Secretion of Gluconic Acid From sp. L1 Is Responsible for Solubilization of Aluminum Phosphate.

Phosphorus (P) deficiency is one of the major factors limiting plant growth in acid soils, where most P is fixed by toxic aluminum (Al). Phosphate-solubilizing bacteria (PSBs) are important for the solubilization of fixed P in soils. Many PSBs have been isolated from neutral and calcareous soils, where calcium phosphate is the main P form, whereas PSBs in acid soils have received relatively little attention. In this study, we isolated a PSB strain from the rhizosphere of , a plant well adapted to acid soils. On the basis of its 16S rRNA gene sequence, this strain was identified as a species and named L1. After incubation of sp. L1 for 48 h in a culture medium containing AlPO as the sole P source, the concentration of available P increased from 10 to 225 mg L, and the pH decreased from 5.5 to 2.5. sp. L1 exhibited poor FePO solubilization ability. When the pH of non-PSB-inoculated medium was manually adjusted from 5.5 to 2.5, the concentration of available P only increased from 6 to 65 mg L, which indicates that growth medium acidification was not the main contributor to the solubilization of AlPO by sp. L1. In the presence of glucose, but not fructose, sp. L1 released large amounts of gluconic acid to solubilize AlPO. Furthermore, external addition of gluconic acid enhanced AlPO solubilization and reduced Al toxicity to plants. We conclude that secretion of gluconic acid by sp. L1, which is dependent on glucose supply, is responsible for AlPO solubilization as well as the alleviation of Al phytotoxicity by this bacterial strain.