CabHLH18-like Gene Promotes Leaf Yellowing and Directly Activates CaCLH1 in Pepper

Yellow leaf phenotypes represent an important trait in pepper and enrich genetic germplasm resources. Previous studies have primarily examined leaf yellowing caused by defects in chlorophyll biosynthesis, whereas the functions of chlorophyll degradation-related genes remain less well characterized. In this study, a yellow leaf phenotype (NY) was identified. Transmission electron microscopy and RNA-seq showed that differentially expressed genes were concentrated in photosynthesis- and chlorophyll-related pathways. CaCLH1 was then demonstrated to promote chlorophyll degradation. Yeast one-hybrid analysis identified the CabHLH18-like gene as a direct interactor with the CaCLH1 promoter and as a positive regulator of CaCLH1 transcription. The CabHLH18-like gene was highly expressed in yellow leaf pepper. The CabHLH18-like gene was subsequently isolated and characterized, revealing nuclear localization and transcription factor activity. Functional analysis showed that silencing the CabHLH18-like gene increased chlorophyll content, whereas its overexpression reduced chlorophyll content. These findings indicate that CaCLH1 is a direct downstream target of the CabHLH18-like gene and contributes to CabHLH18-like-gene-mediated regulation of leaf yellowing in pepper. This study refines mechanistic understanding of leaf yellowing and provides a foundation for future research and breeding applications in pepper.