Proteomics Analysis of the Response Mechanism of the Antioxidant System of Broccoli Heads under High O₂ Stress

In order to explore the molecular mechanism of the response of the antioxidant system in broccoli heads to high O₂ stress, the changes in the antioxidant system and related protein expression in broccoli heads under different O₂ concentrations (5% O₂ + 5% CO₂, 20% O₂ + 5% CO₂ and 40% O₂ + 5% CO₂) were investigated by physiological and biochemical analysis and isobaric tags for relative and absolute quantitation (iTRAQ) proteomics. The gene ontology (GO) annotation, the clusters of orthologous groups of proteins (COG) analysis and the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that a total of 76 differentially expressed proteins related to the antioxidant system were identified in this study, which were involved in the ascorbate-glutathione cycle, glucosinolate degradation, and the metabolism of polyphenols, flavonoids and antioxidant enzymes. The proteomic analysis showed that the 40% O₂ + 5% CO₂ treatment significantly inhibited the activity of an ascorbate peroxidase and three glutathione peroxidases, thereby inhibiting the ascorbate-glutathione cycle in broccoli compared with the 5% O₂ + 5% CO₂ treatment. Meanwhile, the 40% O₂ + 5% CO₂ treatment significantly increased the activities of 16 myrosinases and promoted the degradation of glucosinolate. In addition, the expression of shikimate dehydrogenase in the 40% O₂ + 5% CO₂ treatment group decreased by 23% compared with that in the 5% O₂ + 5% CO₂ treatment group, inhibiting shikimic acid synthesis and the accumulation of polyphenols and flavonoids. This study has revealed the cause of rapid yellowing and senescence in broccoli heads under high O₂ stress, which provides a theoretical basis for the development of new broccoli preservation technology and the remission of high O₂ stress.