Photosynthetic imbalances result in stronger generation of reactive oxygen species (ROS). The chloroplast antioxidant system controls the ROS-levels at the main site of ROS-generation. We hypothesise that it serves as a priming hub in plant stress management and controls activation of extra-plastidic signalling cascades.
In phase I, we studied first regulation of the chloroplast antioxidant system during and after a 2 week cold period in 10 Arabidopsis thaliana accessions. Afterwards, we investigated the effect of 24 h and 2 week cold priming (short-term and long-term priming) on the response to triggering by cold or moderately elevated light intensity. Cold shifted the de novo support of the antioxidant capacity from thylakoids to the chloroplast stroma and decreased expression of the main plastid superoxide dismutase. In response to long-term priming, the intensity of disregulation of the chloroplast antioxidant system negatively correlated with the capability to acquire freezing tolerance. In the post-stress phase, the O2-/H2O2 ratio increased. In response to the triggering stimulus, short- and long-term cold priming resulted in weaker induction of markers for extra-plastidic ROS signalling and differential regulation of genes for chloroplast ascorbate peroxidases. The effects of cold triggering were stronger after cold priming than of excess light triggering. After short-term priming, which does not induce freezing tolerance, the priming effects on the chloroplast antioxidant system were stronger and that on extra-plastidic ROS-signalling less in accessions originating from cold habitats with shorter vegetation periods than in Col-0. We conclude that priming of the ROS-control is a strategy preferentially used to cope with repetitive short stresses which are too weak to activate persisting acclimation responses. As benefits of priming, faster activation of photochemical quenching, less energy loss in non-photochemical quenching, positive biomass effects and higher seed numbers were observed.
In phase II we will concentrate on the mechanisms of priming and analysis of the stability of the memory. Relative protection of the stroma and the thylakoids and O2- detoxification will be modified in transgenic lines by induced expression and suppression of the key enzymes and their regulators. Using the information of phase I on natural differences in the primability of Arabidopsis accessions, we will screen for novel regulators of priming and identify them by DNA-Seq based mapping.