The SnRK1 (SNF1-related kinase 1) kinases are the plant cellular fuel gauges, activated in response to energy-depleting stress conditions to maintain energy homeostasis while also gatekeeping important developmental transitions for optimal growth and survival. Similar to their opisthokont counterparts (animal AMP-activated kinase, AMPK, and yeast Sucrose Non-Fermenting 1, SNF), they function as heterotrimeric complexes with a catalytic (kinase) α subunit and regulatory β and γ subunits. Although the overall configuration of the kinase complexes is well conserved, plant-specific structural modifications (including a unique hybrid βγ subunit) and associated differences in regulation reflect evolutionary divergence in response to fundamentally different lifestyles. While AMP is the key metabolic signal activating AMPK in animals, the plant kinases appear to be allosterically inhibited by sugar-phosphates. Their function is further fine-tuned by differential subunit expression, localization, and diverse post-translational modifications. The SnRK1 kinases act by direct phosphorylation of key metabolic enzymes and regulatory proteins, extensive transcriptional regulation (e.g. through bZIP transcription factors), and down-regulation of TOR (target of rapamycin) kinase signaling. Significant progress has been made in recent years. New tools and more directed approaches will help answer important fundamental questions regarding their structure, regulation, and function, as well as explore their potential as targets for selection and modification for improved plant performance in a changing environment.

Energy homeostasis is vital to all living organisms. In eukaryotes, this process is controlled by fuel gauging protein kinases: AMP-activated kinase in mammals, Sucrose Non-Fermenting1 (SNF1) in yeast (Saccharomyces cerevisiae), and SNF1-related kinase1 (SnRK1) in plants. These kinases are highly conserved in structure and function and (according to this paradigm) operate as heterotrimeric complexes of catalytic-a and regulatory band g-subunits, responding to low cellular nucleotide charge. Here, we determined that the Arabidopsis (Arabidopsis thaliana) SnRK1 catalytic a-subunit has regulatory subunitindependent activity, which is consistent with default activation (and thus controlled repression), a strategy more generally used by plants. Low energy stress (caused by darkness, inhibited photosynthesis, or hypoxia) also triggers SnRK1a nuclear translocation, thereby controlling induced but not repressed target gene expression to replenish cellular energy for plant survival. The myristoylated and membrane-associated regulatory b-subunits restrict nuclear localization and inhibit target gene induction. Transgenic plants with forced SnRK1a-subunit localization consistently were affected in metabolic stress responses, but their analysis also revealed key roles for nuclear SnRK1 in leaf and root growth and development. Our findings suggest that plants have modified the ancient, highly conserved eukaryotic energy sensor to better fit their unique lifestyle and to more effectively cope with changing environmental conditions.

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Here, we used a hxk1 mutant in the Col-0 background. We demonstrated that HXK1 regulates cell proliferation and expansion early during leaf development, and that HXK1 is involved in sucrose-induced leaf growth stimulation independent of GPT2. Furthermore, we identified KINγ as a novel HXK1-interacting protein possibly regulating HXK1 function in plant growth.

SummaryPlants have evolved an extensive specialized secondary metabolism. The colorful flavonoid anthocyanins, for example, not only stimulate flower pollination and seed dispersal, but also protect different tissues against high light, UV and oxidative stress. Their biosynthesis is highly regulated by environmental and developmental cues and induced by high sucrose levels. Expression of the biosynthetic enzymes involved is controlled by a transcriptional MBW complex, comprising (R2R3) MYB‐ and bHLH‐type transcription factors and the WD40 repeat protein TTG1. Anthocyanin biosynthesis is not only useful, but also carbon‐ and energy‐intensive and non‐vital. Consistently, the SnRK1 protein kinase, a metabolic sensor activated in carbon‐ and energy‐depleting stress conditions, represses anthocyanin biosynthesis. Here we show that Arabidopsis SnRK1 represses MBW complex activity both at the transcriptional and post‐translational level. In addition to repressing expression of the key transcription factor MYB75/PAP1, SnRK1 activity triggers MBW complex dissociation, associated with loss of target promoter binding, MYB75 protein degradation and nuclear export of TTG1. We also provide evidence for direct interaction with and phosphorylation of multiple MBW complex proteins. These results indicate that repression of expensive anthocyanin biosynthesis is an important strategy to save energy and redirect carbon flow to more essential processes for survival in metabolic stress conditions.

Plants have evolved an extensive specialized secondary metabolism. The colorful flavonoid anthocyanins, for example, not only stimulate flower pollination and seed dispersal but also protect different tissues against high light, UV- and oxidative stress. Their biosynthesis is highly regulated by environmental and developmental cues and induced by high sucrose levels. Expression of the biosynthetic enzymes involved is controlled by a transcriptional MBW complex, comprising (R2R3) MYB- and bHLH-type transcription factors (TF) and the WD40 repeat protein TTG1. Anthocyanin biosynthesis is obviously useful but also carbon- and energy-intensive and non-vital. Consistently, the SnRK1 protein kinase, a metabolic sensor activated in carbon- and energy-depleting stress conditions, represses anthocyanin biosynthesis. Here we show that Arabidopsis SnRK1 represses MBW complex activity both at the transcriptional and post-translational level. In addition to repressing expression of the key transcription factor MYB75/PAP1, SnRK1 activity triggers MBW complex dissociation, associated with loss of target promoter binding, MYB75 protein degradation and nuclear export of TTG1. We also provide evidence for direct interaction with and phosphorylation of multiple MBW complex proteins. These results indicate that repression of expensive anthocyanin biosynthesis is an important strategy to save energy and redirect carbon flow to more essential processes for survival in metabolic stress conditions.