Genetic mapping of a spontaneous short-grain mutation reveals a novel loss-of-function allele of SRS3 in rice

Spontaneous mutations are a rare but important source of novel genetic variation, yet their detection and characterization within active breeding programs are seldom documented at gene-level resolution. Grain size and shape are key determinants of rice quality, yield, and market classification. Here, we report the discovery and genetic characterization of a spontaneous short-grain (SG) mutation arising in the long-grain wild-type (WT) advanced breeding line RU2002174 from the LSU AgCenter Rice Breeding Program. The SG phenotype was first observed in 2019 and segregated in subsequent generations as a single recessive gene across both indica and japonica genetic backgrounds. Genetic mapping localized the mutation to a 41.6 kb interval on chromosome 5. Whole-genome sequencing identified a single candidate causal variant: a G[->]T transversion in exon 4 of SRS3 (Os05g06280), introducing a premature stop codon and resulting in a truncated protein. This allele was absent from representative U.S. breeding germplasm and the IRRI 3K SNP database, demonstrating that it represents a novel spontaneous loss-of-function allele of a previously characterized grain-size gene. These findings document the real-time emergence of functional genetic variation in elite rice germplasm and highlight the importance of monitoring off-types during seed increase and purification in breeding programs. They also provide additional insight into the role of kinesin-mediated cell elongation in determining rice grain architecture.