Rolling Leaf 2 Controls Leaf Rolling by Regulating Adaxial-Side Bulliform Cell Number and Size in Rice

Leaves represent an important organ in plant photosynthesis, and moderately rolled leaves would be beneficial in establishing an ideal plant architecture and thereby increasing rice yields. In this study, a stable inherited rolled leaf mutant was obtained via ethyl methanesulfonate (EMS) mutagenesis from japonica variety WYJ27, which was named rll2 (rolling leaf 2). rll2 showed a leaf-rolling phenotype at the seedling stage, which increased with growth. Compared with the wild type, the leaves at all levels of rll2 were significantly shorter and narrower, and the leaf-rolling index gradually decreased from the highest leaf to the third-highest leaf. Semi-thin sections showed that the bulliform cells of rll2 were significantly larger than those of the wild type, and the number of cells was significantly higher than that of the wild type. Genetic analysis showed that rll2 is controlled by a pair of recessive nuclear genes. Map-based cloning revealed that RLL2 encodes a conserved and plant-specific calpain-like cysteine proteinase. RLL2 was mainly expressed in young roots, shoots, spikelets, and panicles. Transcriptome sequencing showed that a total of 104 genes were differentially expressed in the wild type and rll2. Moreover, several transcription factor genes were significantly altered in the rll2 mutant. Taken together, our findings indicate that RLL2 plays an important role in leaf rolling by regulating bulliform cells, which may be useful in breeding rice with an ideal plant architecture.