Microbial role in regulatory mechanism of plants. The document discusses the important role that microbes play in plant growth regulation and how they can help address issues of increasing global population and food security. Microbes like plant growth promoting bacteria and rhizobacteria can fix nitrogen, solubilize phosphate, sequester iron and modulate plant hormone levels to facilitate plant growth. They also act through indirect mechanisms like producing antibiotics, siderophores, competing with pathogens, and helping plants withstand environmental stresses. The document provides examples and explanations of these various microbial mechanisms.

1. Microbial role in regulatory mechanism of plants
Regulatory Mechanism:
Regulatory mechanisms are those that are systems of control in keeping the internal
environment relatively stable and maintained within narrow limits, despite external
environment change.
Why microbial role is important?
There are currently around 7 billion people in the world and this is expected to increase to
approximately 8 billion sometime around the year 2020. When one considers both the
expected worldwide population increase and the increasing environmental damage that is a
consequence of ever greater levels of industrialization, it is clear that in the next ten to
twenty years it will be a significant challenge to feed all of the world’s people, a problem
that will only increase with time.
There is absolutely no time to lose; to feed this growing population, the world needs to
begin to greatly increase agricultural productivity, and to do so in a sustainable and
environmentally friendly manner. To feed the growing world, it is necessary to re-examine
many of the existing approaches to agriculture that includes the use of chemical fertilizers,
herbicides, fungicides, and insecticides. Instead, sustainable agriculture will likely make
much greater use of both transgenic plants and plant growth-promoting bacteria, or PGPB.
One way to address this problem is through the use of phytoremediation, the purposeful
use of plants to take up and concentrate or degrade a wide range of environmental
pollutants. Moreover, the addition of PGPB to plants that are used in phytoremediation
protocols typically makes the entire remediation process much more efficacious.
PGPB
These are plant growth promoting bacteria.
Microbes in soil: -
Soil is replete with microscopic life forms including bacteria, fungi, actinomycetes, protozoa,
and algae. Of these different microorganisms, bacteria are by far the most common (i.e.,
95%). It has been known for some time that the soil hosts a large number of bacteria (often
around 108 to 109 cells per gram of soil) and that the number of culturable bacterial cells in
soil is generally only about 1% of the total number of cells present.
However, in environmentally stressed soils the number of culturable bacteria may be as low
as 104 cells per gram of soil. Both the number and the type of bacteria that are found in
different soils are influenced by the soil conditions including temperature, moisture, and the

2. presence of salt and other chemicals as well as by the number and types of plants found in
those soils. In addition, bacteria are generally not evenly distributed in soil.
Example:-
Thus, for example, an IAA overproducing mutant of the bacterium Pseudomonas
fluorescence BSP53a stimulated root development in blackcurrant cuttings while inhibiting
the development of roots in cherry cuttings. This observation may be interpreted as
indicating that the blackcurrant cuttings contained a suboptimal level of IAA that was
enhanced by the presence of the bacterium. On the other hand, with the cherry cuttings
the IAA level was optimal prior to the addition of the bacterium and the additional IAA
provided by the bacterium became inhibitory. Notwithstanding these caveats, it is usually a
straightforward matter to decide whether a bacterium either promotes or inhibits plant
growth.
PGPR
These are plant growth promoting rhizobacter.
Two groups
Symbiotic bacteria
Free living rhizo bacteria
It can also be divided into two groups according to their residing sites.
IPGPR inside the plant cells
EPGPR outside the plant cells
Some important bacteria and their function: -
Free living soil bacteria beneficial to plant growth are usually reffered to as plant growth
promoting rhizobacteria PGPR capable of promoting plant growth by colonizing the plant
root.
Non-symbiotic nitrogen fixing bacteria such as Azobacter,Bacillus are also used to inoculate
a large area of arable land in the world with the aim of enhancing plant productivity.
Phosphate solubilizing bacteria such as species of Bacillus and Paenibacillus have been
applied to soils to specifically phosphorus status of plants.
Direct Mechanisms

3. Facilitating Resource Acquisition
The best-studied mechanisms of bacterial plant growth promotion include providing plants
with resources/nutrients that they lack such as fixed nitrogen, iron, and phosphorus.
It would obviously be advantageous if efficient biological means of providing nitrogen and
phosphorus to plants could be used to substitute for at least a portion of the chemical
nitrogen and phosphorus that is currently used.
Mechanisms involved in Facilitating Resource Acquisition are:
1. Nitrogen Fixation
2. Phosphate Solubilization
3. Sequestering Iron
Nitrogen Fixation
Nitrogen fixation is a process in which nitrogen (N2) in the atmosphere is converted into
ammonia (NH3). Atmospheric nitrogen (N2) is relatively inert, it does not easily react with
other chemicals to form new compounds.
N-fixation ability is limited to few bacteria, either as free-living organisms or in symbiosis
with higher plants.
Nitrogen fixation is done by way of nitrogenase metalloenzymes which contain iron,
molybdenum, or vanadium.
Nitrogenase enzyme converts (N2) into ammonia (NH3).
The process of nitrogen fixation requires a large amount of energy in the form of ATP.

