Effects of suppression of phosphate transporter 4;4 on CO2 assimilation in rice.

Homeostasis of inorganic phosphate (Pi) in the chloroplasts is essential for healthy CO2 assimilation. When Pi in chloroplasts is insufficient, the increase in the CO2 assimilation rate (A) with an increase in CO2 level is restricted, whereas A per unit total protein level moderately decreases under low-to-normal CO2 levels. Some phosphate transporters (PHT) are localized in the chloroplast envelope; however, their contribution to the maintenance of Pi homeostasis for CO2 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 CO2 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 CO2 level, and typically increased to saturation with increasing CO2 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 Pi-limited CO2 assimilation. Therefore, PHT4;4 is suggested to be involved in the maintenance of chloroplast Pi homeostasis for healthy CO2 assimilation, although its contribution is minor.