Diseases and Management of Crops under Protected Cultivation

(Diseases and Management of Crops under Protected Cultivation)
Once biocontrol bacteria are established in the rhizosphere, a wide variety of mechanisms can
result in suppression of plant pathogens. Accordingly, except in the case of induced resistance, a
biocontrol agent must occupy an ecological niche similar to that of the plant pathogen and its
mode of action (competition, parasitism, antibiosis, induction of SAR) must interfere both spatially
and temporally with precise steps in the development of the pathogen.
Interactions between plants and rhizobacteria: The rhizosphere is a narrow zone of soil that is
influenced by root secretions. The structure of rhizobacterial communities is determined by the
plant species, and differences in the composition and amounts of root exudates probably account
for the differences in microbial populations. Root exudates offer a carbon-rich diet to the
rhizosphere microorganisms which includes organic acids such as citrate, malate, succinate,
pyruvate, fumarate, oxalate and acetate and sugars such as glucose, xylose, fructose, maltose,
sucrose, galactose and ribose, constitute the ‘main course’, whereas variable amounts of amino
acids, nucleobases and vitamins (such as thiamin and biotin) provide the ‘entrée’ or ‘dessert’. The
ability of rhizobacteria to use organic acids as carbon sources correlates with rhizosphere
competence. The rhizosphere, which is the narrow zone of soil that is influenced by root
secretions, can contain up to 10 11 microbial cells per gram root and more than 30,000 prokaryotic
species. The collective genome of this microbial community is much larger than that of the plant
and is also referred to as the plant’s second genome. Chemotaxis, flagellar mobility,
lipopolysaccharide (LPS) structure, the outer membrane protein OprF and, to a lesser extent, pili
are all important for competitive root colonization. A root glycoprotein complex, agglutinin involved
in the short-term adherence of Pseudomonads to the plant roots. Once biocontrol Pseudomonads
have moved and attached to a root zone, microcolonies form in a few days. Other bacteria can
reach the same site at a later time and intermingle with pre-existing microcolonies. The root collar,
where the root joins the main stem, is a site of intense exudation and is more strongly colonized by
bacteria than is the root tip.
The rhizosphere microbiome: The microflora of most soils is carbon starved. Because plants
secrete up to 40% of their photosynthates into the rhizosphere, the microbial population densities
in the rhizosphere are much higher than in the surrounding bulk soil. This phenomenon is known
as the ‘rhizosphere effect’. In general, rhizosphere microbial communities are less diverse than
those of the bulk soil. It appears that, from the reservoir of microbial diversity that the bulk soil
comprises, plant roots select for specific microorganisms to prosper in the rhizosphere. Together
with the plant genotype, soil type is an important driver of the microbial community composition in
the rhizosphere. Plants can determine the composition of the root microbiome by active secretion
of compounds that specifically stimulate or repress members of the microbial community. Recent
evidences suggest that plant genotype also determines microbiome composition. Differences in a
single gene between plant genotypes can have a significant impact on the rhizosphere
microbiome. The production of a single exogenous glucosinolate significantly altered the microbial
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