As fundamental nutrients, amino acids are important for rice (Oryza sativa) growth and development. Here, we identified the amino acid permease 5 (OsAAP5), that regulates tiller number and grain yield in rice. The OsAAP5 promoter sequence differed between indica and japonica rice varieties. Lower expression of OsAAP5 in the young leaf blade in indica varieties than in japonica varieties was associated with more tillers in indica than in japonica Down-regulation of OsAAP5 expression in japonica using RNA interference (RNAi) and clustered regularly interspaced short palindromic repeats led to increases in tiller number and grain yield, whereas OsAAP5 overexpression (OE) had the opposite effect. Both a protoplast amino acid uptake assay and HPLC analysis indicated that more basic (Lys, Arg) and neutral (Val, Ala) amino acids were transported and accumulated in the OE lines than in the wild type, but the opposite was observed in the RNAi lines. Furthermore, exogenous application of Lys, Arg, Val, and Ala in the OE lines substantially inhibited tiller bud elongation, but the effect was lost in the RNAi lines. Notably, concentrations of the cytokinins cis-zeatin and dihydrozeatin were much lower in the OE lines than in the wild type, whereas concentrations in the RNAi lines were higher. Thus, OsAAP5 could regulate tiller bud outgrowth by affecting cytokinin levels, and knockout of OsAAP5 could be valuable for japonica breeding programs seeking high yield and grain quality.
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