6-Benzylaminopurine and Gibberellic Acid Mitigate the Yellowing of Pak Choi (<i>Brassica rapa</i> Subsp. <i>Chinensis</i>) During Storage by Regulating Sugar Scarcity-Induced Chlorophagy

This study investigated the effects of 6-benzylaminopurine (6-BA) and gibberellic acid (GA) on the yellowing of pak choi associated with sugar and protein metabolism during storage at 20 °C. 6-BA, GA, and 6-BA+GA prevented leaf yellowing, the decline in chlorophyll content, and changes in color parameters. All treatments activated sucrose phosphate synthase and sucrose synthase-synthesis in addition to inhibiting the decrease of the amylase, glucokinase, and UDP-glucose pyrophosphorylase activities. These treatments also maintain high levels of sugars during storage. Besides, the three treatments increased the transcript of sucrose synthesis genes and suppressed the expression of genes associated with chlorophyll catabolism, amylolysis, protein degradation, autophagy, and sugar transport, sensing and signaling. Sugar scarcity may activate <i>BcATGs</i> expression, which triggers Rubisco degradation and chlorophagy, ultimately leading to chlorophyll breakdown, whereas 6-BA and GA may slow sugar breakdown and thus its adverse effects. Our findings demonstrate the ability of 6-BA/GA to delay the yellowing induced by autophagy and to prevent sugar and protein metabolism in leaf vegetables.