Protein phosphorylation and calcium signals in chloroplasts

Chloroplasts are essential plant organelles of endosymbiotic origin that carry out a wide range of important metabolic pathways such as photosynthesis, the biosynthesis of amino acids, vitamins, or lipids. These processes are ultimately linked to plant productivity and a deeper understanding of those mechanisms is essential to be able to improve future yields and performance of crops. For a fast regulation in response to changing environmental conditions, chloroplasts are integrated into the signalling network of the entire cell. In addition to the predominant redox regulation in chloroplasts, reversible protein phosphorylation is a key mechanism for the regulation of cellular processes and for signal transduction in response to environmental changes. This project aims at the functional characterization of two newly identified regulatory proteins in chloroplasts: One protein kinase, which is localized inside of chloroplasts, and one calcium-binding protein, which is localized at the outer chloroplast membrane. Furthermore we will investigate the impact of calcium-dependent phosphorylation on the chloroplast protease Var1, which plays an important role in regulation of turnover of a photosynthetic reaction center protein. We will perform a functional characterization of knockout and overexpressor plant lines and screen for targets of those proteins in order to understand fundamental regulatory principles in chloroplast metabolism and photosynthesis.

Plants are able to build complex organic molecules using CO2 from the air and simple nutrients from the soil. This occurs via a process called photosynthesis in the chloroplast of the leaves. Here, the light energy is absorbed by chlorophyll molecules and then used to build organic material, while oxygen is released as a by-product. Because the environmental conditions are frequently changing, the plant needs to adjust its light-harvesting or water use efficiency to these fluctuating environmental conditions. To this end, plants employ molecular switches, so-called protein kinases as well as calcium signals. These two mechanisms of signal transduction can even be linked, for example when calcium signal lead to activation of protein kinases. These signals are generally well understood today, in animals as well as in plant systems. However, particularly in chloroplasts much less is known about these processes, which is remarkable as they provide the basis of plant productivity, and in the broader sense, also the basis for all life on earth. In this project, three components, which are involved in chloroplast signalling in response to environmental changes, were investigated: One protein kinase, one calcium-binding protein, which is localized within the chloroplast, and one calcium-binding protein, which is localized at the outer chloroplast membrane, the so-called envelope membrane. The characterization of the two calcium-binding proteins revealed that calcium does indeed play a major role in the development and stress response of chloroplasts. Plants, which had either too much or none of those calcium-binding proteins, exhibited slow growth and yellowish coloured leaves, which indicates problems with photosynthesis. In fact the photosynthetic performance of those mutant plants was reduced and also the phosphorylation patterns of thylakoid proteins were dramatically reduced, thus showing the strong relation between calcium and protein phosphorylation in chloroplasts. Plants with altered levels of the calcium-binding protein in the chloroplast envelope were more susceptible to herbivore attack. Preliminary data indicate that the regulation of plant defense responses against herbivore attack is altered in these mutants, where chloroplasts play also an important role. Plant defense against pathogens and herbivores includes different metabolic pathways, which are localized at different places in the cell. It seems that the calcium-binding membrane protein, we studied here, plays an important role in coordinating the different metabolic activities between the different parts of the plant cell, particularly those of the chloroplast with those of the cytosol and other organelles.