Regulation of tiller angle by gravity-sensing genes in rice

Endodermal cells and starch-accumulating amyloplasts are well-known gravity sensors initiating shoot gravitropism in Arabidopsis thaliana. The transcription factors SHR and SGR1 regulate endodermal cell formation, while PGM has been demonstrated to regulate starch biosynthesis within chloroplasts, which eventually leads to starch accumulation in amyloplasts. However, the molecular mechanisms of gravity sensing in monocot shoots remain largely unexplored. In this study, we investigated the roles of these genes in rice (Oryza sativa), a model monocot, using CRISPR-Cas9 to generate single, double, and higher-order mutants. The rice genome harbors two orthologs each of SHR and SGR and a single ortholog of PGM. Our results revealed that single mutants of OsPGM, but not OsSHR or OsSGR, showed compromised shoot gravitropism. However, double mutants shr1shr2 and sgr1sgr2 displayed wider tiller angles and reduced gravity sensing, suggesting functional redundancy within each gene pair. Higher-order mutants exhibited progressively severe phenotypes, with quintuple mutants almost unresponsive to gravity stimulation. These findings suggest that these genes act additively through distinct but converging pathways in shoot gravitropism regulation. This study provides novel insights into the molecular mechanisms underlying gravity sensing in monocots and offers valuable knowledge for precision breeding to optimize rice architecture.