Transitory starch in rice leaves is essential for temporary carbon storage, plant growth development, and subsequent grain filling, yet its regulatory mechanisms for transitory starch biosynthesis remain unclear. Here, we found that Leaf Starch Deficiency 3 (LSD3), as a rice glutamyl-tRNA reductase (GluTR), interacts with GluTRBP (GluTR-binding protein), and GluTRBP interacts with granule-bound starch synthase II (GBSSII), the key enzyme for leaf amylose synthesis. LSD3-GluTRBP, as a molecular module, can maintain the protein stability and enzymatic activity of GBSSII. The lsd3 and glutrbp mutants exhibited lower GBSSII enzymatic activity, leading to significant reductions in leaf starch content. In addition, lsd3 and glutrbp mutants displayed severe defects in carbon sources transport from leaves to developing grains, with significantly down-regulated expression of genes associated with carbon source transport. Finally, the lsd3 and glutrbp mutants showed significantly reduced endosperm storage starch content and severely compromised grain quality and yield. Notably, overexpression of GBSSII partially rescued the defective phenotypes in both leaves and grains of lsd3 and glutrbp mutants. In summary, our results establish the LSD3-GluTRBP module functions upstream of GBSSII and plays a crucial role in transitory starch biosynthesis and the determination of grain quality and yield in rice. These findings provide new molecular targets for simultaneous improvement of grain yield and quality in rice.
Duan et al. (Mon,) studied this question.