Maternal effect contributes to grain-filling defects of Ospho1;2 rice mutants
In seeds, filial and maternal tissues are symplastically isolated. Nutrient delivery through plasma membrane-localized transporters at the junction of filial and maternal tissues is indispensable for sustaining endosperm and embryo development. Phosphorus (P), an essential element, is transported primarily in the form of inorganic phosphate (Pi) across cell membranes. Successful P delivery is achieved by Pi efflux from maternal cells followed by Pi influx into filial cells. Recent studies have reported the importance of PHO1 Pi transporters in transferring Pi from maternal to filial tissues in developing seeds. Loss-of-function Ospho1;2 mutants produced small and shrunken rice seeds with reduced starch content, resulting in grain chalkiness, highlighting the crucial role of OsPHO1;2 in determining grain quality and yield. OsPHO1;2 is predominantly expressed in the nucellar epidermis, the innermost cell layer of the caryopsis coat, where nutrients are unloaded from maternal to filial tissues. Thus, it is rational to envision that the grain-filling defects in Ospho1;2 mutants result from a reduced level of Pi in the starchy endosperm owing to impaired Pi unloading from maternal tissues. However, a previous model proposed that an elevated Pi level in the Ospho1;2 starchy endosperm cells obstructs starch synthesis because of reduced Pi efflux from the inner starchy endosperm cells. In the present study, we carefully examined the expression of OsPHO1;2 and total P and Pi distributions in the grains. We also analyzed the relevance of OsPHO1;2 in grain development through reciprocal crossing between Ospho1;2 mutants and wild-type rice. Our results provide unequivocal evidence that a maternal effect contributes to the grain phenotypes of Ospho1;2 mutants. Loss of function of OsPHO1;2 in the nucellar epidermis of the caryopsis coat rather than the endosperm is the primary cause of grain-filling defects observed in Ospho1;2 mutants. Through this study, we have clarified this point and presented a revised model.
Swee-Suak Ko1,2,†,*, Wen-Chien Lu1,†, Jo-Chi Hung1,2,†, Hsin-Fang Chang1, Min-Jeng Li1,2, Kuo-Chen Yeh1 and Tzyy-Jen Chiou1* (2024) Maternal effect contributes to grain-filling defects of Ospho1;2 rice mutants New Phytologist, 244(2): 351-357