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

植物生命的开始以一个主要的阶段转变为特征：种子萌发和随后的幼苗建立完全是异养的，种子储备迅速转化为碳水化合物，以满足代谢需求并安装自养生活方式。因此，幼苗建立是一个非常适合研究发育过渡过程中代谢调节的模型系统。尽管控制种子代谢的酶途径已经确立，但它们的调控在很大程度上仍然是难以捉摸的。我们之前的工作揭示了进化上保守的调节激酶SnRK1 （Snf1相关蛋白激酶）在协调储存化合物动员，下胚轴伸长和光合系统建立方面的关键功能。重要的是，葡萄糖喂养在很大程度上恢复了生长缺陷和在激酶突变体中观察到的基因调控改变，支持SnRK1在资源动员中的主要功能。代谢物研究表明，蔗糖是早期幼苗建立的主要资源，但与snrk1无关。随后，SnRK1在有限数量的瓶颈基因的转录以及翻译后水平上协调三酰甘油（TAGs）的分解代谢。此外，SnRK1对种子贮藏蛋白动员和氨基酸（AA）代谢有重要影响。伴随转录组分析定义了AA代谢中大量依赖snrk1的基因调控。引人注目的是，SnRK1控制着两个基因的转录，这些基因编码促进糖异生的关键代谢中心酶：TAG分解代谢的PCK1 （PY-RUVATE CARBOXYKINASE1）和AA分解的cyPPDK（胞质丙酮酸正磷酸盐二激酶）。作为一个典型的例子，我们确定了转录因子bZIP63 (BASIC LEUCINE ZIPPER63)，它被SnRK1磷酸化，直接靶向并与SnRK1协同激活cyPPDK启动子。本研究旨在扩展储藏化合物动员控制的研究，以及表征SnRK1在幼苗发育中的功能。(a)使用筛选工具，我们将识别和表征控制脂质代谢中心瓶颈基因的转录调节因子，以促进糖异生（例如PCK1）。(b)此外，我们将应用磷酸化蛋白质组学方法，比较野生型和SnRK1突变型幼苗，以确定TAG分解代谢中的SnRK1靶酶和下胚轴伸长中的SnRK1依赖转录因子。(c)最后，我们提出在幼苗建立过程中阐明碳水化合物有效性与SnRK1激活之间的机制联系，重点研究低丰度信号化合物海藻糖6-磷酸（T6P）的调控功能。综上所述，该项目将为幼苗建立的代谢控制提供深入的见解。这些知识对于基础科学和未来的作物改良都非常重要。

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.