Effects of suppression of phosphate transporter 4;4 on CO assimilation in rice.

Homeostasis of inorganic phosphate (P) in the chloroplasts is essential for healthy CO assimilation. When P in chloroplasts is insufficient, the increase in the CO assimilation rate (A) with an increase in CO level is restricted, whereas A per unit total protein level moderately decreases under low-to-normal CO levels. Some phosphate transporters (PHT) are localized in the chloroplast envelope; however, their contribution to the maintenance of P homeostasis for CO assimilation has rarely been reported. In this study, we generated transgenic rice plants with RNAi-suppressed PHT4;4, one of the two genes of chloroplast envelope-localized PHT, and examined the changes in the characteristics of CO assimilation. In three transgenic lines, the mRNA levels of PHT4;4 decreased by approximately 80% without a notable decrease in total leaf-P levels or total leaf-N levels, which is thought to approximately correspond to total protein levels. A in the transgenic plants tended to be slightly lower than that in the wild-type plants, irrespective of the CO level, and typically increased to saturation with increasing CO levels. A per unit total leaf-N level in transgenic plants tended to be slightly lower than that in wild-type plants. These results indicate that substantial PHT4;4 suppression caused slight symptoms of P-limited CO assimilation. Therefore, PHT4;4 is suggested to be involved in the maintenance of chloroplast P homeostasis for healthy CO assimilation, although its contribution is minor.