Function of the ascorbate peroxidase (APX) in fruits and their modulation by reactive species

Ascorbate peroxidase (APX) is one of the enzymes of the ascorbate-glutathione cycle and is the key enzyme that breaks down H2O2 with the aid of ascorbate as an electron source, although other enzymes also break down H2O2 such as catalase, peroxiredoxins, among others. APX is present in all photosynthetic eukaryotes from algae to higher plants and at the cellular level, it is localized in all subcellular compartments where H2O2 is generated, including apoplast, cytosol, plastids, mitochondria, and peroxisomes, either in soluble form or attached to the organelle membranes. The APX activity can be modulated by various post-translational modifications (PTMs) including tyrosine nitration, S-nitrosation, persulfidation, and S-sulfenylation among others. This allows the connection of the H2O2 metabolism with other relevant signaling molecules such as NO and H2S thus building a complex coordination system. In both climacteric and non-climacteric fruits, APX plays a key role during the ripening process as well as during postharvest, since it participates in the regulation of both H2O2 and ascorbate levels affecting fruit quality. Currently, the exogenous application of molecules such as NO, H2S, H2O2, and more recently melatonin has been seen as a new alternative to maintain and extend the shelf life and quality of the fruits because these molecules can modulate APX activity as well as other antioxidant systems. Therefore, these molecules are being considered new biotechnological tools to improve crop quality in the horticultural industry.