadenanthera peregrina Burkholderia
Desmanthus illinoensis Rhizobium
Desmanthus paspalaceus Mesorhizobium, Rhizobium
Desmanthus virgatus Rhizobium
Leucaena leucocephala Ensifer, Bradyrhizobium,Rhizobium
Microlobius foetidus Bradyrhizobium, Rhizobium
~50 Mimosa spp. Burkholderia
Mimosa affinis Rhizobium
Mimosa albida, M. Ensifer, Rhizobium
biuncifera, M.borealis,
M. dysocarpa, M.
polyantha, M. tricephala,
Mimosa sp
Mimosa asperata Cupriavidus

Mimosa benthamii, M. Rhizobium

goldmanii, M.
monancistra, M. robusta,
M. tequilana

16
Mimosa borealis, M. Ensifer
lacerata, M. luisana, M.
similis
Mimosa ceratonia Rhizobium

Mimosa cruenta, M. Cupriavidus

magentea, M. ramulosa,
M. reptans, M.
schleidenii
Mimosa diplotricha Burkholderia, Cupriavidus, Rhizobium

Mimosa hamata, Ensifer

M.himalayana
Mimosa invisa Rhizobium
Mimosa pigra Burkholderia, Cupriavidus
Mimosa polyantha Rhizobium
Mimosa pudica Bradyrhizobium, Burkholderia, Cupriavidus, Rhizobium
Mimosa skinneri Burkholderia, Rhizobium
Mimosa strigillosa Ensifer
Neptunia natans Allorhizobium, Devosia
Parapiptadenia Burkholderia
pterosperma
Parapiptadenia rigida Burkholderia, Cupriavidus,Rhizobium
Piptadenia adiantoides, Rhizobium
P. flava
Piptadenia gonoacantha, Burkholderia, Rhizobium
P. paniculata
Piptadenia stipulacea, P. Burkholderia
trisperma, P. vividiflora
Prosopis Bradyrhizobium, Ensifer, Mesorhizobium, Rhizobium
Prosopis chilensis Ensifer
Prosopis cineraria Ensifer
Prosopis farcta Ensifer, Mesorhizobium
Prosopis juliflora Ensifer, Rhizobium
Pseudopiptadenia burkholderia
contorta
17
Stryphnodendron sp. Bradirhizobium
Vachellia abyssinica Mesorhizobium, Ensifer
Vachellia cochliacantha, Ensifer
V. farnesiana, V.
pennatula
Vachellia gummifera Ensifer
Vachellia horrida Ensifer
Vachellia jacquemontii Ensifer
Vachellia macracantha Ensifer, Rhizobium
Vachellia nubica Bradyrhizobium
Vachellia seyal Rhizobium, Ensifer
Vachellia tortilis Ensifer, Mesorhizobium, Rhizobium
Vachellia xanthophloea Mesorhizobium
Xylia xylocarpa Bradyrhizobium

2) Nodulation genetics in bacteria:

In general, three sorts of symbiotic genes are involved within the process of nodulation and
BNF in bacteria. They are: the nod genes necessary for nodulation, the nif genes coding for
nitrogenase and also the fix genes essential BNF Nod and Nif proteins are encoded by
accessory genes of bacteria that are placed in transmissible genetic elements like plasmids and
chromids. Actually, these sets may be transferred horizontally in high frequencies within the
species of a bacterial genus and less often between genera. (MERCADO-BLANCO and
TORO, 1996; NOEL, 2009; Remigi and al, 2016)

In the genera Bradyrhizobium, Azorhizobium and in the species Mesorhizobium loti, these
symbiotic genes are located on the chromosome. (Sharma and al., 1993)

a) Nod genes:

Nodulation genes or nod genes include common genes and genes specific to the host to be
infected (hsn genes). (BROUGHTON et al., 2000 ; SPAINK, 2000)The majority of rhizobia
tested had nod genes. However, some Bradyrhizobium strains, that don’t possess nodABC
genes, can form N2 fixing nodules on particular Aeschynomene spp. Bacterial species from a
variety of genera within the α-proteobacteria (most commonly Bradyrhizobium, Ensifer
18
(Sinorhizobium), Mesorhizobium and Rhizobium) and two genera within the β-proteobacteria
(Burkholderia (Paraburkholderia) and Cupriavidus) can form functional (N2 fixing) nodules on
specific legumes. (Mitchell Andrews , 2017)

 Gene nod D

The nod D gene negotiates the first steps of nodulation in bacteria. It is consequently the first
nod gene to be transcribed during the nodulation process. Its transcription is done in a
constitutive way (GEURTS and BISSELING, 2002) and the activation of its product, Nod D,
is carried out by flavonoids which are molecules emitted by the roots of leguminous plants.
Once activated, Nod D is linked to the regulatory sites of the operons carrying the other nod
genes and activates their transcription.

