SnRK1 and the low energy signaling network

Abstract

Climate change, increasing population, and diversion of cereals to biofuels are amongst the major challenges that are increasing the pressure on agriculture. Extensive losses in agricultural production worldwide are due to stress factors that impact negatively on growth and crop productivity. Research is mainly focusing on strategies for target manipulation into developing crops with improved resistance to nutrient limitation and abiotic stresses . Stress and energy signaling pathways are particularly important, and in this context, given its central role at the interface between metabolic and stress signaling, the SnRK1 pathway has been extensively studied, and its great potential for potential manipulation has been highlighted . The studies on the Low Energy Syndrome (LES) underline the central role of this network composed of SnRK1, TOR, their targets, and several sugars and metabolites in the control of growth and development. Exposure of plants to conditions that challenge their energy homeostasis results in significant metabolic reprogramming to prevent damage to cells, tissues, and organs. However, the complexity inherent to energy signaling and the regulation of growth and development is vast, and much work is needed to fill in the current gaps in the knowledge available on these pathways. The SnRK1 pathway plays a central role at the interface between metabolic and stress signaling, thus detailed understanding of this pathway under normal and stress conditions is crucial. Our objective was to contribute to the current understanding of the SnRK1 pathway by identifying novel SnRK1 target genes and provide avenues for further research. We produced inducible over-expressor and artificial microRNA lines targeting the A. thaliana SnRK1 α subunits KIN10 and KIN11, and studied their transcriptional and metabolic response under light, dark, non-induced, and induced conditions. Our results suggest that the inducible variation of the expression of the SnRK1 catalytic subunit KIN10 does not induce major transcript or metabolic changes, contrary to the dark treatment, that leads to major transcript and metabolic changes. Additionally, we combined a robust co-regulation approach with a newly developed phenotypic assay to identify novel SnRK1 target genes. We identified three co-regulated genes with a phenotypic response to low energy conditions, that may have potential links to the SnRK1 pathway and warrant further investigation. One of the identified targets, SEX4, has known links to starch metabolism, and we therefore investigated the link between starch metabolism and the SnRK1. Our phenotypic data suggests that SEX4 is important for starch mobilization during starvation. We also showed that SEX4 wild-type protein is phosphorylated in vitro by KIN10 and KIN11. The SnRK1 pathway is in an excellent position for target discovery and crop improvement. There are still many open questions regarding the biological context and outcome of the LES network that require further investigation, in parallel with progress in the current experimental procedures. Fundamental discoveries, including phosphorylation of specific targets such as SEX4, and the use of inducible lines to validate molecular hypothesis have therefore great potential to contribute for increased knowledge and future development of improved crops.