4. Phosphate Solubilization
Solubilization and mineralization of phosphorus by phosphate-solubilizing bacteria is an
important trait in PGPB.
The solubilization of inorganic phosphorus occurs as a consequence of the action of low
molecular weight organic acids such as gluconic and citric acid, both of which are
synthesized by various soil bacteria.
The mineralization of organic phosphorus occurs through the synthesis of a variety of
different phosphatases, catalyzing the hydrolysis of phosphoric esters.
Phosphate solubilization and mineralization can coexist in the same bacterial strain.
Sequestering Iron
Despite the fact that iron is the fourth most abundant element on earth, in aerobic soils,
iron is not readily assimilated by either bacteria or plants because ferric ion or Fe+3.
Fe+3 is the predominant form in nature, and is only sparingly soluble so that the amount of
iron available for assimilation by living organisms is extremely low.
Both microorganisms and plants require a high level of iron, and obtaining sufficient iron is
even more problematic in the rhizosphere where plant, bacteria and fungi compete for iron.
In order to survive such low amounts of bacteria low molecular mass siderophores which
have high affinity for iron are made. Membrane receptors are also required to facilitate
uptake of Fe-siderophore complex by microorganisms.
ModulatingPhytohormoneLevels:
Plant hormones play key roles in plant growth and development and in the response of
plants to their environment. Moreover, during its lifetime, a plant is often subjected to
a number of nonlethal stresses that can limit its growth until either the stress is
removed or the plant is able to adjust its metabolism to overcome the effects of the
stress. When plants encounter growth limiting environmental conditions, they often
attempt to adjust the levels of their endogenous phytohormones in order to decrease
the negative effects of the environmental stressors.
Cytokinins and Gibberellins:
Several studies have shown that many soil bacteria in general, and PGPB in particular,
can produce either cytokinins or gibberellins or both. Moreover, plant growth
promotion by some cytokinin- or gibberellin-producing PGPB has been reported.
Some strains of phytopathogens can also synthesize cytokinins. However, it appears
that PGPB produce lower cytokinin levels compared to phytopathogens so that the

5. effect of the PGPB on plant growth is stimulatory while the effect of the cytokinins
from pathogens is inhibitory.
IndoleaceticAcid:
It has been known for more than 70 years that different IAA concentrations affect the
physiology of plants in dramatically different ways. Plant responses to IAA vary from
one type of plant to another, where some plants are more or less sensitive to IAA than
other plants; according to the particular tissue involved, for example, in roots versus
shoots (the optimal level of IAA for supporting plant growth is 5 orders of magnitude
lower for roots than for shoots); and as a function of the developmental stage of the
plant. However, the endogenous pool of plant IAA may be altered by the acquisition
of IAA that has been secreted by soil bacteria. In this regard, the level of IAA
synthesized by the plant is important in determining whether bacterial IAA stimulates
or suppresses plant growth.
Overall, bacterial IAA increases root surface area and length, and thereby provides the
plant has greater access to soil nutrients.
Ethylene
Ethylene can affect plant growth and development in a large number of different ways
including promoting root initiation, inhibiting root elongation, promoting fruit
ripening, promoting flower wilting, stimulating seed germination, promoting leaf
abscission, activating the synthesis of other plant hormones, inhibiting Rhizobia spp.
nodule formation, inhibiting mycorrhizae-plant interaction, and responding to both
biotic and abiotic stresses. The ethylene that is synthesized as a response to various
stresses is called “stress ethylene” and it describes the increase in ethylene synthesis
that is typically associated with various environmental stresses including extremes of
temperature; high light; flooding; drought; the presence of toxic metals and organic
pollutants; radiation; wounding; insect predation; high salt; various pathogens
including viruses, bacteria, and fungi.
INDIRECT MECHANISM OF MICROBES:
Antibiotics and Lytic Enzymes
The synthesis of a range of diverse antibiotics is the PGPB trait that is most often associated
with the ability of the bacterium to prevent the proliferation of plant pathogens (generally
fungi). One problem with depending too much on antibiotic-producing bacteria as
biocontrol agents is that with the increased use of these strains, some phytopathogens may
develop resistance to specific antibitoics. To prevent this from happening, some researchers
have utilized biocontrol strains that synthesize hydrogen cyanide as well as one or more
antibiotics. This approach is effective because, while hydrogen cyanide may not have much
biocontrol activity by itself, it appears to act synergistically with bacterially encoded
antibiotics.
Siderophores:

