A glutamyl-tRNA reductase and its binding protein promote transitory starch biosynthesis and enhance grain quality...

Transitory starch in rice leaves is critical for temporary carbon storage, plant growth, and subsequent grain filling, yet the regulatory mechanisms underlying its biosynthesis remain poorly understood. Here, we found that Leaf Starch Deficiency 3 (LSD3), a rice glutamyl-tRNA reductase (GluTR), interacts with GluTR-binding protein (GluTRBP), which in turn associates with granule-bound starch synthase II (GBSSII), the key enzyme responsible for amylose synthesis in leaves. The LSD3-GluTRBP module maintains both the protein stability and enzymatic activity of GBSSII. The lsd3 and glutrbp mutants exhibited markedly reduced GBSSII activity, resulting in significant decreases in leaf starch accumulation. Moreover, these mutants displayed severe defects in carbon-source transport from leaves to developing grains, accompanied by strong downregulation of genes associated with carbon-source allocation. Consequently, the lsd3 and glutrbp mutants showed reduced endosperm-storage starch content and severely compromised grain quality and yield. Notably, overexpression of GBSSII partially rescued defective phenotypes in both leaves and grains of the mutants. In summary, our findings establish that the LSD3-GluTRBP module acts upstream of GBSSII and plays a central role in transitory starch biosynthesis as well as in determining grain quality and yield in rice. These findings provide new molecular targets for simultaneously improving both grain yield and quality in rice.