The role of calcium signaling in plants: from Arabidopsis to barley

Abstract

Ca²⁺ is an important intracellular secondary messenger involved in many signal transduction pathways in plants. During abiotic and biotic stresses, specific Ca²⁺ signatures are formed within the plant cell causing a spike in the Ca²⁺ concentration that have a particular frequency, shape, and duration. Studying these Ca²⁺ signatures in detail will give a closer look into the plant's defense mechanisms caused by different gene regulations. Although, a lot of research on cytosolic and organellar Ca²⁺ transients has been performed in <em>Arabidopsis thaliana</em> in response to various stimuli, very little is known about the Ca²⁺ transients in crops such as <em>Hordeum vulgare</em> (barley). Therefore, the main aim of this project was to study the cytosolic Ca²⁺ transients caused in barley in response to oxidative and drought stress. For the first time, in this study, it was possible to target apoaequorin into the cytosol of barley to monitor the Ca²⁺ transients. The results showed stimulus specific Ca²⁺ signatures in barley and these signatures were compared with Arabidopsis. It was also revealed that these Ca²⁺ transients are tissue specific and dependent on the age of the barley seedling as well. By using inhibitors of the Ca²⁺ transients, it could be seen that the Ca²⁺ transients were caused by the influx of Ca²⁺ across the plasma membrane and from the Ca²⁺ release from internal stores like the endoplasmic reticulum. Furthermore, to investigate the gene expression involved in causing the Ca²⁺ transients in response to oxidative stress, RNA-sequencing was done using the leaf and root samples in the presence and absence of the Ca²⁺ channel inhibitor, lanthanum chloride. The results showed that there is a clear differentiation between stress responses that require a Ca²⁺ signal and those that are Ca²⁺ independent. <br> Ca²⁺ sensors, such as calmodulin (CaM) and CaM-like proteins (CML) transduce Ca²⁺ signals into a cellular response which are activated by different target proteins. In the second part of this work, a previously identified CaM/Ca²⁺ binding target protein called the Rieske iron-sulfur protein (RISP) was analyzed for its functionality and topology. To that end, the Ca²⁺-dependent CaM binding property of RISP was confirmed and the localization of the N- and C- terminus of the protein was found to be in the mitochondrial matrix indicating that the CaM/Ca²⁺ regulation occurs within the matrix. However, there still exists a possibility that the localization of the N-terminus of RISP is not exclusively fixed to the matrix but also to the inner membrane space.