Elucidating the impact of the central metabolic kinase SnRK1 and its downstream en-zymes and transcription factors in controlling the phase transition from heterotrophy to autotrophy during Arabidopsis seedling establishment

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The onset of plant life is characterised by a major phase transition: seed germination and sub-sequent seedling establishment are entirely heterotrophic and seed reserves are rapidly con-verted into carbohydrates to fuel metabolic demands and to install the autotrophic life style. Hence, seedling establishment is a well-suited model system to study the regulation of me-tabolism during developmental transitions. Whereas the enzymatic pathways controlling seed-ling metabolism are well-established, their regulation remains largely elusive. Our previous work unravelled a crucial function the evolutionarily conserved regulatory kinase SnRK1 (Snf1 RELATED PROTEIN KINASE) in orchestrating storage compound mobilization, hypo-cotyl elongation and establishment of the photosynthetic system. Importantly, glucose feeding largely restores growth defects and altered gene regulation observed in the kinase mutant, supporting a major SnRK1 function in resource mobilization. Metabolite studies revealed su-crose as a primary resource in early seedling establishment, however in a SnRK1-independent manner. Later on, SnRK1 orchestrates catabolism of triacylglycerols (TAGs) on transcription of a limit number of bottleneck genes as well as on post-translational level. Moreover, a major impact of SnRK1 was observed on seed storage protein mobilization and amino acid (AA) metabolism. Concomitant transcriptome profiling defines a massive SnRK1-dependent regulation of genes in AA metabolism. Strikingly, SnRK1 controls transcription of two genes encoding crucial metabolic hub enzymes fuelling gluconeogenesis: PCK1 (PY-RUVATE CARBOXYKINASE1) in TAG catabolism and cyPPDK (cytosolic PYRUVATE ORTHOPHOSPHATE DIKINASE) in breakdown of AA. As a prototypic example, we identi-fied the transcription factor bZIP63 (BASIC LEUCINE ZIPPER63), which is phosphorylated by SnRK1 and directly targets and activates the cyPPDK promoter in a synergistic manner with SnRK1. The current proposal aims at extending the work on control of storage com-pound mobilization, as well as characterizing SnRK1 function in seedling development. (a) Using screening tools, we will identify and characterize transcriptional regulators controlling central bottleneck genes in lipid metabolism to fuel gluconeogenesis (e.g. PCK1). (b) Moreo-ver, applying a phosphoproteomics approach, we will compare wild-type and SnRK1 mutant seedlings to define SnRK1 target enzymes in TAG catabolism and SnRK1 dependent tran-scription factors in hypocotyl elongation. (c) Finally, we propose to elucidate the mechanistic link between carbohydrate availability and SnRK1 activation during seedling establishment, focusing on the regulatory function of the low abundant signaling compound trehalose 6-phosphate (T6P). Taken together, this project will provide a deep insight in the metabolic con-trol of seedling establishment. This knowledge is of major importance, both for basic science and future crop improvement.