Wall-associated kinase-like gene RL1 contributes to red leaves in sorghum

Red leaves are common in trees but rare in cereal crops. Red leaves can be used as raw materials for anthocyanin extraction and may have some adaptive significance for plants. In this study, we discovered a red leaf phenotype in the F₁ hybrids derived from a cross between two sorghum accessions with green leaf. Histological analysis of red leaves and green leaves showed that red compounds accumulate in mesophyll cells and gradually spreads to the entire leaf blade. In addition, we found chloroplasts degraded more quickly in red leaves than in green leaves based on transmission electron microscopy. Metabolic analysis revealed that flavonoids including six anthocyanins are more abundant in red leaves. Moreover, transcriptome analysis revealed that expression of flavonoid biosynthesis genes was upregulated in red leaves. These observations indicate that flavonoids and anthocyanins in particular, are ideal candidates for the red compounds accumulating in red leaves. Segregation analysis of the red leaf phenotype suggested a genetic architecture consisting of three dominant genes, one (RL1 for RED LEAF1) of which we mapped to a 55-kb region on chromosome 7 containing seven genes. Sequencing, reverse transcription-polymerase chain reaction, and transcriptome analysis suggested Sobic.007G214300, encoding a wall-associated kinase, as the most likely candidate for RL1. Fine mapping the red leaf gene and identifying the metabolites that cause red leaf in sorghum provide us with a better understanding of the red leaf phenotype in the natural population of sorghum.