Reactive oxygen species and transcript analysis upon excess light treatment in wild-type Arabidopsis thaliana vs a photosensitive mutant lacking zeaxanthin and lutein

Background: Reactive oxygen species (ROS) are unavoidable by-products of oxygenic photosynthesis, causingprogressive oxidative damage and ultimately cell death. Despite their destructive activity they are also signallingmolecules, priming the acclimatory response to stress stimuli.Results: To investigate this role further, we exposed wild type Arabidopsis thaliana plants and the double mutantnpq1lut2 to excess light. The mutant does not produce the xanthophylls lutein and zeaxanthin, whose key rolesinclude ROS scavenging and prevention of ROS synthesis. Biochemical analysis revealed that singlet oxygen (1O2)accumulated to higher levels in the mutant while other ROS were unaffected, allowing to define the transcriptomicsignature of the acclimatory response mediated by 1O2 which is enhanced by the lack of these xanthophyllsspecies. The group of genes differentially regulated in npq1lut2 is enriched in sequences encoding chloroplastproteins involved in cell protection against the damaging effect of ROS. Among the early fine-tuned components,are proteins involved in tetrapyrrole biosynthesis, chlorophyll catabolism, protein import, folding and turnover,synthesis and membrane insertion of photosynthetic subunits. Up to now, the flu mutant was the only biologicalsystem adopted to define the regulation of gene expression by 1O2. In this work, we propose the use of mutantsaccumulating 1O2 by mechanisms different from those activated in flu to better identify ROS signalling.Conclusions: We propose that the lack of zeaxanthin and lutein leads to 1O2 accumulation and this represents asignalling pathway in the early stages of stress acclimation, beside the response to ADP/ATP ratio and to the redoxstate of both plastoquinone pool. Chloroplasts respond to 1O2 accumulation by undergoing a significant change incomposition and function towards a fast acclimatory response. The physiological implications of this signallingspecificity are discussed.