Grain size is one of the most important traits that influence grain yield in rice. Here we report the isolation and characterization of the long grain1 (lin1) mutant, which exhibited long grains and increased grain weight. A field trial conducted over 2 years indicated that the grain weight increased 3.0–6.6% in the lin1 mutant compared with that of the wild-type Nipponbare. Whole-genome sequencing and genetic linkage analysis indicated that a 1-bp deletion within the coding sequence of Os06g0675200, which showed no homology to previously characterized genes, is responsible for the lin1 phenotype. Quantitative real-time PCR indicated that LIN1 transcripts were more abundant in young panicles than at advanced inflorescence developmental stages, which indicated that LIN1 controls grain length mainly at early stages of grain development. Sequence polymorphism analysis of LIN1 showed that all 15 temperate japonica cultivars tested as well as six out of the nine indica cultivars tested possessed the Nipponbare-type LIN1 allele, whereas the remaining three indica cultivars and one aus cultivar tested harbored an identical missense mutation in LIN1. These results revealed that the mutant allele of LIN1 has not been widely utilized in breeding temperate japonica cultivars currently in cultivation. Our findings indicate that the lin1 mutation may be useful to further improve grain length and presumably grain yield in temperate japonica and indica rice cultivars that harbor the Nipponbare-type LIN1 allele.
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