 Common nod genes (ABCIJ)

The common nod genes (nod A, B, C, I and J) are nodulation genes common to all rhizobia
studied so far. They are located on an operon of which the ABC nodes are essential for
nodulation. These are physically and functionally conserved in all rhizobia species and their
mutations cause a complete loss of the ability to infect and nodulate host plants. Indeed, these
genes are essential to induce the deformation of the absorbent hairs into the shape of a stick
and induce plant cells to divide. (BREWIN and al., 1992 ; PELMONT, 1995)

 Specific nod genes (nod H, nod FE, nod G, nod MN, nod Q etc.)

Specific nod genes have been shown to be major determinants of legume host specificity
(PELMONT, 1995). These hsn (host specific nodulation) genes are specific genes of the plant
to be infected. They are not necessarily present or functionally conserved in all rhizobia. These
genes are responsible for host specificity and recognition between the bacterium and the plant,
a step prior to infection. Their mutation causes a delay, a reduction in nodulation or a
modification of their host specificity. (DEBELLE and SHARMA, 1986; DEBELLE and
al., 1986; HORVATH and al., 1986 ; CERVANTES et al., 1989).

* After their transcription, the nod genes produce enzymes which control the synthesis of
molecules essential to the nodulation process. These signals, which are generally known as nod
factors, are lipo-chito-oligosaccharide molecules (LCOs). The synthesis of the
lipooligosaccharide nucleus of these molecules is controlled by the ABCD nod genes while the
hsn genes provide the various substitutions of these molecules. (DEBELLE et al., 2001)
19
*Nod factors are responsible for the specificity of the recognition between symbionts and the
triggering of nodular organogenesis. Indeed, even at minimal concentrations, these signals can
trigger symbiotic responses in the plant such as root hair deformation, cortical cell division and
primordial nodule formation. The identification of the Nod signal is an essential step in
establishing the molecular dialogue at the origin of the symbiosis between legumes and
nitrogen-fixing bacteria. (DEBELLE and al., 2001)

b) Nif genes :

The nif genes exist in several bacteria including rhizobia. (YOUNG and HAUKKA, 1996) In
rhizobia, these genes code for the synthesis of an enzyme complex that catalyzes the reduction
of nitrogen and is known as nitrogenase or dinitrogenase. (HOPKINS, 2003) This enzyme
complex is made up of two metalloproteins of different sizes: the site of the substrate reduction
corresponding to the MoFe-protein or dinitrogenase and the electron donor corresponding to
the Fe-protein or dinitrogenase reductase.

Under the catalysis of this enzyme complex, the reduction of molecular nitrogen takes place in
two steps. In the first step, Fe-protein is reduced by a primary electron donor, usually
ferredoxin. In the second step, the reduced Fe-protein transfers electrons to the Mo-Fe-protein
which catalyzes both the reduction of the gaseous nitrogen and the production of hydrogen.
(HOPKINS, 2003)

The ATP in the reaction comes from the aerobic respiration of the bacteroids. It reacts with
reduced Fe-protein and is involved in the transfer of electrons between Fe-protein and Mo-Fe-
protein. For each reduced dinitrogen molecule, at least 16ATPs are required, two per electron
(HOPKINS, 2003). In total, legumes use up to 22% of the energy from their photosynthesis to
achieve nitrogen fixation, the general equation of which is as follows:

N2 + 8H+ + 8 e- + 16 ATP  2NH3 + H2 + 16 ADP + 16Pi

At the end of the reaction, the fixed ammonia is converted into glutamine, asparagine, ureides
etc. before being transported into the sap of the xylem for assimilation by the host plant. This
conversion is possible thanks to the nif and fix genes, etc. which partly code for the synthesis
of different catalyst enzymes such as glutamate dehydrogenase, glutamine synthetase (GS) etc.

c) Fix genes:

20
The fix genes are additional genes, specific to symbiotic fixers and involved in the late stages
of nodule development during symbiotic nitrogen fixation (BREWIN and al., 1992;
HOPKINS, 2003; NOEL, 2009). Some of these genes (fix L, fix J) are regulators of nif A
gene synthesis, involved in the regulation of nitrogenase synthesis. (NOEL, 2009)Yet others
(fix NOPQ) code for the synthesis of enzymes catalyzing the regulation of oxygen during
fixation.

