Rice grain yield is determined by three major “visible” morphological traits: grain weight, grain number per panicle, and effective tiller number, which are affected by a series of “invisible” physiological factors including nutrient use efficiency and photosynthetic efficiency. In the past few decades, substantial progress has been made on elucidating the molecular mechanisms underlying grain yield formation, laying a solid foundation for improving rice yield by molecular breeding. This review outlines our current understanding of the three morphological yield-determining components and summarizes major progress in decoding physiological traits such as nutrient use efficiency and photosynthetic efficiency. It also discusses the integration of current knowledge about yield formation and crop improvement strategies including genome editing with conventional and molecular breeding.

Heat stress (HS) caused by rapidly warming climate has become a serious threat to global food security. Rice (Oryza sativa L.) is a staple food crop for over half of the world’s population, and its yield and quality are often reduced by HS. There is an urgent need for breeding heat-tolerant rice cultivars. Rice plants show various morphological and physiological symptoms under HS. Precise analysis of the symptoms (phenotyping) is essential for the selection of elite germplasm and the identification of thermotolerance genes. In response to HS, rice plants trigger a cascade of events and activate complex transcriptional regulatory networks. Protein homeostasis under HS is especially important for rice thermotolerance, which is affected by protein quality control, effective elimination of toxic proteins, and translational regulation. Although some agronomic and genetic approaches for improving heat tolerance have been adopted in rice, the molecular mechanisms underlying rice response to HS are still elusive, and success in engineering rice thermotolerance in breeding has been limited. In this review, we summarize HS-caused symptoms in rice and progress in heat-stress sensing and signal cascade research, and propose approaches for improving rice thermotolerance in future.