Molecular, cellular and Yin-Yang regulation of grain size and number in rice

Final grain size and grain number per plant, representing the sink capacity, are two major developmental traits determining grain yield of rice and other cereals. Understanding the regulatory mechanisms controlling grain size and number has become an important research field in plant science. Grain number and size are determined by the activity, size and number of inflorescence meristems, branch meristems and floral meristems, which are coordinately regulated by both rate and duration of cell division, cell expansion and cell differentiation. Current advances have revealed several signaling pathways controlling grain size and number, including G protein signaling, biosynthesis and signaling of BR, CK and IAA, MAPK signaling, peptide signaling, the ubiquitin-proteasome degradation pathway, epigenetic pathways and transcriptional regulation. This review provides a general overview of recent findings and future perspectives in molecular, cellular and Yin-Yang regulatory mechanisms of coordinating grain size and number in rice, which has substantial implications for breaking the yield bottleneck caused by the trade-off of grain size and number in crop genetic improvement. We also propose 16 key scientific questions underlying grain size and number in rice and other crops to call for an international coordinated effort in studying and solving these problems in the form of a project named GSGN16.