Authors:
Dr Rhitu Rai- Scientist, National Research Centre on Plant Biotechnology (NRCPB), LBS building, New Delhi-110012
Dr P K Dash- Senior Scientist, National Research Centre on Plant Biotechnology (NRCPB), LBS building, New Delhi-110012

The Plant Growth Promoting Rhizobacteria (PGPR) are root colonizing bacteria that exert beneficial effects on plant development. They enhance the plant growth directly by producing phytohormones like, indole acetic acid (IAA), gibberellic acid and also by improving nutrient availability to the plant through production of siderophores. phosphorus solubilization etc. The PGPRs indirectly promote plant growth via suppression of phyto pathogens by production of antibiotics, toxins, biosurfactants, lytic enzymes and also through induction of systemic resistance in crop the host plants. However, production of antibiotics is known to be the primary mechanism involved in disease suppression by PGPRs. Among the many antibiotics produced, 2, 4-diacetylphloroglucinol (2,4-DAPG), a polyketide compound, has received particular attention because of its broad-spectrum activities like antiviral, antifungal, antibacterial, antitumor and phytotoxic properties . This metabolite is of particular significance to agriculture as a wide range of pseudomonads produce it and its anti-pathogenic activity in situ against a variety of root and seedling pathogens of plants has been established. Hence, it has been incorporated as a major ingredient in biocontrol agents such as Pseudomonas fluorescens strains Q2-87, F113, CHA0 and 2P24, used to protect crops against diseases like wheat take all caused by Gaeumannomyces graminis, sugar beet seedlings damping off caused by Pythium ultimatum, cotton rhizoctoniosis caused by Rhizoctonia solani and tomato bacterial wilt caused by Ralstonia solanacearum. Also, 2, 4-DAPG producing microorganisms aggressively colonise roots, a trait that further contributes in their ability to suppress pathogens through competition. Evidence for an important role of 2, 4-DAPG in plant protection comes from studies on 2, 4-DAPG-negative mutants of P. fluorescens, which lack antibacterial properties. When they are transformed with clones containing 2, 4-DAPG biosynthetic genes they imparted resistance against wilt pathogen. In situ detection of 2, 4-DAPG at the site of disease suppression (rhizosphere) of plants further supports the role of the metabolite in plant protection.

The genes coding for biosynthesis of 2, 4-DAPG are conserved across species. The biosynthetic genes phlACBD flanked by regulator (phlF) and an efflux protein (phlE) have been identified. PHL is a polyketide synthesized by condensation of three molecules of acetyl coenzyme A with one molecule of malonyl coenzyme A to produce the precursor monoacetylphloroglucinol, which is subsequently transacetylated to generate PHL. Nucleotide sequence analysis revealed six genes organized in three transcriptional units. Four genes comprise the operon phlACBD, and expression studies indicated that the protein products of all four are required for the synthesis of both MAPG and 2, 4-DAPG. The products of phlACBD resemble neither type I nor type II PKS enzyme systems. Rather, phlD exhibits striking homology to plant chalcone synthases, indicating that phloroglucinol synthesis is mediated by a novel kind of PKS not previously described in microorganisms. The, detection of the phlD gene correlates with the capacity to produce 2, 4-DAPG and hence phlD+ fluorescent Pseudomonas spp." and "2, 4-DAPG producer" are used in synonimity. The biosynthetic operon is flanked on either side by the separately transcribed genes phlE and phlF, which code respectively for putative efflux and regulatory (repressor) proteins that are not absolutely required for phloroglucinol production. 2, 4-DAPG positively controls its own biosynthesis by preventing the binding of the repressor PhlF to the operator regions upstream of phlA; thus, 2, 4-DAPG from one isolate can affect production of the antibiotic by a neighboring isolate. phl F mutants are derepressed for 2,4-DAPG production.

Given the importance of these antibiotic producing rhizobacteria, although the number of commercially available biocontrol products is steadily increasing, inconsistent performance by biocontrol agents remains a major impediment in their successful use as a common practice in agriculture for disease control. This inconsistency has been attributed to a wide range of biotic and abiotic factors in the soil environment that can adversely impact root colonization, expression of genes involved in biocontrol and/or activity of biocontrol metabolites. Some phlD+ genotypes have a preference for the roots of certain crop species and this trait can be utilized as an advantage in the development of biocontrol agents viz. these strains can be applied at very low doses and will maintain threshold population densities in the rhizosphere on inoculated crops throughout the growing season and during subsequent seasons.

The DAPG producing PGPRs offer a promising and sustainable alternative for biological control of the soil borne plant pathogens. However, for their effective use in agriculture, it will be a prerequisite to study each plant-PGPR-soil system individually.

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