6. Some bacterial strains that do not employ any other means of biocontrol can act as
biocontrol agents using the siderophores that they produce. In this case, siderophores from
PGPB can prevent some phytopathogens from acquiring a sufficient amount of iron thereby
limiting their ability to proliferate. It has been suggested that this mechanism is effective
because biocontrol PGPB produce siderophores that have a much greater affinity for iron
than do fungal pathogens ,so that the fungal pathogens are unable to proliferate in the
rhizosphere of the roots of the host plant because of a lack of iron .In this model, the
biocontrol PGPB effectively out-compete fungal pathogens for available iron.
Competition
Although it is difficult to demonstrate directly, some indirect evidence indicates that
competition between pathogens and nonpathogens (PGPB) can limit disease incidence and
severity. Thus, for example, abundant nonpathogenic soil microbes rapidly colonize plant
surfaces and use most of the available nutrients, making it difficult for pathogens to grow.
For example, in one series of experiments, researchers demonstrated that treatment of
plants with the leaf bacterium Sphingomonas sp. prevented the bacterial pathogen
Pseudomonas syringae pv. tomato from causing pathogenic symptoms.
Modulating the effects of environmental stress
Ethylene
Most of the aforementionedenvironmentalstressesresultdue to the productionof inhibitorylevels
of stressethylene. As the stress ethylene produced as a consequence of phytopathogen infection,
highlevelsof ethyleneandthe damage thatit causesmaybe at leastpartiallyavoidedby employing
ACCdeaminase-containingPGPB.Some of the abioticstresseswhose effectscanbe improved inthis
wayinclude temperature extremes,flooding,drought , metals and metaloids ,hypoxia and organic
contaminants.
IAA
In the more recent research, it has been suggested that PGPB may help plants to overcome abiotic
stresses by providing the plant with IAA that directly stimulates plant growth, even the inhibitory
compounds are present or not.
The bacteriathat most effectivelyprotectplants against a wide range of different stresses produce
both IAA and ACC deaminase [65, 177–179].through tryptophan dependent pathway, the
amino acid tryptophan is excluded by plant roots and then taken up by PGPB bound to the roots,
where itisconvertedintoIAA.The bacteriallyproducedIAA issecreted, taken up by plant cells and,
together with the plant’s pool of IAA stimulates an auxin signal transduction pathway, including
variousauxinresponse factors.Asaconsequence,plantcellsgrow andproliferate;atthe same time,
some of the IAA promotes transcription of the gene encoding the enzyme ACC synthase, thereby
yieldinganincreasedconcentrationof ACCandeventuallyethylene(ascatalyzedbythe enzyme ACC
oxidase since ACCisthe immediate precursor of ethylene). Various biotic and abiotic stresses may

7. alsoeitherincrease the synthesisof IAA orstimulate the transcriptionof the gene for ACC synthase.
In the presence of a bacterium that contains the enzyme ACC deaminase,
some ACC may be taken up the PGPB bound to the plant, and degraded to ammonia and α-
ketobutyrate. Thus, an ACC deaminase-containing PGPB acts as a sink for ACC with the
consequence that,followingan environmental stress, a lower level of ethylene is produced by the
plantand the stressresponse of the plantisdecreased. Asthe levelof ethylene in a plant increases,
the transcription of auxin response factors is inhibited .
Antifreeze
To function effectively in the field, a PGPB(plant growth promoting bacteria) must be to
persist and proliferate in the environment. In addition, in cold and temperate climates many
fungal phytopathogens are most destructive when the soil temperature is low. In those
environments, cold tolerant (psychrotrophic) biocontrol PGPB work efficiently than
mesophilic biocontrol strains. Moreover, in countries such as Canada, Sweden, Finland, and
Russia, PGPB must be functional at the cool soil temperatures that are common in the
spring (i.e., 5–10°C).
Antifreeze proteins:
Antifreeze proteins are characterized by their ability to control the size and shape of ice
crystals. Typically secreted, these proteins control extracellular ice by inhibiting ice
growth.In the cool environments, cold tolerant (psychrotrophic) biocontrol PGPB are likely
to be more effective in the field than mesophilic biocontrol strains. Moreover, in countries
such as Canada, Sweden, Finland, and Russia, PGPB must be functional at the cool soil
temperatures that are common in the spring (i.e., 5–10°C).
Cytokinin
Cytokinins are compounds with a structure resembling adenine this name is given to them
due to their ability to promote cytokinesis or cell division in plants. They are produced by
plants, some yeast strains and by a number of soil bacteria, including PGPB [66, 68].
Transgenic plants that overproduce cytokinins, especially during periods of abiotic stress,
are significantly protected from the deleterious effects of those stresses. Unfortunately,
there are not yet any definitive studies indicating whether bacterially-produced cytokinins
can also protect plants from abiotic stresses. This would involve a detailed comparison of
the biological activity of cytokinin-producing PGPB with cytokinin minus mutants of those
bacteria.
“Biological Control”
Definitions;
The terms “biological control” and its abbreviated synonym “biocontrol” have been used in
different fields of biology, most notably entomology and plant pathology. In entomology, it

