The dark side of violaxanthin cycle

Abstract

Based on a fortuitous finding, this thesis challenged the dogma about the light-dependence of violaxanthin (V)-cycle activity. And eventually the dogma was weakened. As initially proved on a desiccation-tolerant fern and later on a wide range of photosynthetic macroorganisms, the dehydration of vegetative tissues induces the de-epoxidation of V in darkness. The reaction is catalysed by V de-epoxidase (VDE) and occurs even after several days in the dry state, when the water content is close to zero and the cytoplasm is in the glassy state. The resultant high content of zeaxanthin (Z) enhances thermal dissipation of energy and fitness of tissues during the following rehydration, acting as a pre-emptive response triggered during the desiccation. The conversion of V to Z in darkness is also induced under different abiotic stresses, such as anoxia or supraoptimal temperatures, suggesting it is a common response to most stressors that has been conserved tenaciously during the evolution to protect thylakoid membranes and photosynthetic machinery under hazardous conditions, even when these occur in the dark. The synthesised Z is reversibly converted to V after the cessation of stress confirming the complete operation of the cycle in the dark. This new understanding of V-cycle operation was completed with unprecedented results that suggest the inhibition of Z-epoxidase as the key factor on regulating the activity of V-cycle in darkness. The ¿new light¿ provided in this work on the dark-side of V-cycle, opens exciting unexplored gates among which one of the most promising aspects would be the enhancement of Z content in edible plants.