Although not usually referred to as fix genes, dct genes (Dicarboxylate transport genes) fit the
definition of fix genes. They are necessary for bacteria to assimilate dicarboxylic acids
(succinate, malate) from carbon compounds (glucose, fructose) supplied by the plant during
nitrogen fixation. The mutation of these genes in bacteria leads to a weak proliferation of
bacteroids and a deficiency of these during nitrogen fixation. (NOEL, 2009)

3) Plant immunity and host range restriction:

In parallel with Nod factor-dependent partner selection, the plant immune system helps
exclude other soil microorganisms from legume roots (Zipfel and Oldroyd, 235 2017). Partner
selection is a continuous process. The host legume employs multiple checkpoints throughout
this process to discriminate between symbionts and pathoge

21
 B. Mycorrhizal Symbiosis
1) Definition:

The term mycorrhizae, "mukes": fungus and "rhiza": root, refers to the interaction between the
roots of a plant and a soil fungus. This type of symbiosis is found in natural environments
since it is estimated that more than 90% of the families of Terrestrial plants have at least one
representative capable of mycorrhizing. They exist different types of mycorrhizae, depending
on the morphology of the interaction, characterized by by a superficial (ectomycorrhizae) or
internal (endomycorrhizae) interaction.

2) The different types of mycorrhizal symbiosis

Mycorrhizae begins with the meeting of a fungus with the roots of a plant.

a. Ectomycorrhizae: are mainly found in trees and shrubs and involve a very large variety of
Basidiomycetes fungi, Ascomycetes or Zygomycetes. During the interaction, the fungal
partner forms a sleeve of hyphae around certain roots, and insinuates itself between the
cells of the epidermis and the external cortex without ever penetrating the plant cells.
b. Endomycorrhizae:

On the contrary, they are characterized by the presence of fungal structures within certain cells
of the host plant. Depending on the morphology of the intracellular fungal apparatus, several
types of endomycorrhizae are differentiated.

 In the case of the mycorrhizae of Ericaceae and Orchids, the hyphae of the fungus
(Ascomycete or Basidiomycete) forms a cluster within a cell.
 Whereas in arbuscular mycorrhizal symbiosis, the fungus develops from intercellular
hyphae, highly branched intracellular structures, the arbuscles.
 In both cases, clusters and arbuscules, these intracellular structures are the site of nutrient
exchanges between the two partners).(Bonfante,2010)

c. Ectendomycorrhizae: a third type of mycorrhizae, where both processes coexist.

22
 These three main types of mycorrhizal symbioses have different geographical distributions
depending on latitude, altitude and soil type. (Bonfante,2010)

3) The vegetal partner

Approximately 6,000 species of terrestrial plants have ectotrophic status. Ectotrophic plant
species are gymnosperms and especially angiosperms. Trees are mainly involved in this
symbiosis, which also involves shrubs, lianas and herbaceous plants. Trees are mainly
represented in families or subfamilies of Caesalpinioideae, Dipterocarpaceae, Fagaceae,
Myrtaceae, Papilionoideae and Pinaceae. Ectotrophic trees are frequently associated with with
endomycorrhizogenic symbionts and/or, more rarely, with symbionts forming
ectendomycorrhizae, especially in Ericaceae. For example, in the genus Eucalyptus,
Helianthemum and Quercus, arbuscular mycorrhizae are dominant on young seedlings, while
ectomycorrhizae are mainly detected in adult trees.

