Postharvest senescence of green vegetables and the role of glutathione metabolism: current state of knowledge and future research opportunities.

The rapid progression of senescence contributes to the postharvest perishability of green vegetables including Brassicaceae commodities. Tissue yellowing due to chlorophyll degradation is a hallmark feature of senescing green vegetables and is associated with the accumulation of reactive oxygen species. Moreover, oxidative stress metabolism in vegetables is exacerbated by postharvest temperature abuse and the senescence-related phytohormone ethylene. Glutathione is a primary antioxidant that detoxifies reactive oxygen species in plants. Glutathione status in postharvest vegetables is partially dependent upon the degree to which it is oxidized to glutathione disulfide, as well as whether glutathione is recycled from glutathione disulfide via the ascorbate-glutathione recycling pathway. Apart from reactive oxygen species management, the concentration of glutathione within Brassicaceae commodities is a consequence of its participation in non-antioxidant metabolism such as glucosinolate biosynthesis and protein glutathionylation. Although not well described for postharvest vegetables, glutathione status in postharvest vegetables is postulated to be impacted by catabolism to its constituent amino acids. In the model Brassicaceae plant Arabidopsis thaliana, glutathione is catabolized to the metabolic intermediates cysteinylglycine and 5-oxoproline, and finally to cysteine, glycine and glutamate. Improved knowledge of these metabolic events in commercially relevant Brassicaceae commodities could provide novel biochemical targets for offsetting postharvest senescence of their edible vegetables. In addition, we discuss the current state of knowledge on the application of exogenous glutathione prior to postharvest storage as a strategy to lessen the negative impact of oxidative stress mediated deterioration of highly perishable vegetables.