The HD-ZIP I transcription factor ClLL1 regulates lobed leaf development through the auxin pathway in watermelon

Watermelon exhibits deeply lobed leaves, distinct from its close cucurbit relatives such as melon and cucumber. However, the genetic mechanisms underlying lobed-leaf development in watermelon remain elusive. In this study, we successfully delimited the lobed-leaf regulatory gene, ClLL1, into a 29.14-Kb interval on chromosome 4 and verified that its loss-function results in non-lobed leaves. Phylogenetic analysis revealed that ClLL1 is a putative orthologue of AtHB22 within the class I of HD-ZIP transcription factor family, distinct from the well-characterized LMI1/RCO lineage. Notably, ClLL1 displays exceptional sequence conservation among watermelon lobed-leaf germplasms. Mechanistically, we demonstrated that ClLL1 forms homodimers and autoregulates its own expression through direct promoter binding, establishing a dose-dependent relationship with leaf lobation severity. Furthermore, ClLL1 directly activates the auxin efflux carrier gene ClPIN1, linking leaf-shape determination to auxin signaling. This regulatory network was corroborated by exogenous application of the auxin transport inhibitor NPA and auxin (IAA), which significantly altered leaf margin phenotypes. Our results establish a ClLL1-mediated auxin pathway in watermelon leaf-shape formation, providing new insights into the functions of the ClLL1 lineage in Cucurbitaceae.