4) The Fungal Partner

The diversity of ectomycorrhizal fungi is estimated to be between 20,000 and 25,000 species
(0.5 to 0.7% of total fungal diversity). These fungal symbionts belong to mainly to
Basidiomycetes and Ascomycetes and more rarely to Glomeromyomycetes. The diversity of
ectomycorrhizal fungi would be less important in tropical regions than in temperate regions.
This finding would be underestimated because many fungal species have not yet been
identified in the tropical. (Bonfante,2010)

 Lifestyle: Symbiotes

Mycorrhizal interaction is characterized by a bi-directional transfer of nutrients. The fungi, in

exchange of the carbon provided by the plant, improves the plant's water and mineral nutrition,
particularly in terms of phosphorus (P) and nitrogen (N). These reciprocal nutritional
exchanges are at the center of the mycorrhizal association and act as regulatory components
ensuring the proper functioning of the symbiosis.

 These trophic exchanges not only allow a better growth of the two symbiotic partners but
also a better resistance of the plant to environmental biotic and abiotic stress.

23
 5) Host specificity

A given plant can be mycorhized by different species of fungi not necessarily belonging to the
same genus. Similarly, a species of fungus is generally capable of mycorrhizing a wide variety
of plants not belonging to the same taxon. In addition to that, the same fungal individual is
capable of mycorrhizing simultaneously two adjacent plants of different species.

The efficiency of symbiosis can vary from one pair of partners to the other, thus, some fungi
will develop more in certain plant species, probably because the plant is better adapted to the
species in question.

To ensure its multiplication, the individual is constrained to establish a symbiotic interaction

with the plants in its immediate environment, which favours a broad spectrum of host plants.
The heterogeneity of the genetic material of mycorrhizal fungi is one of the main reasons
ensuring a broad host spectrum for these organisms, allowing diversity in the recognition and
signalling systems between partners. Similarly, the nutrient transfer is affected by the nature of
the symbionts and the benefit that can be derived from the plant of the mycorrhization varies
according to the species of the fungus. (Bonfante,2010)

6) Implementation of symbiotic interaction

Penetration of the fungus into the roots:

The infection of a plant's roots with the fungus can occur through two distinct pathways:

 The so-called primary infection is made from pre-symbiotic fungal hyphae derived
from a germinating spore.
 The so-called secondary infection involves the extra-radicular mycelium of a fungus
that has already invaded the root system of a host plant. This type of secondary
infection can be done between roots of the same root system or between root systems of
two different plants.

7) Interests of mycorrhizal symbiosis :

Mycorrhizal symbiosis plays a crucial role in the mineral nutrition of plants. For example, the
inorganic phosphate PO4- present in the soil is often chelated to metal cations such as iron or

24
aluminum, which makes it a little mobile. It forms thus a very phosphate-poor zone around the
roots and absorbent hairs of the plants.

The interaction with a mycorrhizal fungus increases soil volume accessible to the plant. The
same applies to other mineral compounds transported by the fungus, including water, allowing
better resistance of the mycorhized plants to water stress. However, the interaction with the
fungus has a cost since the plant supplies it with sugars from photosynthesis.

Mycorrhizal symbiosis has a bioprotective effect through a reduction in the effect of

pathogenicity of certain phytoparasites and improves mycorhized plants tolerance to stresses
induced by the trace of metal elements or polycyclic aromatic hydrocarbons. (BONFANTE,
2010)

 The beneficial character of the mycorrhizal symbiosis for the plant depends on the balance
between mineral nutrition benefits and photosynthetic costs

C. Interests of root symbioses:

 All biogeochemical cycles involve plant and microbial components and the services
resulting from these cycles cannot be provided without the intervention of both partners.
Thus, the atmospheric nitrogen fixation by plants is made possible through the association
of these plants with specific bacteria and allows the enrichment of soils with nitrogen,
without the need for the addition of nitrogen fertilizers. In the same way, AMF are able to
reduce up to 40 times the nitrogen losses through rainwater leaching of soils (Asghari &
Cavagnaro 2012) and thus keep it available for plants that do not require exogenous
addition of this element in agricultural soils.
 Microorganisms provide other services through plants in agriculture; we can speak of agro-
ecosystemic services (Cortois & De Deyn 2012; Wu et al 2009). Crop protection,
imeaning the ability of microorganisms to protect the plant against pathogens which is an
important agro-ecosystemic service because it makes it possible to avoid chemical
treatments (Kuiper et al 2004).

