Calcium dependent protein kinases in Arabidopsis thaliana

Plants as sessile organisms have to respond to various external stimuli such as different forms of stress, for instance pathogens attacks, drought and salt stress, or different light intensities. In order to adapt their metabolism to these changes in growth conditions, plants have developed a number of biochemical adaptation strategies, which depend on the recognition of such extracellular changes (stimulus) and subsequent translation into a cellular response. Many extracellular signals elicit changes in the cellular Ca2+ concentrations in plants. Calcium is a ubiquitous second messenger in eukaryotic signal transduction and is the most dominant second messenger in plants. Changes in the cellular calcium concentration lead to the activation of a signal transduction cascade, which is mediated by protein kinases such as the calcium dependent protein kinases (CDPKs). The small plant in the mustard family Arabidopsis thaliana is used as a model organism to study such processes with biochemical and genetic approaches. Sequencing of the Arabidopsis thaliana genome revealed the presence of 34 different (CDPKs). A number of these kinases have a distinct localization within the cell. We investigate the role of a chloroplast associated protein kinase in plants in response to extracellular stimuli using both biochemical and gentic approaches. The understanding of the mechanisms by which plants percive signals and transmit these signals to the cellular machinery to activate adaptive responses is of fundamental importance to biology.

This project aimed at the elucidation of molecular signalling events in plants responses to external stimuli such as changing growth conditions or stresses, which involve calcium signals. Those signals are generated by sudden changes in the free calcium concentration in the cell and they activate calcium dependent protein kinases (CDPKs). These protein kinases can function as molecular switches of cellular response reactions by activation or deactivation of certain processes. In this project one particular kinase was characterized at the molecular level. In order to understand how these kinases work it is first necessary to know where they are localized in the plant, and second which molecular targets they regulate. We found that CDPK3 is preferentially bound to cell membranes in Arabidopsis and modifies a transcription factor by phosphorylation. The important role of this kinase in plants stress response is illustrated by the phenotype of knockout or overexpressor plants. If this CDPK is not functional any more (knocked out), the plant is not able to adapt to higher salt concentrations, whereas the overexpressor line, where this CDPK is more active as compared to normal (wild type) plants, is much more robust in response to these stress. This information could be used in the future in plant breeding programmes for generation of more tolerant plants.