Class III peroxidases CgPRX24/41/65 regulate postharvest lignification in granulated juice sacs during citrus fruit senescence

Abstract

Granulation, a physiological disorder in citrus fruit characterized by progressive lignification of juice sacs during ripening and postharvest senescence, significantly impacts fruit quality. While class III peroxidases (PRXs) are recognized as crucial enzymes for lignin monomer polymerization, their specific regulatory roles in citrus granulation remain unclear. In this study, we performed a comprehensive analysis of the PRX gene family in Citrus grandis (pomelo) to explore their role in lignification during postharvest granulation. A total of 88 PRX genes were identified, and phylogenetic and expression pattern analyses revealed that four PRX genes—CgPRX24, CgPRX41, CgPRX64, and CgPRX65—were upregulated during the granulation process across multiple growing seasons. Further functional studies through transient overexpression in citrus fruit pericarp demonstrated that CgPRX24, CgPRX41, and CgPRX65 significantly promoted lignification, enhancing pericarp firmness and lignin content in both pomelo and sweet orange. In contrast, CgPRX64 had no significant effect on lignification. Conserved upregulation patterns and high sequence similarity (more than 93 %) of homologs in Citrus sinensis suggest evolutionary conservation of these PRXs' lignification function across citrus species. Our findings establish CgPRX24/41/65 as phylogenetically conserved central regulators of juice sac lignification during postharvest citrus senescence. The present study not only functionally validates the "Disorder of cell wall metabolism inducing granulation" hypothesis but also provides valuable insights into the molecular mechanism underlying fruit senescence.