Default activation and nuclear translocation of the plant cellular energy sensor SnRK1 regulate metabolic stress responses and development.

Energy homeostasisis vital to all living organisms and in eukaryotic cells is controlled by fuel gaugingprotein kinases: AMP-activated kinase (AMPK) in mammals, Sucrose Non-Fermenting1 (SNF1) in yeast, and SnRK1 (SNF1-related kinase1) in plants. These kinases are conserved in structure and function and according to the current paradigm operate as heterotrimeric complexes of catalytic α and regulatory β and γ subunits, responding to low cellular nucleotide charge. Our analyses indicate that the plant SnRK1 catalytic α subunit has regulatory subunit-independent activity, consistent with default activation (and active repression), a strategy that is more generally used by plants. Low energy stress (darkness, inhibition of photosynthesis, hypoxia) also triggers SnRK1α nuclear translocation, controlling induced but not repressed target gene expression to replenish cellular energy crucial for plant survival. The myristoylatedand membrane-associated regulatory β subunits restrict nuclear localization and inhibit target gene induction. Transgenic plants with forced SnRK1α subunit localization are affected in metabolic stress responses, but also reveal novel key functions for nuclear SnRK1 in leaf and root growth and development. Our findings indicate that plants have modified the ancient and highly conservedeukaryotic energysensor to better fit their uniue lifestyle and more effectively cope with changing environmental conditions.