Discovering and characterizing a protein involved in endosperm starch formation in rice

Rice is a globally important crop, with most of its calorific component coming from starch. Despite its importance, we do not fully understand the mechanism of starch formation in the rice endosperm. To discover novel factors involved in this process, Yan et al. performed a forward genetics screen and identified a FLOURY ENDOSPERM 9 (flo9) mutant that had a 32% reduction in starch accumulation. Genetic mapping revealed that flo9 plants had a point mutation in LIKE EARLY STARVATION1 (LESV1) which leads to a premature stop codon and a severely truncated LESV1 protein. LESV1 is a non-enzymatic protein, so the authors investigated whether it interacted with the starch debranching enzyme ISOAMYLASE (ISA1), as isa1 mutants are phenotypically similar to flo9 plants. Yeast 2-hybrid and luciferase complementation assays indeed demonstrated a physical interaction between LESV1 and ISA1. The authors then tested whether the flo9 mutation affects ISA1 localization. Expressing ISA1-GFP in wild type rice protoplasts caused a proportion of ISA1-GFP to be localized to starch granules. Whereas, when repeated with flo9 protoplasts, much less ISA1-GFP localised to starch granules. Hence LESV1 interacts with and targets ISA1 to starch granules and this discovery greatly enhances our understanding of rice endosperm starch formation. (Summary by Rose McNelly @Rose_McN) Plant Cell 10.1093/plcell/koae006