The response of the phosphate uptake system and the organic acid exudation system to phosphate starvation in Sesbania rostrata

It is well known that the P-i uptake system via the high-affinity P-i transporter and the organic acid exudation system via PEPC are enhanced in the roots of P-i-starved plants. In this paper, we compared the expression of these two systems in Sesbania rostrata, a leguminous plant, on whose roots and stems it forms nodules. When S. rostrata plants were transferred to a 0 muM P-i nutrient solution, the expression of both the high-affinity Pi transporter and PEPC was enhanced within 2 d. The enhancement of the expression of the high-affinity P-i transporter genes and the PEPC gene coordinated with the increases in the P-i uptake rate and the PEPC activity, respectively. This suggests that the expression of the high-affinity P-i transporters and PEPC is regulated in part at the transcript level. Furthermore, we examined which of the environmental or the endogenous P-i level regulates the expression of these two systems. The P-i content in the 6-day-old plants decreased to a lower level than that in the 15-day-old plants when grown in a 30 muM P-i solution. At that time, the expression of the high-affinity P-i transporters and PEPC was enhanced only in the 6-day-old plants. Moreover, the P-i content in plants forming many nodules on their stems decreased. The expression of the high-affinity P-i transporters and PEPC was then enhanced in the nodulated plants. These facts suggest that the expression of these two systems may be regulated by the P-i content in the plants, not by the P-i concentration in the soil.