Physiological and iTRAQ-based proteomic analyses for yellowing of postharvest broccoli heads under elevated O2 controlled atmosphere

Controlled atmosphere storage has been proved to be an effective measure to regulate chlorophyll degradation. The obvious inhibitory effect of CO2 on chlorophyll degradation was widely studied, but information available on the promotion of chlorophyll degradation by elevated O2 was limited. The present work aimed to investigate the regulation of different O2 concentrations on yellowing of broccoli heads during storage by physiology and proteomics. As expected, 40% O2 + 5% CO2 treatment promoted the chlorophyll degradation and carotenoid synthesis, and thus to accelerate the yellowing of broccoli heads. In addition, elevated O2 decreased the ascorbic acid, glucosinolate, and flavonoid contents and increased the MDA content. A total of 189 proteins related to the yellowing of broccoli heads were identified as differentially expressed proteins (fold change >= 1.20 or <= 0.83). These proteins were mainly associated with pigment metabolism, thylakoid structure, photosynthetic system, chloroplast ribosome, antioxidant substance metabolism, and antioxidant enzyme metabolism. The proteomics results showed 40% O2 + 5% CO2 treatment disrupted antioxidant metabolism and aggravated oxidative stress compared with 5% O2 + 5% CO2 and 20% O2 + 5% CO2 treatments, thus leading to the destruction of photosystems I and II, thylakoid membrane, and chloroplast ribosome in broccoli heads. In addition, 40% O2 + 5% CO2 treatment accelerated the yellowing of postharvest broccoli heads by down-regulating chlorophyll biosynthetic pathway and the expression of apolipoproteins (light-harvesting complex I, II, and oxygen-evolving enhancer protein) and ribulose-1,5-bisphosphate carboxylase (Rubisco), and up-regulating chlorophyll degradation and carotenoid biosynthetic pathways. This study might improve our understanding of yellowing acceleration mechanisms of broccoli heads by elevated O2, thereby promoting the development of new broccoli heads preservation technology.

Automated summary

Controlled atmosphere storage is effective in regulating chlorophyll degradation in broccoli heads. Elevated O2 levels were found to promote chlorophyll degradation and carotenoid synthesis, accelerating yellowing. Proteomics analysis revealed that high O2 levels disrupted antioxidant metabolism, leading to damage in photosystems and ribosomes in broccoli heads. The study provides insights into the mechanisms of yellowing acceleration in broccoli heads under elevated O2, potentially leading to new preservation technologies.