Grain Shape Genes: Shaping the Future of Rice Breeding

The main goals of rice breeding nowadays include increasing yield, improving grain quality, and promoting complete mechanized production to save labor costs. Rice grain shape, specified by three dimensions, including grain length, width and thickness, has a more precise meaning than grain size, contributing to grain appearance quality as well as grain weight and thus yield. Furthermore, the divergence of grain shape characters could be utilized in mechanical seed sorting in hybrid rice breeding systems, which has been succeeded in utilizing heterosis to achieve substantial increase in rice yield in the past decades. Several signaling pathways that regulate rice grain shape have been elucidated, including G protein signaling, ubiquitination-related pathway, mitogen-activated protein kinase signaling, phytohormone biosynthesis and signaling, microRNA process, and some other transcriptional regulatory pathways and regulators. This review summarized the recent progress on molecular mechanisms underlying rice grain shape determination and the potential of major genes in future breeding applications.