A rice receptor-like kinase gene OSBBS1/OsRLCK109 was identified; this gene played vital roles in leaf senescence and the salt stress response. Early leaf senescence can cause negative effects on rice yield, but the underlying molecular regulation is not fully understood. bilateral blade senescence 1 (bbs1), an early leaf senescence mutant with a premature senescence phenotype that occurs mainly performing at the leaf margins, was isolated from a rice mutant population generated by ethylmethane sulfonate (EMS) treatment. The mutant showed premature leaf senescence beginning at the tillering stage and exhibited severe symptoms at the late grain-filling stage. bbs1 showed accelerated dark-induced leaf senescence. The OsBBS1 gene was cloned by a map-based cloning strategy, and a guanine (G) insertion was found in the first exon of LOC_Os03g24930. This gene encodes a receptor-like cytoplasmic kinase and was named OsRLCK109 in a previous study. Transgenic LOC_Os03g24930 knockout plants generated by a CRISPR/Cas9 strategy exhibited similar early leaf senescence phenotypes as did the bbs1 mutant, which confirmed that LOC_Os03g24930 was the OsBBS1 gene. OsBBS1/OsRLCK109 was expressed in all detected tissues and was predominantly expressed in the main vein region of mature leaves. The expression of OsBBS1 could be greatly induced by salt stress, and the bbs1 mutant exhibited hypersensitivity to salt stress. In conclusion, this is the first identification of OsRLCKs participating in leaf senescence and playing critical roles in the salt stress response in rice (Oryza sativa L.).

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Panicle architecture and grain size are two important agronomic traits which determine grain yield directly in rice. In the present study, a mutant named ltbsg1 (longer top branch and shorter grain 1) was isolated from the cultivar “Zhenong 34” (Oryza sativa L. ssp. indica) by ethyl methane sulfonate (EMS) mutagenesis. The target gene was studied through phenotype observation, genetic analysis, map-based cloning and functional analysis. The histocytological analysis indicated that the elongated top branch and shorter grain of mutant ltbsg1 were caused from the defects of cell elongation. The ltbsg1 gene in mutant revealed a single nucleotide substitution (G-A) in the exon 2 of LOC_Os10g25780, causing an amino acid variation (Glycine-Arginine) in the FAD (Flavin-adenine dinucleotide)-binding domain of delta (24)-sterol reductase, which was involved in the brassinosteroid (BR) biosynthesis. LTBSG1 was constitutively expressed and the protein was widely localized in chloroplast, nucleus and cytomembrane. The ltbsg1 seedlings had a lower endogenous BR level and could be restored to the phenotype of wild type by exogenous BR. The LTBSG1 knock-out lines showed similar phenotype defects as mutant ltbsg1, which confirmed that LTBSG1 was responsible for top branch elongation and grain size reduction. Furthermore, LTBSG1 along with other BR-related genes were feedback-regulated due to their obvious altered expression in mutant ltbsg1. This study demonstrated that LTBSG1 could play a new role in regulating panicle and grain development by BR biosynthetic pathway.

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