Crc Regulates Succinate-Mediated Repression of Mineral Phosphate Solubilization in Acinetobacter sp. SK2 by Modulating Membrane Glucose Dehydrogenase

The plant growth-promoting Acinetobacter sp. SK2 isolated from Vigna radiata rhizosphere was characterized for mineral phosphate solubilization (MPS). To understand the contribution of the membrane glucose dehydrogenase (mGDH) and soluble glucose dehydrogenase (sGDH) in glucose oxidation and MPS, insertional inactivation of the corresponding genes was carried out. The disruption of mGDH encoding gene gdhA resulted in complete loss of mGDH activity, which confirmed its role in periplasmic glucose oxidation and gluconate-mediated MPS phenotype. The inactivation of sGDH encoding gene gdhB resulted in loss of sGDH activity, which did not alter the MPS or mGDH activity. Thus, it was also concluded that the sGDH was dispensable in gluconate-mediated MPS. Supplementation of succinate in glucose-containing medium suppressed the activity of mGDH (and sGDH) and therefore repressed the MPS phenotype. The catabolite repression control protein (Crc) of Pseudomonas was implicated in Acinetobacter sp. for a similar function in the presence of preferred and non-preferred carbon sources. To understand the regulatory linkage between Crc and genes for glucose oxidation, crc mutants were generated. The inactivation of crc resulted in increased activity of the mGDH in glucose + succinate-grown cells, indicating derepression. An increase in phosphate solubilization up to 44% in glucose + succinate-grown crc ^- compared with glucose-grown cells was recorded, which was significantly repressed in the wild-type strain under similar conditions. It is therefore proposed that in Acinetobacter sp. SK2, Crc is involved in the succinate-provoked repression of the MPS phenotype. The gene expression data indicated that Hfq may also have a regulating role in preferential utilization of carbon source by perhaps modulating Crc-Hfq functionality. V. radiata plants inoculated with the wild type improved both root and shoot length by 1.3 to 1.4-fold. However, crc ^- increased the root and shoot length by 1.6-fold, compared with the uninoculated controls. In mimicking the soil condition (in the presence of multiple carbon sources, e.g., succinate along with glucose), the crc ^- strain of Acinetobacter sp. SK2 performed better in supporting the growth of V. radiata in pot experiments.

Modulation of PQQ-dependent glucose dehydrogenase (mGDH and sGDH) activity by succinate in phosphate solubilizing plant growth promoting Acinetobacter sp. SK2

Prospective plant growth promoting rhizobacteria isolated from the rhizosphere of Vigna radiata was identified as Acinetobacter sp. SK2 that solubilized 682 μg ml^-1 of tricalcium phosphate (TCP) and 86 μg ml^-1 of rock phosphate (RP) with concomitant decrease in pH up to 4 due to the production of gluconate. The biochemical basis of the P solubilization suggested that the gluconate production was mediated by mGDH and sGDH enzymes. Our results illustrate the role of succinate in repression of P solubilization via suppression of mGDH and sGDH activity which correlated with repression of expression of respective genes, gdhA and gdhB. SK2 also produced IAA (117 μg ml^-1), siderophore (87% units), HCN, ammonia and solubilized minerals of Zn and K. Our findings imply that it is important to understand the cause of failure of several phosphate solubilizing bacteria in field conditions where catabolite repression may control the expression of several genes and pathways including that of mineral phosphate solubilization. Furthermore, Acinetobacter sp. SK2 bearing two glucose dehydrogenase (gdhA and gdhB) genes was recognized as promising strain for P biofortification and enhanced plant growth promotion.