Mutant Ppbbx24-del gene positively regulates light-induced anthocyanin accumulation in the red pear

Anthocyanins are pigments and nutrients in red pears regulated by BBX family genes. Herein, we characterized a 14-nucleotide deletion mutation in the coding region of the PpBBX24 gene from Red Zaosu pear (Pyrus pyrifolia White Pear Group), named Ppbbx24-del. Genetic and biochemical approaches were used to compare the roles of PpBBX24 and Ppbbx24-del in anthocyanin accumulation. Ppbbx24-del played a positive role in anthocyanin biosynthesis of the Red Zaosu pear peel by light treatment. Functional analyses based on overexpression in tobacco and transient overexpression in pear fruit peels showed that Ppbbx24-del promoted anthocyanin accumulation. Cyanidin and peonidin were major differentially expressed anthocyanins, and transcript levels of some structural genes in the anthocyanin biosynthesis pathway were significantly increased. Protein interaction assays showed that PpBBX24 was located in the nucleus and interacted with PpHY5, whereas Ppbbx24-del was colocalized in the nucleoplasm and did not interact with PpHY5. PpHY5 and Ppbbx24-del had positive regulatory effects on the expression of PpCHS, PpCHI, and PpMYB10 when acting alone, but had cumulative effects on gene activation when acting simultaneously. Alone, PpBBX24 had no significant effect on the expression of PpCHS, PpCHI, or PpMYB10, whereas it inhibited the activation effects of PpHY5 on downstream genes when it existed with PpHY5. Our study demonstrated that mutant Ppbbx24-del positively regulates the anthocyanin accumulation in pear. The results of this study clarify the mechanism and enrich the regulatory network of anthocyanin biosynthesis, which lays a theoretical foundation for Ppbbx24-del use to create red pear cultivars.