Revisiting the role of ascorbate oxidase in plant systems.

Ascorbic acid (AsA) plays an indispensable role in plants, serving as both an antioxidant and a master regulator of the cellular redox balance. Ascorbate oxidase (AO) is a blue copper oxidase, responsible for the oxidation of AsA with the simultaneous production of water. For many decades, AO was erroneously postulated as an enzyme without any obvious advantage, as it decreases the AsA pool size and thus it is expected to weaken plant stress resistance. It was only a decade ago that this perspective has shifted towards the fundamental role of AO in orchestrating both AsA and oxygen levels, by influencing the overall redox balance in the extracellular matrix. Consistent with its localization in the apoplast, AO is involved in cell expansion, division, resource allocation, and overall plant yield. The increasing number of transgenic studies demonstrated that AO can also facilitate communication between the surrounding environment and the cell, as its gene expression is highly responsive to factors such as hormonal signaling, oxidative stress and mechanical injuries. This review aims to dip into the multiple functions of AO in plant growth, development, stress resilience, and explore any additional roles the enzyme might have in fruits through the course of ripening.