摘要Grain size is an important agronomic trait affecting grain yield,but the underlying molecular mechanisms remain to be elucidated.Here,we isolated a dominant mutant,big grain3 (bg3-D),which exhibits a remarkable increase of grain size caused by activation of the PURINE PERMEASE gene,OsPUP4.BG3/OsPUP4 is predominantly expressed in vascular tissues and is specifically suppressed by exogenous cytokinin application.Hormone profiling revealed that the distribution of different cytokinin forms,in roots and shoots of the bg3-D mutant,is altered.Quantitative reverse transcription-PCR (qRT-PCR) analysis indicated that expression of rice cytokinin type-A RESPONSE REGULATOR (OsRR) genes is enhanced in the roots of the bg3-D mutant.These results suggest that OsPUP4 might contribute to the long-distance transport of cytokinin,by reinforcing cytokinin loading into vascular bundle cells.Furthermore,plants overexpressing OsPUP7,the closest homolog of OsPUP4,also exhibited a similar phenotype to the bg3-D mutant.Interestingly,subcellular localization demonstrated that OsPUP4 was localized on the plasma membrane,whereas OsPUP7 was localized to the endoplasmic reticulum.Based on these findings,we propose that OsPUP4 and OsPUP7 function in a linear pathway to direct cytokinin cell-to-cell transport,affecting both its long-distance movement and local allocation.