Abstract
The xanthophyll cycle (Xc), which involves violaxanthin de-epoxidase (VDE) and the zeaxanthin epoxidase (ZEP), is one of the most rapid and efficient responses of plant and algae to high irradiance. High light intensity can activate VDE to convert violaxanthin (Vx) to zeaxanthin (Zx) via antheraxanthin (Ax). However, it remains unclear whether VDE remains active under low light or dark conditions when there is no significant accumulation of Ax and Zx, and if so, how the ΔpH required for activation of VDE is built. In this study, we used salicylaldoxime (SA) to inhibit ZEP activity in the intertidal macro-algae Ulva sp. (Ulvales, Chlorophyta) and then characterized VDE under low light and dark conditions with various metabolic inhibitors. With inhibition of ZEP by SA, VDE remained active under low light and dark conditions, as indicated by large accumulations of Ax and Zx at the expense of Vx. When PSII-mediated linear electron transport systems were completely inhibited by SA and DCMU, alternative electron transport systems (i.e., cyclic electron transport and chlororespiration) could maintain VDE activity. Furthermore, accumulations of Ax and Zx decreased significantly when SA, DCMU, or DBMIB together with an inhibitor of chlororespiration (i.e., propyl gallate (PG)) were applied to Ulva sp. This result suggests that chlororespiration not only participates in the build-up of the necessary ΔpH, but that it also possibly influences VDE activity indirectly by diminishing the oxygen level in the chloroplast.
Highlights
Light is essential for photosynthesis, yet it may be potentially harmful to plants
It is known that high light intensity can activate Violaxanthin de-epoxidase (VDE) to convert Vx to Zx via Ax, it is unclear whether VDE remains active under low light or dark conditions when there is no significant accumulation of Ax and Zx, and if so, how the necessary DpH across the thylakoid membrane for the activity of VDE is built
Ulva sp. was used to investigate the operation of the xanthophyll cycle (Xc) under low light intensity in vivo in the presence of salicylaldoxime (SA), which inhibits the catalytic ability of Zx epoxidase (ZEP). We propose that both VDE and ZEP are permanently operating in Ulva sp. under low light and dark conditions, and, in the latter case, the required DpH can be built by chlororespiration-mediated electron transport
Summary
Light is essential for photosynthesis, yet it may be potentially harmful to plants. To prevent excess absorption of light energy and consequent oxidative damage to the photosynthetic apparatus, higher plants and most algae evolved various photoprotection mechanisms. The enzyme VDE has been purified in higher plants and comprehensively investigated in vitro [6,7], some questions still remain. ZEP mutants have been obtained in some higher plants [8,9,10] and green algae [11], these mutants are not very suitable for studying VDE in vivo because of deficiency of Vx in LHCII [2]. Inhibiting ZEP using inhibitors in wild type organisms may be an alternative method for characterizing VDE activity in vivo. It is known that high light intensity can activate VDE to convert Vx to Zx via Ax, it is unclear whether VDE remains active under low light or dark conditions when there is no significant accumulation of Ax and Zx, and if so, how the necessary DpH across the thylakoid membrane for the activity of VDE is built
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