Rice grain size: current regulatory mechanisms and future perspectives

Rice is a staple food for over half of the world's population. To feed the growing population, molecular breeders aim to increase grain yield. Grain size is an important factor for crop productivity, and it has been extensively studied. However, molecular breeders face a major challenge in further improving crop productivity in terms of grain yield and quality. Grain size is a complex trait controlled by multiple genes. Over the past few decades, genetic studies have identified various gene families involved in grain size development. The list of molecular mechanisms, and key regulators involved in grain size development is constantly expanding, making it difficult to understand the main regulators that play crucial roles in grain development. In this review, we focus on the major regulators of grain size, including G-protein signaling, the mitogen-activated protein kinase (MAPK) pathway, transcriptional regulation, the ubiquitin-proteasome degradation (UPD) pathway, and phytohormone signaling. These molecular mechanisms directly or indirectly regulate grain size. We provided a comprehensive understanding of the genes involved in these mechanisms and cross discussions about how these mechanisms are interlinked. This review serves as a valuable resource for understanding the molecular mechanisms that govern grain development and can aid in the development of molecular breeding strategies.

Variations in CYP78A13 coding region influence grain size and yield in rice

Grain size is one of the most important determinants of crop yield in cereals. Here, we identified a dominant mutant, big grain2 (bg2-D) from our enhancer-trapping population. Genetic analysis and SiteFinding PCR (polymerase chain reaction) revealed that BG2 encodes a cytochrome P450, OsCYP78A13. Sequence search revealed that CYP78A13 has a paralogue Grain Length 3.2 (GL3.2, LOC_Os03g30420) in rice with distinct expression patterns, analysis of transgenic plants harbouring either CYP78A13 or GL3.2 showed that both can promote grain growth. Sequence polymorphism analysis with 1529 rice varieties showed that the nucleotide diversity at CYP78A13 gene body and the 20 kb flanking region in the indica varieties were markedly higher than those in japonica varieties. Further, comparison of the genomic sequence of CYP78A13 in the japonica cultivar Nipponbare and the indica cultivar 9311 showed that there were three InDels in the promoter region and eight SNPs (single nucleotide polymorphism) in its coding sequence. Detailed examination of the transgenic plants with chimaeric constructs suggested that variation in CYP78A13 coding region is responsible for the variation of grain yield. Taken together, our results suggest that the variations in CYP78A13 in the indica varieties hold potential in rice breeding for application of grain yield improvement.