Some genetic manipulations of phosphate solubilizing bacteria
Phosphate solubilizing bacteria (PSB) are important bioinoculants used in ecofriendly sustainable agriculture. Crop applications of PSB fertilizers have resulted in increased grain yield, reduced the use of chemical phosphate fertilizers, reduction in constraints of agricultural economy and control of environmental pollution. Genetic engineering techniques are being employed to improve microbial phosphate solubilization. We know that the dissolution of phosphatic compounds is an important property of plant growth promoting rhizobacteria (PGPR). Therefore the basic approach of genetic manipulation is to induce and /or improve phosphate solubilization efficiency of PGPR.
Genetics of mineral phosphate solubilization and mineralization could be studied once the biochemical mechanism of solubilization was known. Information regarding structural organization of genes involved in phosphate solubilization (mps) and their regulation is still very scanty. But some genetic transformation experiments done in various laboratories were successful and proved to be effective in improvement of strains of PGPR that constitute phosphatic biofertilizers. Mps genes of Erwinia herbicola involved in solubilization mediated by organic acid production were for the first time cloned in 1987 by scientists Goldstein and Liu. It was found that organic acids, particularly gluconic (and 2-ketogluconic) acids (GA) are produced from direct glucose oxidation mechanism catalyzed by enzyme glucose dehydrogenase (GDH). GDH controls electron transfer from glucose to electron transport chain but in presence of cofactor pyrroloquinoline quinone (PQQ). Mps genes of Erwinia herbicola were also expressed in phosphate non-solubilizing strain of Escherichia coli which can synthesize GDH but without PQQ. Therefore, E. coli is unable to produce GA, even if GDH is available and hence cannot dissolve phosphates. Incorporation and expression of mps genes made possible the phosphate solubilization by E. coli. Until gabY, pKKY, pKG3791 and pcc genes from Pseudomonas cepacia, Enterobacter agglomerans, Serratia marcescens and Synechococcus sp. concerned to phosphate solubilization have been isolated and cloned. However, not all the genes like pcc and pKKY produce acid but involved in phosphate solubilization process. This finding supports the fact that organic acid production is not the sole mechanism of phosphate solubilization.
Genes involved in phosphate mineralization have also been isolated. Genes of mineralization encodes important phosphatase enzymes like phosphoesterases and phytases. Phosphoesterase enzymes catalyze dephosphorylation of phosphor-ester bonds in organic matter. Genes acp code for acid phosphatase and pho genes encode alkaline phosphatase enzymes which are involved in phosphorus transport to the cells of PSB. Acp genes are found usually in Gram negative bacteria and functionally active at pH 6 and temperature of 30˚C. AcpA genes from Francisella sp., phoC and NapA from Morganella morganii have been isolated. Their gene functions are found to be active when soil or medium is deprived of soluble phosphate. NapA of M. morganii has been successfully transferred to PGPR strains of Burkholderia and Azospirillum using suitable delivery systems. Thermostable and functionally active enzymes in absence of soluble phosphate have been characterized. Genes encoding such enzymes have been isolated from Rhizobium meliloti and different species of Bacillus like licheniformis, subtilis and amyloliquifaciens. Phytase enzymes are also predominant like phosphoesterases. They are high molecular weight acid phosphatases and known as myo-inositol hexakisphosphate phosphohydrolase. They release phosphorus from phytic acid generally present in plant debris and soil. The phy genes encode the functions of phytase enzymes. Both plant and animals are deficient in phytase activity and hence deprived of organic phosphorus nutrition. Arabidopsis plants have been genetically transformed with phytase gene (phyA) from Aspergillus niger. Improvement in phosphorus nutrition was found when phyA transformed plants were supplemented with phytic acid. Some PSB express phytase activity and therefore capable of providing required organic assimiable phosphorus to plants. Such PSB are being exploited for higher phytase production and reducing phosphate pollution in soil. The study of phy genes from PSB and their transformation to plant species yet remains to be investigated.
Investigation and exploitation of mps genes from different PSB presents great potential for future improvement of PGPR strains as efficient bioinoculants for sustainable agriculture.
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