Analysis of xanthophylle cycle gene expression during the adaptation of Arabidopsis to excess light and drought stress: changes in RNA steady-state levels do not contribute to short-term responses

To protect the photosynthetic apparatus from excess light violaxanthin de-epoxidase (VDE) converts violaxanthin (V) to antheraxanthin then zeaxanthin (Z). the reverse reactions are catalysed by zeaxanthin epoxidase (ZEP) when light conditions become non-saturating. The control of VDE and ZEP genes in response to excess light conditions has been investigated in Arabidopsis thaliana (L.) Heynh. In leaves, both ZEP and VDE transcript levels exhibited diurnal variations even in non-saturating light, in both wild type and the mutants npq1-2 and abal-5 that accumulate V or Z. respectively. The biphasic pattern of VDE transcript oscillations indicated that VDE expression was controlled by light and a second unidentified factor. In agreement with their opposing enzymatic activities, the times of peak VDE expression were the converse of that for ZEP. Pigment analysis confirmed that VDE and ZEP activities also varied diurnally, but as activity peaks did not correlate with RNA and protein steady-state levels the short-term induction of the xanthophyll cycle in high light (HL) only seems to involve post-translational regulation. Nonetheless, lower levels of both VDE RNA and protein were observed on prolonged HL exposure, which correlated with the reduction in de-epoxidated xanthophylls observed on acclimation. The npq1-2 and abal-5 mutants exhibited wild type ZEP or VDE expression profiles, respectively, indicating the absence of feedback control on gene expression via pigment levels. Decreased ZEP transcript abundance in roots indicates feedback to long-distance signalling initiated in leaves exposed to excess light. Finally, expression changes observed when water stress was combined with excess light found that drought response was predominant.