Identification of Putative Regulatory Genes involved in Succinate Mediated Catabolite Repression (SMCR) of Phosphate Solubilization in Acinetobacter. SK2

Abstract

Mineral phosphate solubilizing (MPS) bacteria SK2, obtained from the rhizosphere of Vigna radiata (mung bean) and identified to be a strain of Acinetobacter sp. (Bharwad and Rajkumar 2020) showed other plant growth promoting (PGP) activities such as production of IAA (117 μg/ml), siderophore (87% units), HCN, ammonia and solubilize Zn and K minerals as well. It solubilized 682 μg/ml of tri calcium phosphate (TCP) and 86 μg/ml of rock phosphate (RP) with a simultaneous decrease in pH up to 4 due to the production of gluconic acid. In the presence of succinate, the P solubilizing ability of this strain was significantly reduced. This was correlated to nearly 80% decrease in phosphate solubilization in Pikovyskia (PVK) and TRP media. The organic acid mediated mineral phosphate solubilization (MPS) is the main mechanism of phosphate solubilization in most bacteria including Acinetobacter, which was constitutive in glucose grown cells but repressed in the presence of succinate. This strain and many other phosphate solubilizing microorganisms (PSMs) are applied in soil as biofertilizers, but may fail as phosphate solubilizers, because the presence of succinate in root exudates may hinder the MPS ability due to succinate mediated catabolite repression (SMCR). PQQ cofactor-dependent enzymes mGDH (encoded by gdhA) and sGDH (encoded by gdhB) oxidize glucose into gluconate, due to which the surrounding pH lowers down, ultimately leading to P solubilization. Regulator proteins crc and hfq are known to govern SMCR by binding to ribosomal-binding sites on target mRNAs (here gdhA and gdhB) and obstructing their translation in Pseudomonas. Identification and confirmation of genes governing SMCR of MPS phenotype was done in Acinetobacter sp. SK2 – crc and hfq genes influenced the repression of MPS phenotype by repressing gdhA, gdhB, rpoN and pqq operon genes essential for P solubilization. Knockout of genes responsible for CCR may help in restricting repression and this agenda can be exploited in other PSMs for enabling constitutive phosphate (P) solubilization.