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Background  Reactive oxygen species (ROS) are unavoidable by-products of oxygenic photosynthesis, causing progressive oxidative damage and ultimately cell death. Despite their destructive activity they are also signalling molecules, priming the acclimatory response to stress stimuli.    Results  To investigate this role further, we exposed wild type  Arabidopsis thaliana  plants and the double mutant  npq1lut2  to excess light. The mutant does not produce the xanthophylls lutein and zeaxanthin, whose key roles include ROS scavenging and prevention of ROS synthesis. Biochemical analysis revealed that singlet oxygen ( 1 O 2 ) accumulated to higher levels in the mutant while other ROS were unaffected, allowing to define the transcriptomic signature of the acclimatory response mediated by  1 O 2  which is enhanced by the lack of these xanthophylls species. The group of genes differentially regulated in  npq1lut2  is enriched in sequences encoding chloroplast proteins 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 biological system adopted to define the regulation of gene expression by  1 O 2 . In this work, we propose the use of mutants accumulating  1 O 2  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  1 O 2  accumulation and this represents a signalling pathway in the early stages of stress acclimation, beside the response to ADP/ATP ratio and to the redox state of both plastoquinone pool. Chloroplasts respond to  1 O 2  accumulation by undergoing a significant change in composition and function towards a fast acclimatory response. The physiological implications of this signalling specificity are discussed.

bmc plant biology

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