8. has been used to describe the use of live predatory insects, entomopathogenic nematodes,
or microbial pathogens to suppress populations of different pest insects.
In both fields, the organism that suppresses the pest or pathogen is referred to as the
biological control agent (BCA).
Explanation;
More broadly, the term biological control also has been applied to the use of the natural
products extracted or fermented from various sources. These formulations may be very
simple mixtures of natural ingredients with specific activities or complex mixtures with
multiple effects on the host as well as the target pest or pathogen. And, while such inputs
may mimic the activities of living organisms, nonliving inputs should more properly be
referred to as bio pesticides or bio fertilizers, depending on the primary benefit provided to
the host plant. The various definitions offered in the scientific literature have sometimes
caused confusion and controversy.
Mechanisms:
Plant Disease Suppression
Beneficial microbes help to control plant diseases by the following mechanisms:
 Predation and Hyperparasitism = feeding on pathogens
 Antagonism, competitive exclusion and microbiostasis = competing for nutrients or
space by producing metabolites that kill pathogens or inhibit their growth and
movement;
 Rhizospere competency = blocking pathogen access to plant roots;
 Induced systemic resistance and systemic acquired resistance = stimulating or
priming the plant’s own natural defense system As a general rule, disease-
suppressive microorganisms work best at preventing rather than curing diseases.
Iron-chelating compounds;
Synthesis of iron-chelating compounds, such as siderophores, by Pseudomonas is a
characteristic feature visible in some isolates from bulk or rhizosphere soils. In culture
media with trace amounts of iron, a yellow-green halo can be observed, which may be
fluorescent under ultraviolet light. A pioneering work by Kloepper proposed that
siderophores might be involved in biocontrol of plant pathogens and in plant growth
promotion. Since then, the role of siderophores as chelating agents depriving soil pathogens
of iron, an essential element for growth without which the survival of many micro-
organisms is affected, has been widely recognised.
Antibiotic-mediated suppression
Antibiotics are microbial toxins that can, at low concentrations, poison or kill other
microorganisms. Most microbes produce and secrete one or more compounds with
antibioticactivity. In some instances, antibiotics produced by microorganisms have been

9. shown to be particularly effective at suppressing plant pathogens and the diseases they
cause.
Bacteria That Infect Plants
Some bacteria produce enzymes that break down the cell walls of plants
anywhere in the plant. They do this to break open to cell to gain access to the
nutrients inside but the plant cells affected die quickly, causing parts of the
plant to start rotting.
This is why plant diseases that develop due to cell wall degrading enzymes are
generally known as a type of 'rot'
o Some bacteria produce toxins that are damaging to plant tissues
generally, usually causing early death of the plant.
o Others produce large amounts of polysaccharide sugars that have long
chains and are very sticky. As these travel in the water carrying vessels,
the xylem, they block the narrow channels, preventing water getting
from the plant roots up to the shoots and leaves, again causing rapid
death of the plant.
o Finally, some bacteria produce proteins that mimic plant hormones.
These lead to overgrowth of plant tissue and tumors form. These grow
rapidly, taking up valuable nutrients and energy resources that the plant
would otherwise usefor its own tissuegrowth and it becomes weakened
and susceptible to attack by other pathogens such as fungi.
o One of the main types of bacteria that cause plant disease are the
Proteobacteria.
o The disease symptoms progress from discoloured leaves to extensive
wilting of all of the stems and leaves, with all the leaves eventually
turning yellow. Parts of the plant start to die and rot and the plant dies
completely shortly afterwards.
o Some species of Mycoplasma-like species of bacteria also cause serious
plant disease. These are bacteria that do not have cell walls and they are

10. introduced into the phloem tissue of plants by insects such as aphids
that suck plant sap.
o The Mycoplasma bacteria grow inside the aphids and other sap-sucking
insects and are then introduced into their plant host by a very effective
insect hypodermic.
o Plants that produce large amounts of sugar are the worst affected – for
example coconut plants and sugar cane. The overall effect of the
infection is to weaken the plant, opening it up to other diseases. The
leaves usually lose their colour and another common symptom is green
flowers, or an absence of flowers.
References
http://archive.bio.ed.ac.uk/jdeacon/microbes/nitrogen.htm
http://www.nature.com/scitable/knowledge/library/biological-nitrogen-fixation-
23570419
http://www.hindawi.com/journals/scientifica/2012/963401/
www.nature.com/scitable/knowledge/library/biological-nitrogen-fixation-23570419
www.ncbi.nlm.nih.gov › NCBI › Literature › PubMed Central (PMC).
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