Regulatory effects of gibberellin and cytokinin on citrus peel cell wall metabolism

During citrus fruit development, exogenous gibberellin (GA) and 6-benzylaminopurine (6-BA, a synthetic cytokinin (CTK)) are both known to promote citrus peel thickness; however, the differences in their regulatory mechanisms on cell wall metabolism in citrus peels remain unclear. In this study, we found that GA treatment significantly increased cell wall polysaccharides in citrus peels, such as pectin and cellulose, whereas 6-BA treatment led to a notable accumulation of lignin. RNA-sequencing data revealed that several fruit ripening-related cell wall degradation genes, such as PME3, PL18, and EXPA1/8, exhibited decreased expression levels in both GA and 6-BA treatments. Additionally, a set of cell wall polysaccharide synthesis genes was upregulated in response to GA treatment but was largely downregulated in 6-BA-treated peels. Conversely, a group of lignin biosynthesis genes was upregulated in 6-BA-treated peels. GA treatment inhibited DELLA proteins (encoded by RGA and GAI) in the GA signaling pathway, whereas 6-BA treatment increased the expression of B-ARRs (ARR1 and ARR2) in the CTK signaling pathway. Furthermore, GA treatment elevated endogenous CTK levels, while 6-BA treatment also enhanced endogenous GA content, suggesting a reciprocal interaction between these two hormonal pathways.