Endosperm sugar accumulation caused by mutation of PHS8/ISA1 leads to pre‐harvest sprouting in rice

Summary Pre‐harvest sprouting ( PHS ) is an unfavorable trait in cereal crops that could seriously decrease grain yield and quality. Although some PHS ‐associated quantitative trait loci or genes in cereals have been reported, the molecular mechanism underlying PHS remains largely elusive. Here, we characterized a rice mutant, phs8 , which exhibits PHS phenotype accompanied by sugary endosperm. Map‐based cloning revealed that PHS 8 encodes a starch debranching enzyme named isoamylase1. Mutation in PHS 8 resulted in the phytoglycogen breakdown and sugar accumulation in the endosperm. Intriguingly, with increase of sugar contents, decreased expression of Os ABI 3 and Os ABI 5 as well as reduced sensitivity to abscisic acid ( ABA ) were found in the phs8 mutant. Using rice suspension cell system, we confirmed that exogenous sugar is sufficient to suppress the expression of both Os ABI 3 and Os ABI 5 . Furthermore, overexpression of Os ABI 3 or Os ABI 5 could partially rescue the PHS phenotype of phs8 . Therefore, our study presents important evidence supporting that endosperm sugar not only acts as an essential energy source for seed germination but also determines seed dormancy and germination by affecting ABA signaling.