The of various approaches of biological control

The soil and plant system is a natural reservoir of
a rich biodiversity of microbes. These microorganisms are known to play an important
role in organic and nature farming systems. There are many ways in which one
can understand organic farming but most systems rely on the following: legumes
to supply nitrogen (N) to crops instead of synthetic fertilizers; use of crop
residues, crop rotations and cycling of animal manures, green manures, composts
and off-farm wastes to provide nutrients and improve soil health; use of
various approaches of biological control to control insects, weeds and other
pests. All these processes require the micro organisms as the mediator to carry
out soil biochemical reactions and functions. Microorganisms
which live in soil are algae, bacteria, actinomycetes, bacteriophages,
protozoa, nema­todes and fungi. Of these microorganisms, bacteria are
most common. Amongst them, Gram positive Actinobacteria constitute one
of the largest bacterial phyla. The word “Actinomycetes” is derived from Greek
word “atkis” (a ray) and “mykes” (fungus), having characteristics of both
bacteria and fungi. Most of the Actinobacteria (the Streptomycetes in
particular) are saprophytic, soil-dwelling organisms. They spend the majority
of their life cycles as semi-dormant spores, especially under nutrient limited conditions.
Actinobacteria have a mycelial lifestyle and undergo complex morphological
differentiation. Hence, the phylum is found in wide range of ecological
environments.

The
actinomycetes include Actinomyces itself, which forms spores on sporophores.
Among them, Streptomyces is the
predominant genus followed by Actinomadura,
Microbispora, Micromonospora, Nocardia, Nonomurea, Mycobacterium, Frankia,
Actinoplanes, Saccharopolyspora, and
Verrucosispora.

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            Actinobacteria has both direct and
indirect mechanisms to influence the plant growth and protection. The direct
mechanisms involve the production of vital factors for crop growth such as
growth hormones and the assistive actions on nitrogen fixation, phosphate solubilization,
and iron acquisition. The actinobacteria indirectly influence the plant growth
by controlling and minimizing the deleterious effects of external stresses of
either biotic or abiotic sources through the following modes: competition for
nutrients, cell-wall degrading enzymes, and antibiotics, in which the latter
two are the key phenomenon deployed by the actinobacterial community.

Microorganisms
play an important role in the recycling of agricultural wastes. Bacteria are important
in agricultural soils because they contribute to the carbon cycle by fixation
 (photosynthesis) and decomposition. Some bacteria are important decomposers and
others such as actinomycetes are particularly effective at breaking down tough
substances. Actinomycetes
are the main group of soil microorganisms that play a major role in recycling
of organic matters in environment by production of hydrolytic enzymes. As a
decomposer, the actinomycetes specialize in breaking down tough cellulose,
lignin and the chitin. The breakdown of these materials makes nutrients
once again available to plants. During the process
of composting mainly thermophilic (adapted to high temperatures) and
thermo tolerant actinomycetes are responsible for decomposition of the organic
matter at elevated temperatures.

Organic residues added
to soil are first attacked by bacteria and fungi and later by actinomycetes,
because they are slow in activity and growth than bacteria and fungi. They
decompose the more resistant and indecomposable organic substance/matter and
produce a number of dark black to brown pigments which contribute to
the dark colour of soil humus. They are also responsible for subsequent further
decomposition of humus (resistant material) in soil.

Actinomycetes
form associations with some non-leguminous plants and fix N, which is then
available to both the host and other plants in the near vicinity. The natural
nitrogen cycle relies on nitrogen fixing bacteria like those found in the Frankia family
of actinobacteria, to supply the fixed nitrogen. Actinobacteria, Arthrobacter,
Rhodococcus, Gordonia, Streptomyces, and Micromonospora have been reported
for phosphate solubilization in vitro and glass house conditions.

Actinobacteria
commonly inhabit the rhizosphere, being an essential part of this environment
due to their interactions with plants. Such interactions have made possible to
characterize them as plant growth-promoting rhizobacteria (PGPR).
Actinobacteria apart from improving the availability of nutrients and minerals,
they synthesize plant growth regulators, are capable of inhibiting
phytopathogens and supporting plants under abiotic stress as well.  Iron in soil is known for its un-availability
to both plants and microbes due to its normal presence as insoluble hydroxides
and oxyhydroxides. This is made available by the synthesis of siderophores.
Besides the context of plant nutrition, siderophore also offers for plant
protection through the control of phytopathogens. They acquire iron thereby
create a competitive environment for other pathogenic microbes in the root
vicinity.

Actinobacteria
produce are saprophytic in nature and synthesize different enzymes like  nucleases, lipases, glucanases, xylanases,
amylases, proteinases, peptidases, peroxidases, phenyl ammonia lyase,
chitinases, cellulases, ligninases, pectinase, hemicellulase, and keratinase.
These enzymes have biocontrol potential as it degrades the cell wall of
pathogenic fungus and bacteria. Actinobacteria account for about two-third of antibiotics production. These antibiotics
also are involved in plant disease control. The production of these molecules
contributing to plant resistance to diseases is termed as induced systemic
resistance (ISR).

 Actinobacteria produce phytohormones such as
indole acetic acid and giberrellic acid which help in plant growth promotion.
The root elongation in plants helps in absorption of nutrients from deeper
strata of soil, which in turn improves plant yield. Increased root biomass can
help in efficient water uptake thereby supporting survival of plants under
water stressed conditions. Also, actinomycetes have been known to support crop
tolerance to saline conditions. Ethylene is produced in plants under abiotic
stress conditions which affects plant survival. Association of actinobacteria
mitigates abiotic stress through ACC deaminase by lowering ethylene production.
The exopolysaccharide production, IAA production, biofilm formation, production
of volatiles, glycine betaine and such molecules supports plant survival under
abiotic stress; which is termed as induced systemic tolerance.    

Molecular
studies through metagenomics, have shown that incorporation of organic
cultivation increases actinobacterial population in soil and improves plant
health and yields. Transcriptomic analysis of plant samples inoculated with
actinobacteria under biotic stress have shown increased expression of genes
involved in ISR and plant resistance to different pathogens. Analysis of
actinobacterial secondary metabolome showed presence of molecules involved in
anti-bacterial, anti-fungal activity and plant growth promotion. Under water
stress and salt stress, inoculation of actinobacteria to plants improved root
growth promotion and help in abiotic stress tolerance. 

Thus,
use of actinomycetes can be promoted in organic cultivation to improve soil
health, and their presence in rhizosphere can help in crop tolerance to biotic
and abiotic stress.