The saprophytic microflora of the rhizosphere includes both deleterious and beneficial elements have the potential to influence the plant growth and yield, significantly. Their deleterious activities include alterations of the supply of water, ions and plant growth substances by changing root functions or limiting their growth. The beneficial bacteria affect plant growth positively by promoting the availability and uptake of mineral nutrients, provision of plant-growth substances as well as suppression of deleterious rhizosphere microorganisms. Deleterious rhizosphere microorganisms (DRMO) are the minor pathogenic rhizosphere microorganisms that affect plants by their metabolites without parasitizing plant tissue. These include deleterious rhizobacteria and rhizofungi.
DRMO may affect plant growth by interfering with the delivery of plant growth substances or nutrients to plants by free-living rhizosphere microorganisms. However, increase in solute concentrations of poorly soluble compounds such as phosphate or iron salts by free living rhizosphere microorganisms are not considered to be of great importance for plant growth as plants itself can increase the concentrations of these materials by changing the rhizosphere pH or excreting chelators. Continued root growth in soil, their length and number of root hairs as well as an efficient energy metabolism of root cells are important to enable plants in obtaining water and ions such as phosphorus (P) and potassium (K) having low diffusion constants. Microorganisms producing metabolites that inhibit these processes may have an effect on crop production. Many rhizosphere microorganisms produce metabolites like auxin, ethylene, cytokinin, vitamin and other plant growth substances. The negative and positive effects of auxins and ethylene on root growth and morphology and their mode of action are well reviewed. But, whether microbial sources of plant growth substances have negative or positive effects on plant depends on their total and relative concentrations. Most of these metabolites are organic acids e.g. HCN. It is easily inactivated by soil components or assimilated by soil microorganisms. Fluorescent Pseudomonads produces a variety of secondary metabolites including plant growth substances and antibiotics. Production of ATP, mediated by cytochrome oxidase respiration, can be inhibited by cyanide (-CN). This inhibition causes electrons released by oxidation of NADH in mitochondria to follow the alternative cyanide resistant respiratory pathway to oxygen. Much energy is thereby lost as heat instead of being used for phosphorylation activity of ADP. Cyanide is a secondary metabolite of several microorganisms. It can be produced directly from glycine and proline or from cyanogenic glycosides all of which occurs in root exudates. Cyanide production by isolates of Pseudomonas spp. was found to depend on Fe3+ availability and such microorganisms might be influenced by competition for iron with PGPR. Host specificity of deleterious rhizosphere pseudomonads has been studied for wheat and citrus at the cultivar level. DRMO probably survive on root residues that provide a source of inoculum for root colonization for the next potato crop, as stated for other rhizophere microorganisms. Accumulation of factors in soil that stimulate the production of their toxic metabolic may be involved and such factors could be a precursor for cyanide production (e.g. glycine) and or availability of the ferric ion (Fe++). Glycine and to a lesser extent proline, enhances cyanide production by fluorescent pseudomonads and both are constituents of plant root exudates. The increase in iron availability with increasing frequency of certain crop would stimulate the production of certain toxic metabolites including HCN and reduce competition for iron and siderophore production by the DRMO. The possible role of deleterious rhizobacteria on the other hand is to enhance the plant growth resulting from the introduction of specific root colonizing fluorescent pseudomonads into the rhizosphere, and may be due to the production of siderophores under iron-limited conditions and consequent competition with DRMO in the rhizosphere.
Various cultivars may differ in their sensitivity to DRMO or in the exudation of compounds that stimulate their deleterious activity. Cultivar rotation, then could be considered as a means to limit yield losses due to high frequency of that crop residues. Considerable retardation in plant growth and decreased crop yield may result from the deleterious activity of saprophytic rhizosphere microorganism. This activity increases with certain cropping practices such as growing the same crop frequently in the same field. The deleterious effects seem to be due to microbial metabolites that affect physiological processes in root cells. Increase in production of HCN with increasing cropping frequency is possibly derives from the accumulation of HCN precursors in soil as well as may be due to increased availability of iron in soil for the deleterious Pseudomonas strains, which is required for HCN production. The deleterious effect is host specific. They are amenable to control by siderophore-mediated iron competition by PGPR. The major factor in failures of PGPR to increase yields seems to be in adequate root colonization. The presence or gradual increase of DRMO that can use the Fe(III)-siderophore complex of particular PGPR may also be involved. Some HCN-producing pseudomonads have this ability.
Thus, the negative influence of saprophytic rhizosphere microorganism on crop production is in need of greater research effort in plant pathology. The fundamental research on the mechanisms involved is needed to control the particular and important group of plant pathogens intelligently either by plant breeding, cropping practices or use of PGPR.
About Author / Additional Info: