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

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Abstract
It is well known that the Pi uptake system via the high-affinity Pi transporter and the organic acid exudation system via PEPC are enhanced in the roots of Pi-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 µM Pi 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 Pi transporter genes and the PEPC gene coordinated with the increases in the Pi uptake rate and the PEPC activity, respectively. This suggests that the expression of the high-affinity Pi transporters and PEPC is regulated in part at the transcript level. Furthermore, we examined which of the environmental or the endogenous Pi level regulates the expression of these two systems. The Pi 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 µM Pi solution. At that time, the expression of the high-affinity Pi transporters and PEPC was enhanced only in the 6-day-old plants. Moreover, the Pi content in plants forming many nodules on their stems decreased. The expression of the high-affinity Pi 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 Pi content in the plants, not by the Pi concentration in the soil.