Possible role of cellular phosphate pool and subsequent accumulation of inorganic phosphate on the aluminum tolerance in Bradyrhizobium japonicum

This paper demonstrates the possible role of cellular phosphate on aluminum tolerance in Bradyrhizobium japonicum. Aluminum-tolerant strains accumulated larger amounts of phosphate in the cells than did the sensitive ones when grown in P-rich or P-limited condition. Stored cellular P declined in all strains when exposed to Al-stress (50 μm) in acidic and P-limited condition (pH 5.4; P 5 μm), accompanied by the accumulation of inorganic phosphate (Pi) in the cells. The reduction in the amount of cellular P and the increase in the accumulation of Pi were found to be significantly greater in the tolerant strains than in the sensitive strains, though, the amount of accumulated Pi in the cells was much less than the amount of P released from the cells. In contrast, Al-tolerant strains absorbed less Al compared to the sensitive ones. In Al-tolerant strains, cellular absorption of Al was positively related to the release of P from the cells and an accumulation of intracellular Pi. Thus, the ability to store a higher amount of cellular P and increased accumulation of cellular Pi are considered to be a novel mechanism in B. japonicum to exert tolerance to Al in acidic and extracellular P-limited conditions.