Exogenous 6‐benzylaminopurine application delays leaf yellowing in pak choi ( Brassica rapa subsp. chinensis ) during storage by regulating endogenous N 6 ‐benzyladenine metabolism and chlorophyll catabolism

BACKGROUND: Postharvest yellowing of pak choi is a critical issue affecting its commercial quality. This study investigated the mechanism by which 6-benzylaminopurine (6-BA), a synthetic form of the cytokinin benzyladenine (BA), regulates endogenous cytokinin (CK) metabolism and the signal transduction involved in the yellowing of pak choi during storage at 20 °C. RESULTS: Treatment with 6-BA inhibited the expression of the key chlorophyll catabolic genes BcCLH1 and BcSGR1, retarded chlorophyll breakdown, and eventually mitigated leaf yellowing. The treatment also maintained a high level of BA (the predominant CK in pak choi), enhanced the activity of cytokinin oxidase/dehydrogenase (CKX) and increased the transcript abundance of the associated genes BcCKX3/6/7, and upregulated the expression of the N-glycosylation-related UDP glycosyltransferase (UGT) genes BcUGT75D1/76C2/89A2. These changes accelerated BA degradation and inactivation, contributing to the maintenance of BA homeostasis. This relationship was also associated with the maintenance of greater expression of genes involved in CK transport (BcPUP1/5/9/13 and BcENT3) and signal transduction (BcAHK2/4, BcAHP3, and type A and B Arabidopsis response regulators (ARRs) BcARR4/6/7/9/15 and BcARR11/12/14/18, respectively) in 6-BA-treated pak choi. CONCLUSION: These findings suggest that the protective effects of 6-BA treatment against chlorophyll degradation may be attributed to elevated BA levels, enhanced signal transduction capacity, accelerated BA degradation and inactivation, and feedback regulation between type A and B BcARRs. The findings identify a candidate regulatory pathway and key regulatory genes involved in the CK-mediated delay of pak choi leaf yellowing. © 2026 Society of Chemical Industry.