Effect of Arabinose on Regulation of MPS Phenotype in Rhizobia

Abstract

In soil, rhizobia are one of the most important bacteria present which helps in growth of the plant, they are also known as PGPR. Carbon catabolite repression (CCR) governs uptake and metabolism of preferred and secondary carbon sources. CCR may have effect on carbon source dependent plant growth promoting (PGP) properties like phosphate (P) solubilization. Glucose is by far the best carbon source known for organic acid production linked P solubilization. But catabolite repression may prevent the preferential utilization of glucose or the organic acid production. After glucose, arabinose and xylose are the most abundant sugar presents in the soil that can be utilized. It has been observed that other than glucose, arabinose have also been utilized as a sole carbon source for phosphate solubilization by rhizobia. The metabolism of L-arabinose is also different the fast- and slow-growing rhizobia. From literature review we have come to know that arabinose metabolism links to TCA cycle through α-ketoglutrate (fast growing) and pyruvate (slow growing) and form organic acids like oxalate, lactate, acetate and malate. These organic acids lead to phosphate solubilization. In our study we have used isolates from root nodules. We focused on Isolate CR showed some promising results in solubilization of rock phosphate, further study was carried out with it. Isolate CR which is Rhizobium spp. showed phosphate solubilization when using glucose as sole carbon source and produced gluconic acid. Other than glucose Isolate CR in presence of arabinose as sole carbon source that produced 32mM oxalic acid as organic acid which play an important role in for phosphate solubilization. When combination of arabinose and succinate were used, it showed that both oxalic acid production and P solubilization were repressed by succinate, this shows succinate mediated catabolite repression (SMCR). Enzymes activities of glucose dehydrogenase (GDH; periplasmic glucose oxidation), Isocitrate lyase (ICL) and glyoxylate oxidase (GO) responsible for oxalic acid production and also enzyme involved in arabinose metabolism and its link to TCA cycle, which are arabinose dehydrogenase (ADH) and Glycoalaldehyde dehydrogenase (GADH) were estimated. Enzyme assays shows that arabinose metabolism links with TCA cycle and produced oxalic acid which play an important role in phosphate solubilization. The enzymes GDH, ICL and GO were repressed in succinate. While significant activity of GO, the key enzyme for oxalic acid production during growth on arabinose suggested that it could be inducible in nature, and its inhibition by succinate appeared to be similar to catabolite repression.