Analysis of glycogen synthase kinase/shaggy-like kinases. Novel aspects of salt stress signaling in plants

In many areas of the the world intensified irrigation and fertilization have resulted in high soil salinity becoming an increasingly deleterious obstacle to growth and yield of crop plants. Thus detailed knowledge of high salt signaling hierarchies and the impact of metabolic changes involved in stress responses is of great significance. While physiological studies have been performed extensively, our understanding of the mechanisms of osmotic stress signal transduction is still limited. We have recently found that MsK4, a novel glycogen synthase kinase/shaggy-like protein kinase from Medicago sativa, is involved in the rapid response to salinity. Intriguingly, we found that MsK4 protein kinase is plastid- localized and associates with starch grains. We have isolated the Arabidopsis homolog, AtK-1, that is rapidly activated by high salt stress, too. To unravel crucial processes in high salinity stress signaling and adaptation to high salt conditions we propose 1. to study whether AtK-1 is essential for accurate transmission of high salt stress signals, 2. to place AtK-1 action within the salt stress signaling network, 3. to study cross-talk with other signaling pathways 4. to determine the dynamics of AtK-1 localization under normal and high salt growth conditions and 5. to investigate the putative role of AtK-1 activity in carbon allocation and starch composition under diverse environmental conditions. We seek to address these tasks by a molecular-biochemical approach using Arabidopsis mutants that show enhanced, reduced or no AtK-1 activity as well as by protoplast transient expression analysis.

High soil salinity severely affects agricultural productivity. Salinity is detrimental to plant growth and development, and is a cause of osmotic stress, ion toxicity and nutritional defects. As sessile organisms, plants have evolved various strategies to cope with salinity. Integrated signal transduction systems mediate the perception of different environmental cues and delicately coordinate physiological responses. While progressive global changes call for a better understanding of adaptation processes, our knowledge of how signal transduction coordinates the regulation of metabolic adjustment to stress conditions is scarce. GSK-3/shaggy-like kinases (GSKs) have emerged as novel regulators in plant stress signal transduction. Within the framework of this project, we provided the first evidence of a starch-associated protein kinase that appears to link stress signaling to metabolic adaptation. Starch is the main carbohydrate store in plants, and is at the base of human and animal nutrition. Exposure of plants to environmental stress conditions leads to depletion of this important carbohydrate reserve. Adaptive responses that allow survival and growth under stress conditions include metabolic adjustment, in particular the maintenance of a carbon supply, a process that is still poorly understood at the molecular level. Remarkably, we identified MsK4, a novel GSK, as a plastid-localized protein kinase that is associated with starch granules. Even though starch metabolism is highly dynamic, has been well studied over many years and appears to be regulated by protein phosphorylation, our work on MsK4 is the first to show that a protein kinase localizes to starch grains and regulates starch levels. High salinity conditions induce in vivo MsK4 activity and plants with elevated levels of MsK4 show improved salt tolerance. A metabolomic approach revealed that plants with increased MsK4 activity have significantly more starch and increased carbohydrate metabolism during high salinity stress as compared to wild-type plants. This fact might contribute to the improved fitness under stress conditions and opens new perspectives on how metabolic carbon flux might be regulated in response to environmental stress.