Succinate irrepressible periplasmic glucose dehydrogenase of Rhizobium sp. Td3 and SN1 contributes to its phosphate...

Td3 and SN1 are phosphate-solubilizing nodule rhizobia of Cajanus cajan and Sesbania rostrata, respectively. They solubilized 423 µg/mL and 428 µg/mL phosphate from tricalcium phosphate through the secretion of 19.2 mM and 29.6 mM gluconic acid, respectively, when grown in 100 mM glucose. However, 90% and 80% reduction in phosphate solubilization coupled to the production of 40 mM (Td3) and 28.2 mM (SN1) gluconic acid was observed when the two isolates were grown in 50 mM succinate + 50 mM glucose. Our results illustrate the role of succinate irrepressible glucose dehydrogenase (gcd) in phosphate solubilization and the role of succinate: proton symport in succinate-mediated repression of phosphate solubilization in these rhizobia. Calcium ion supplementation reduced the 88% and 72% repression in P solubilization to 18% and 9% in Td3 and SN1, respectively, coupled to a reduction in media pH from 6.8 to 4.9 in Td3 and 6.3 to 4.8 in SN1. Hence, repression had no genetic basis and is purely due to the biochemical interplay of protons and other cations.