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

It is well known that the P-i uptake system via the high-affinity P-i trans porter and the organic acid exudation system via PEPC are enhanced in the r oots of P-i-starved plants. In this paper, we compared the expression of th ese two systems in Sesbania rostrata, a leguminous plant, on whose roots an d stems it forms nodules. When S. rostrata plants were transferred to a 0 m uM P-i nutrient solution, the expression of both the high-affinity Pi trans porter 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 wi th 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 a nd PEPC is regulated in part at the transcript level. Furthermore, we exami ned which of the environmental or the endogenous P-i level regulates the ex pression of these two systems. The P-i content in the 6-day-old plants decr eased to a lower level than that in the 15-day-old plants when grown in a 3 0 muM P-i solution. At that time, the expression of the high-affinity P-i t ransporters and PEPC was enhanced only in the 6-day-old plants. Moreover, t he 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 enhance d 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.