Genetic analysis of a DROOPING LEAF mutant allele dl-6 associated with a twisted and folded leaf base caused by a deficiency in midrib development in rice.

The failure of midrib formation in rice leaf blades results in the drooping leaf (dl) phenotype. A normal DROOPING LEAF (DL) gene is necessary for leaf homeotic transformation, which affects midrib and pistil development. Genetic analysis was performed on a new drooping leaf (dl) mutant named dl-6 in rice. The dl-6 allelic mutant exhibited drooping leaves that were severely folded and twisted at the base but had normal flower structure. The dl-6 allele is a nuclear recessive trait that fits a 3:1 Mendelian segregation ratio. The dl-6 mutant leaves displayed an abnormal main vein (midrib-less) with undeveloped aerenchyma and vascular bundles, resulting in severe leaf drooping. The lack of a midrib in dl-6 caused weak mechanical support, which resulted in folding at the collar junction of the leaf base and downward bending. Through genetic mapping, the dl-6 allele was identified at approximately 28.2 cM on rice chromosome 3. The allele was caused by mutations within the DL (LOC_Os03g11600.1) gene, with specific amino acid substitutions and additions in the encoded protein of the YABBY transcription factor. The dl-6 mutant is a recessive allele encoding a dysfunctional YABBY transcription factor that regulates leaf midrib development and aerenchymatous clear cell structures, leading to a drooping leaf phenotype in rice.