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 <i>japonica</i> variety WYJ27, which was named <i>rll2</i> (<i>rolling leaf 2</i>). <i>rll2</i> showed a leaf-rolling phenotype at the seedling stage, which increased with growth. Compared with the wild type, the leaves at all levels of <i>rll2</i> 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 <i>rll2</i> 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 <i>rll2</i> is controlled by a pair of recessive nuclear genes. Map-based cloning revealed that <i>RLL2</i> encodes a conserved and plant-specific calpain-like cysteine proteinase. <i>RLL2</i> 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 <i>rll2</i>. Moreover, several transcription factor genes were significantly altered in the <i>rll2</i> mutant. Taken together, our findings indicate that <i>RLL2</i> plays an important role in leaf rolling by regulating bulliform cells, which may be useful in breeding rice with an ideal plant architecture.