Refrigeration, forchlorfenuron, and gibberellic acid treatments differentially regulate chlorophyll catabolic pathway to delay yellowing of broccoli

Postharvest broccoli has a high metabolic rate and senesces quickly. The most critical symptom of senescence is flower bud yellowing due to chlorophyll catabolism. Refrigeration and cytokinin treatment are effective means to retard broccoli yellowing; however, the mechanisms underlie are partially understood. Here, we examined the expression of NON-YELLOW COLORING1 (NYC1), CHLOROPHYLLASE1 (CLH1), CLH2, STAYGREEN1 (SGR1), SGR2, pheophytinase (PPH), pheophorbide a oxygenase (PAO), and other chlorophyll catabolic genes in broccoli and investigated the impact of refrigeration, forchlorfenuron (CPPU), and gibberellic acid (GA3) treatments on the expression of the genes. We found that SGR1 and PAO were most highly expressed, whereas expressions of CLH1 and CLH2 were low in fresh broccoli. Refrigeration and CPPU suppressed NYC1, SGR1, SGR2, and PAO while upregulating CLH1 or CLH2, whereas GA3 induced PPH and repressed CLH2. All 3 treatments retarded broccoli yellowing, and correlation analysis revealed that SGR1 and PAO were highly correlated with yellowing, while CLHs were inversely correlated with yellowing. Application of CPPU and GA3 stimulated mild but significant increases in ethylene production, suggesting cross-regulations between hormones during broccoli senescence. In conclusion, we elucidated the differential regulations of refrigeration, CPPU, and GA3 on chlorophyll catabolic genes and identified SGR1 and PAO as the most highly expressed and yellowing-related genes. These findings strengthen the understanding of broccoli senescence and may elicit new research and development of technologies to manipulate chlorophyll metabolism in postharvest crops.