Photosynthetic signalling during high light stress and recovery: targets and dynamics.

The photosynthetic apparatus is one of the major primary sensors of the plant's external environment. Changes in environmental conditions affect the balance between harvested light energy and the capacity to deal with excited electrons in the stroma, which alters the redox homeostasis of the photosynthetic electron transport chain. Disturbances to redox balance activate photosynthetic regulation mechanisms and trigger signalling cascades that can modify the transcription of nuclear genes. H2O2 and oxylipins have been identified as especially prominent regulators of gene expression in response to excess light stress. This paper explores the hypothesis that photosynthetic imbalance triggers specific signals that target discrete gene profiles and biological processes. Analysis of the major retrograde signalling pathways engaged during high light stress and recovery demonstrates both specificity and overlap in gene targets. This work reveals distinct, time-resolved profiles of gene expression that suggest a regulatory interaction between rapidly activated abiotic stress response and induction of secondary metabolism and detoxification processes during recovery. The findings of this study show that photosynthetic electron transport provides a finely tuned sensor for detecting and responding to the environment through chloroplast retrograde signalling. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.