OsPIL15, a phytochrome-interacting factor-like gene, is involved in the regulation of grain size in rice

As members of the basic helix-loop-helix transcription factor families, phytochrome-interacting factors (PIFs) regulate an array of developmental responses ranging from seed germination to plant growth. However, the role of PIFs on regulating grain size remains unclear. Thus, we analyzed the expression pattern of rice OsPIL genes in grains and found that OsPIL15 may play an important role in grain development. We then generated knock-out (KO) OsPIL15 lines in rice using CRISPR/Cas9, the silencing expression of OsPIL15 led to increased numbers of cells, which thus enhanced grain size and weight. Moreover, overexpression and suppression of OsPIL15 in the rice endosperm resulted in brown rice showing grain sizes and weights that were decreased and increased, respectively. Further studies indicated that OsPIL15 binds to N1-box (CACGCG) motifs of the purine permease gene OsPUP7 promoter. Measurement of isopentenyl adenosine, a bioactive form of cytokinin (CTK), revealed increased contents in the OsPIL15-KO spikelets compared with the wild-type. Overall, our results, for the first time, demonstrate a possible pathway whereby OsPIL15 directly targets OsPUP7, affecting CTK transport and thereby influencing cell division and subsequent grain size. These findings provide a valuable insight into the molecular functions of OsPIL15 in rice grains, highlighting a useful genetic improvement leading to increased rice yield.