Plant Growth Promoting Rhizobacteria

Rhizosphere and the Phyllosphere
Rhizosphere Introduction
Rhizospher Effect
Nitrogen Fixation in the Rhizosphere
Alteration of Rhizosphere Microflora
Associative and Antagonistic Activities in the Rhizosphere
Root Exudates
Organic Compounds Detected in Plant Root Exudates
Fungistasis
Techniques of Rhizosphere
The Soil Dilution and Plate Count Method
The Soil Plate
The Soil Box
The Contact Slide Technique
Artificial Rhizosphere
Model System

Plant Growth Promoting Rhizobacteria

	Plant Growth Promoting Rhizobacteria The use of the term rhizobacteria implies the ability of certain bacteria to colonize the rhizosphere very aggressively. Pseudomonas spp. are receiving world-wide attention under the broad general category known as plant growth promoting rhizobacteria (PGPR). The bacteria exhibit fluorescence under u-v light and hence are also known as fluorescent pseudomonads. Initial observations were based on the ability of P. fluorescence and P. putida when applied to potato seed pieces improved the growth of potatoes. Subsequent field studies have revealed that these species and similar bacteria increased the yield of potato (5.33 per cent), sugarbeet (4-8 t/ha) and radish (6-144 per cent of root weight).
Soil-borne pathogens may be distinguished as major and minor ones. The major ones include the well-known Phytophthora and Fusarium causing root rots and vascular wilts. The minor pathogens cause damage to young tissues of roots with no visual symptoms. The minor pathogens also include, according to recent thinking, certain non-parasitizing deleterious rhizosphere microorganisms (DRMO) which include deleterious rhizobacteria (DRB) and deleterious rhizofungi (DRF). Some soils are conducive to soil-borne diseases whereas others are suppressive to diseases. The reasons behind these observations may lie in the soil structure and or microbial composition. Many genera of soil bacteria have shown great potentiality as biocontrol agents operating not merely by secreting antibiotics but by means of other mechanisms. These genera include Actinoplanes, Agrobacterium, Amarphosporangium, Arthrobacter, Cellulomonas, Bacillus, Azotobacter, Enterobacter, Erwinia, Flavobacterium, Micromonospora, Rhizobium, Bradyrhizobium, Serratia, Streptomyces and Xanthomonas. Agrobacterium radiobacter strain 84 is an excellent example of a biocontrol agent controlling crown gall disease caused by Agrobacterium tumefaciens. Bacillus subtilis, capable of producing endospores and tolerating heat can suppress major and minor soil-borne diseases of carrots, oats and groundnut.
In Netherlands, it has been shown that the frequency of potato cultivation in the same field has a bearing on the yield of potato tubers. When the crop was grown in the same field every third year, yields were reduced by 10-15 per cent from what would be expected if the crop was grown once in six years. The severity of decline in yield appeared to be progressive when yield decrease reached 30 per cent if potato was cropped continuously in the field.
Fluorescent pseudomonads are believed to improve the growth of plants by colonizing the root region aggressively and thus preempt the establishment of DRMO on roots, especially those which produce growth inhibiting cyanide. No such growth promotion was possible in plots where no potato was cultivated probably due to the absence of factors which stimulate the production of toxic substances. Fluorescent pseudomonads have been shown to suppress major plant pathogens like the take-all, a root disease of wheat caused by Gaeumannomyces graminis var tritici, by 11-17 per cent.
	About 10 per cent of bacteria in the rhizosphere appear to be aggressive in reducing the population of DRMOs and there appears to be no relationship between in vitro inhibition and in vivo suppression effects. The field beneficial effects are dependent on soil temperature, pH, moisture and clay content which influence the survival of PGPRs in the rhizosphere. This ecological competence may diminish by repeated subculturing in vitro, possibly related to loss of cell surface structure or reduction in antibiotic and siderophore production as explained hereunder. It has been postulated that hydrocyanic acid (HCN) produced by many DRMOs reduces potato root growth and is responsible for the decreased yield of potato tubers. Pseudomonad PGPRs increase potato yields, according to one theory, by reducing HCN production by DRMOs, through siderophore-mediated competition for Fe(III), which is required for HCN production.

The part played by HCN in reducing yield of potato has become debatable because evidences have also been presented to demonstrate that HCN intact proved beneficial in biological control in other root diseases such as take-all of wheat and black rot of tobacco. Therefore, more critical studies in future may clarify the situation.

Three possible mechanisms have been suggested to explain the beneficial effects of PGPRs in enhancing production. They are competition for substrate and niche exclusion, production of siderophores and antibiotics. However, more than one mechanism may operate for mediating a biological control.

Fluorescent pseudomonads 'mop up' nutrients in the rhizosphere because of their versatility in growth and nutrient absorption. The points of emergence of lateral roots are favourite spots for DRBs and PGPRs appear to compete for these spots very effectively.

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