Control of grain size in rice.

KEY MESSAGE: Summary of rice grain size. Rice is one of the most important crops in the world. Increasing rice yield has been an urgent need to support the rapid growth of global population. The size of grains is one of major components determining rice yield; thus, grain size has been an essential target during rice breeding. Understanding the genetic and molecular mechanisms of grain size control can provide new strategies for yield improvement in rice. In general, the final size of rice grains is coordinately controlled by cell proliferation and cell expansion in the spikelet hull, which sets the storage capacity of the grain and limits grain filling. Recent studies have identified several quantitative trait loci and a number of genes as key grain size regulators. These regulators are involved in G protein signaling, the mitogen-activated protein kinase signaling pathway, the ubiquitin-proteasome pathway, phytohormone signalings, or transcriptional regulation. In this review, we summarize current knowledge on grain size control in rice and discuss the genetic and molecular mechanisms of these grain size regulators.