Ascorbate and glutathione in fruit ripening: Is cycling important for fruit health?

Fruit ripening is an oxidative process and many disorders can be managed through manipulation of one or more of the plant's own antioxidant defences. From the basic plant sciences, we have learnt that ascorbate and glutathione are central antioxidants, linked by a cycle of enzymes, and which comprise a complementary system to detoxify hydrogen peroxide. It has even been suggested that the signalling function of hydrogen peroxide is regulated by the ascorbate/glutathione cycle. While horticultural research has to date been fragmented in its exploration of ascorbate and glutathione, some groups have made considerable progress and their work makes an interesting comparison. In this review, the role and regulation of ascorbate and glutathione in the metabolism of reactive oxygen species (ROS) in fruit ripening is considered. A necessary preamble introduces the nature of ROS, as elucidated from vegetative tissues and considers how the production and metabolism of ROS in fruit ripening may differ. The ascorbate/glutathione pathway is illustrated and progress towards an understanding of its role and regulation in fruit ripening is detailed. While evidence is presented that is suggestive of a key role in the regulation of ROS and their consequences in fruit ripening, the need for further research is discussed. Evidence from a limited number of studies suggests conservation of processes across fruits with contrasting ripening modes. Thus the timeliest advances may be made with the use of so-called model fleshy fruits, such as tomato and grape, exploiting the context of better explored transcriptomes and identifying possible pleiotropic effects on fruit ripening, longevity and nutritional value. The insights and methods gained could then be applied through comparative approaches across a wide range of economically important fruits. The importance of using consistent methodology and terminology to enable such comparisons is highlighted.