Main ConclusionDifferent abiotic stress conditions induce distinct sets of anthocyanins, indicating that anthocyanins have different biological functions, or that decoration patterns of each anthocyanin are used for unique purposes during stress.The induction of anthocyanin accumulation in vegetative tissues is often considered to be a response of plants to biotic or abiotic stress conditions. Arabidopsis thaliana (Arabidopsis) accumulates over 20 anthocyanins derived from the anthocyanidin cyanidin in an organ-specific manner during development, but the anthocyanin chemical diversity for their alleged stress protective functions remains unclear. We show here that, when grown in various abiotic stress conditions, Arabidopsis not only often accumulates significantly higher levels of total anthocyanins, but different stress conditions also favor the accumulation of different sets of anthocyanins. For example, the anthocyanin patterns of seedlings grown at pH 3.3 or in media lacking phosphate are very similar and characterized by relatively high levels of the anthocyanins A8 and A11. In contrast, anthocyanin inductive conditions (AIC) provided by high sucrose media are characterized by high accumulation of A9* and A5 relative to other stress conditions. The modifications present in each condition correlate reasonably well with the induction of the respective anthocyanin modification enzymes. Taken together, our results suggest that Arabidopsis anthocyanin profiles provide ‘fingerprints’ that reflect the stress status of the plants.Electronic supplementary materialThe online version of this article (doi:10.1007/s00425-014-2079-1) contains supplementary material, which is available to authorized users.

Anthocyanins are induced in plants in response to abiotic stresses such as drought, high salinity, excess light, and cold, where they often correlate with enhanced stress tolerance. Numerous roles have been proposed for anthocyanins induced during abiotic stresses including functioning as ROS scavengers, photoprotectants, and stress signals. We have recently found different profiles of anthocyanins in Arabidopsis (Arabidopsis thaliana) plants exposed to different abiotic stresses, suggesting that not all anthocyanins have the same function. Here, we discuss these findings in the context of other studies and show that anthocyanins induced in Arabidopsis in response to various abiotic stresses have different localizations at the organ and tissue levels. These studies provide a basis to clarify the role of particular anthocyanin species during abiotic stress.Anthocyanins are plant pigments of the flavonoid subclass of phenylpropanoids characterized by a 3,5,7-trihydroxylated flavylium backbone. 1 The red-to-purple color imparted by anthocyanins to flowers, fruits, and seeds act as visual deterrents to herbivores, and attractants to pollinators and seed dispersers. Anthocyanins and other flavonoids also contribute to stress tolerance in plants. There is a growing interest in understanding the mechanisms by which anthocyanins help plants cope with abiotic stress, most importantly in the context of crop yield reduction due to global climate change. Anthocyanins are commonly induced in plant vegetative tissues in response to a number of different abiotic stresses including drought, salinity, excess light, sub-or supra-optimal temperatures, and nitrogen and phosphorous deficiency. 2-8 The proposed roles of anthocyanins during abiotic stresses include quenching of ROS, 9,10 photoprotection, 11,12 stress signaling, 13,14 and xenohormesis (i.e., the biological principle that relates bioactive compounds in environmentally stressed plants and the increase in stress resistance and survival in animals that feed from them). 15,16 Plants as a group produce hundreds of structurally distinct anthocyanin species. Arabidopsis (Arabidopsis thaliana) alone produces more than 20 different types of anthocyanins, but whether they have specific functions is unknown. Whereas all anthocyanins could have identical roles, the high metabolic cost of adding numerous decorations (e.g. sugar and acyl groups) to the flavylium backbone in the different anthocyanin species makes this scenario very unlikely.We recently reported that distinct profiles of anthocyanins are induced in seedlings of Arabidopsis in response to different abiotic stresses. 4 We analyzed seedlings grown in 8 abiotic stress conditions, including high salinity, cold, and an artificial stress medium termed anthocyanin induction condition (AIC), which consists of 3% sucrose and no additional nutrients. The fact that distinct profiles of anthocyanins are induced by different abiotic stresses suggested that different anthocyanins, or profiles of anthocyanins, have different function...

Photosynthetic efficiency depends on equal light energy conversion by two spectrally distinct, serially-connected photosystems. The redox state of the plastoquinone pool, located between the two photosystems, is a key regulatory signal that initiates acclimatory changes in the relative abundance of photosystems. The Chloroplast Sensor Kinase (CSK) links the plastoquinone redox signal with photosystem gene expression but the mechanism by which it monitors the plastoquinone redox state is unclear. Here we show that the purified Arabidopsis and Phaeodactylum CSK and the cyanobacterial CSK homologue, Histidine kinase 2 (Hik2), are iron-sulfur proteins. The Fe-S cluster of CSK is further revealed to be a high potential redox-responsive [3Fe-4S] center. CSK responds to redox agents with reduced plastoquinone suppressing its autokinase activity. Redox changes within the CSK iron-sulfur cluster translate into conformational changes in the protein fold. These results provide key insights into redox signal perception and propagation by the CSK-based chloroplast two-component system.

Polycyclic aromatic hydrocarbons (PAHs) are environmental contaminants with cytotoxic, teratogenic and carcinogenic properties. Bioremediation studies with bacteria have led to the identification of dioxygenases (DOXs) in the first step to degrade these recalcitrant compounds. In this study, we characterized the role of the Arabidopsis thaliana AT5G05600, a putative DOX of the flavonol synthase family, in the transformation of PAHs. Phenotypic analysis of loss-of-function mutant lines showed that these plant lines were less sensitive to the toxic effects of phenanthrene, suggesting possible roles of this gene in PAH degradation in vivo. Interestingly, these mutant lines showed less accumulation of H2O2 after PAH exposure. Transgenic lines over-expressing At5g05600 showed a hypersensitive response and more oxidative stress after phenanthrene treatments. Moreover, fluorescence spectra results of biochemical assays with the recombinant His-tagged protein AT5G05600 detected chemical modifications of phenanthrene. Taken together, these results support the hypothesis that AT5G05600 is involved in the catabolism of PAHs and the accumulation of toxic intermediates during PAH biotransformation in plants. This research represents the first step in the design of transgenic plants with the potential to degrade PAHs, leading to the development of vigorous plant varieties that can reduce the levels of these pollutants in the environment.