Big Grain3, encoding a purine permease, regulates grain size via modulating cytokinin transport in rice

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 grainy(bg3-D),which exhibits a remarkable increase of grain size caused by activation of the PURINE PERMEASE gene,OsPUP4.BC3/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 bgj-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.

POLLEN STERILITY,a novel suppressor of cell division,is required for timely tapetal programmed cell death in rice

Timely programmed cell death(PCD) of the tapetum supplying nutrients to microspores is a prerequisite for normal pollen development.Here we identified a unique mutant of rice(Oryza sativa L.),pollen sterility(post),which showed aborted pollens accompanied with extra-large husks.Due to failure of timely PCD of tapetal cells,post exhibited abnormal pollen wall patterning and defective pollen grains.By map-based cloning,we identified a causal gene,POST,encoding a novel protein which is ubiquitously localized in cells.RNA in situ hybridization showed that POST is highly detected in the tapetum and microspores at stages 8 and 9.Transcriptome analysis indicated that POST could function as an important regulator of the metabolic process involved in tapetal PCD.Compared with wild-type rice,post mutant has an increased cell number resulting from elevated expression of cell cycle associated genes in grain husks.Overexpression of POST inhibits grain size in wild type,while appropriate expression of POST in post mutant can recover the seed fertility but has little effect on the large grains,illustrating that fine-tuning of POST expression could be a potential strategy for rice yield improvement.The connection between cell division and cell death conferred by POST provides novel insights into the understanding of the tapetal PCD process.