Integrative analysis of transcriptome and metabolome reveals the possible mechanism of leaf yellowing in pak choi (Brassica rapa subsp. chinensis) with 1-methylcyclopropene treatment during storage at 20 °C

Abstract To understand the regulatory mechanism of leaf yellowing in pak choi with 1-methylcyclopropene (1-MCP) treatment at (20 ± 0.5) °C, integrative analysis of the metabolome and transcriptome profiles was performed. Results showed that treatment with 1-MCP significantly delayed degreening and inhibited the respiration rate and ethylene production of pak choi. Chlorophyllide (Chlide) a, pheophytin (Phy) a and pheophorbide (Pheide) a were the main chlorophyll (Chl) catabolites in pak choi which diminished concomitantly with leaf yellowing. 1-MCP effectively retarded the formation and decrease of Chl intermediate metabolites (Phy a and Pheide a) by suppressing the activities of Chl-degrading enzymes such as Mg-dechelatase (MD) and pheophytinase (PPH), and therefore, maintained high levels of Chl a, Chl b and Chl cycle metabolites. 1-MCP significantly down regulated ethylene related biosynthesis and signal transduction genes including BcMSAMS2, BcACO and BcACO4, together with BcMPK19, BcEIN3 and BcERF073. While expressions of BcACS4, BcCTR1 and BcETR1 were slightly affected by 1-MCP treatment. The treatment also significantly inhibited the expressions of most of Chl degradation associated genes (BcPPH1/2, BcSGR1/2, BcPAO, BcNYC1 and BcNOL) except for BcRCCR. However, chlorophyllase (CLH) activity with 1-MCP treatment were enhanced, as well as BcCLH1/2 expression. Pak choi preserved high Mg-chelating substance (MCS) activity throughout storage, without being affected by 1-MCP. PPH rather than CLH played a central role in Chl catabolism. 1-MCP promoted the conversion of Chl a and Chlide a to Chl b by up-regulating BcCAO, retarding Chl degradation. In conclusion, 1-MCP fumigation significantly attenuated the expressions of genes related in ethylene biosynthesis, signal transduction and Chl degradation, and increased BcCAO expression, ultimately inhibiting Chl degradation and relieving the yellowing of pak choi. The results provide a possible target for delaying the yellowing of green vegetables.