25
 D. Use of symbionts in biofertilization:

Biofertilizers can be defined as products that contain living microorganisms; when applied to
seeds, plant surfaces, or soil, they colonize the rhizosphere or interior of the plant, and promote
plant growth by increasing the supply or availability of primary nutrients to the host plant.
According to Mishra (2013), a biofertilizer is a mixture of live or latent cells encouraging
nitrogen fixing, phosphate solubilizing, or cellulolytic microorganisms used for applications to
soil, seed, roots, or composting areas with the purpose of increasing the quantity of those
mutualistic beneficial microorganisms and accelerating those microbial processes, which
augment the availability of nutrients that can then be easily assimilated and absorbed by the
plants ,Malusá and Vassilev (2014) proposed that a biofertilizer is the formulated product
containing one or more microorganisms that enhance the nutrient status (the growth and yield)
of the plants by either Molecules replacing soil nutrients and/or by making nutrients more
available to plants and/or by increasing plant access to nutrients. Biofertilizer products are
usually based on the plant growth-promoting microorganisms (PGPM). The PGPM can be
classified into three dominant groups of microorganisms: arbuscular mycorrhizal fungi (AMF),
plant growth promoting rhizobacteria (PGPR), and nitrogen fixing rhizobia, which are deemed
to be beneficial to plant growth and nutrition. However, it has been reported that PGPR have
been used worldwide as biofertilizers, contributing to increased crop yields and soil fertility.
Hence, with the potential contribution of the PGPR, this leads to sustainable agriculture and
forestry. (Pravin Vejan, 2016)

Biofertilization has many advantages:

 Increasing crop productivity: Biofertilizers can be used as an economic input to enhance

the productivity of crops; fertilizers could be reduced and nutrients in high quality from
soil can be harvested. When biofertilizers are employed as a soil inoculants they proliferate
and play a role in ecological cycle of nutrients and are proven helpful for the productivity
of crops. (Mishra, D,2013)
 The technology of production in case of biological fertilizers is simple and cost-effective in
comparison to synthetic fertilizer plants. (Mishra, D,2013)
 Growth and yield of plant can be upgraded by frequent inoculation of very potent
rhizobacteria, etc and it was reported that microorganisms such as phosphate solubilizing

26
 bacteria which we use as biofertilizer can increase the yield up to 200-500kg/ha and
therefore about 30- 50kg of superphosphate can be stored. (Mishra, D,2013)
 Role of biofertilizer in photosynthesis: about 90% of plant body growth is achieved
through carbon dioxide assimilation by photosynthesis. It was announced that some strains
of Rhizobia sufficiently increase the plant leaves surface area, plant rate of photosynthesis,
stomatal opening and closing and efficiency of water availability showing that the
photosynthetic ability of plant can be increased by Rhizobial administration. Leaves are a
major part of plant’s photosynthetic process. Therefore the number of leaves is very
important. (Mishra, D,2013)

 The catch here is whether the “living” biofertilizers used could be self-sustaining or
would need to be re-applied to soil on a continual basis, and also whether excessive
usage would destabilize the microorganism interaction in the soil. (Pravin Vejan, 2016)

Conclusion:
Understanding the interactions that bind plants and soil microorganisms is an essential step for
the sustainable management of ecosystems, especially in agriculture. The ecosystem services
resulting from such interactions include plant productivity which responds, in part, to the food
requirements of the world's population and the regulation of biogeochemical cycles. These
ecosystem services depend on trophic links between the two partners in the interaction and can
be represented by a tradeoff between the costs and benefits for each partner. Moreover, As
long as the human population continues to increase, the world will have to withstand the
escalating demand for food. Demand is soon catalyzed by greed to increase the crop yield,
which results in the overexploitation of the soil ecosystem. This has to be put to rest and the
conventional crop approaches cannot be practiced anymore since anthropogenic activities such
as intensive agriculture, crop monocultures, and the use of agrochemicals cause serious
concerns and disturb the ecosystem. Considering the good impact of biofertilization,
biocontrol, and bioremediation, all of which exert a positive influence on crop productivity and
ecosystem functioning, encouragement should be given to its implementation in agriculture.
Hoping for the betterment of technology in developing successful researchs giving birth to
crucial processes that will ensure the stability and productivity of agro-ecosystems, thus
leading us towards an ideal agricultural system.

27
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