Impact of vacuolar monosaccharide accumulation on cellular stress response of Arabidopsis. Function of AtTMT type carriers

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Sugar accumulation in vacuoles is of high importance for plant stress tolerance, especially in respect to cold or osmotic stress stimuli. We identified a first vacuolar sugar transporter (TMT, tonoplast monosaccharide transporter) and showed in the last funding period that TMT acts as a proton-coupled vacuolar sugar antiporter. Moreover, we were able to reveal that TMT activity is critical for intracellular sugar compartmentation, is involved in cellular sugar sensing and governs seed yield in Arabidopsis. In addition, a novel mode of TMT activity regulation controlled by a predicted mitogen-activated triple kinase (MAP3K, previously named VIK, a protein kinase able to bind a VH1/BRL2 receptor-like kinase functioning in vascular development) was identified. VIK belongs to a small subclade (6 members) of the large family of predicted MAP3K in Arabidopsis. Remarkably, VIK knock out lines closely phenocopy Arabidopsis mutants lacking vacuolar TMT activity, VIK phosphorylates the TMT1 loop protein, it interacts with the authentic TMT1 protein and stimulates glucose import into isolated mesophyll vacuoles. In the proposed funding period we will decipher the function of the other 5 members of the corresponding MAP3K subclade, we will identify upstream- and further down-stream partner proteins of VIK, we will search for further proteins interacting with TMT1, and we will study the properties of TMT1 mutants either lacking proposed phosphorylation sites, or mimicking constitutive phosphorylation. Moreover, it appears likely that the unique aspartate cluster located in the central TMT1 loop contributes to the subcellular targeting of TMT1. Thus, detailed analysis of the targeting function of the aspartate cluster is also part of our project. As in the past, all future mutants will be analysed in respect to molecular-, metabolic- and physiological changes induced by abiotic stress stimuli.