Mineral phosphate solubilization activity of gluconacetobacter diazotrophicus under P-limitation and plant root environment

Abstract

The ability to solubilize insoluble inorganic pho- sphate compounds by Gluconacetobacter diazotrophicus was studied using different cul-ture approaches. Qualitative plate assays using tricalcium phosphate as the sole P-source showed that G. diazotrophicus produced solu-bilization only when aldoses were used as the C-source. Extracellular aldose oxidation via a pyrroloquinoline quinone-linked glucose dehy-drogenase (PQQ-GDH) is the main pathway for glucose metabolism in G. diazotrophicus. In batch cultures with 5 g l-1 of hydroxyapatite as the P-source and glucose as the C-source, more than 98% of insoluble P was solubilized. No solubilization was observed neither using glyc-erol nor culturing a PQQ-GDH mutant of G. di-azotrophicus. Solubilizaton was not affected by adding 100 mmol l-1 of MES buffer. Continuous cultures of G. diazotrophicus showed significant activities of PQQ-GDH either under C or P limi-tation. An intense acidification in the root envi-ronment of tomato and wheat seedlings inocu-lated with a G. diazotrophicus PAL5 was ob-served. Seedlings inoculated with a PQQ-GDH mutant strain of G. diazotrophicus showed no acidification. Our results suggest that G. di-azotrophicus is an excellent candidate to be used as biofertilizer because in addition to the already described plant growth-promoting abili-ties of this organism, it shows a significant mineral phosphate solubilization capacity.