Wolfgang Dröge-Laser

Plants exhibit considerable phenotypic plasticity, allowing them to adapt their metabolism to the fluctuating energy supply in a natural environment. Using dark-induced senescence (DIS) as an experimental system, a mutant study combining phenotypic, transcriptomic and chromatin immunoprecipitation sequencing approaches identified distinct members of the Arabidopsis thaliana group S1 basic leucine zipper (bZIP) transcription factors that orchestrate the starvation response. Whereas excluding bZIP2, bZIP11, and bZIP44 to play a major role in DIS, bZIP1, and bZIP53 act partially redundantly to control a coexpression network governing amino acid catabolism and transport, gluconeogenesis and energy homeostasis. Moreover, bZIP1 and bZIP53 regulate genes involved in the asparagine–glutamine balance, two amino acids critical for carbon and nitrogen homeostasis. This transcriptional reprogramming in resource management is required for survival during starvation and regaining meristematic activity during the recovery from stress. Thus, our findings provide insights into the transcriptional control of plant resource and energy management during starvation. Overall, this work sheds light on the discrepancy between in vitro DNA binding and overexpression studies versus mutant analyses and in vivo DNA binding, providing a critical view on how to define specific transcription factor functions within large families.

Summary: Seed dormancy governs germination timing, with both evolutionary and applied consequences. Despite extensive studies on the hormonal and genetic control of these processes, molecular mechanisms directly linking dormancy and germination remain poorly understood. By screening a collection of lines overexpressing Arabidopsis transcription factors, we identified ERF50 as a key gene to control dormancy and germination. To study its regulation, we measured seed‐related physiological parameters in loss‐of‐function mutants and carried out transactivation, protein interaction and ChIP‐PCR analyses. We found direct ERF50‐mediated repression of DOG1 and activation of EXPA2 transcription, which results in enhanced seed germination. Although ERF50 expression is increased by DOG1 in dormant seeds, ERF50 germination‐promoting activity is blocked by RGL2. The physiological, genetic and molecular evidence gathered here supports that ERF50 controls germination timing by regulating DOG1 levels to leverage its role as enhancer of seed germination, via RGL2 antagonism on EXPA2 expression. Our results highlight the central role of ERF50 as a feedback regulator to couple and fine‐tune seed dormancy and germination.

In plants, sucrose nonfermenting 1 (SNF1)-related protein kinase 1 (SnRK1) is a key energy sensor that orchestrates large-scale transcriptional reprograming to maintain cellular homeostasis under energy deficit. SnRK1 activity is under tight negative control, although the exact mechanisms leading to its activation are not well understood. We show that the Arabidopsis (Arabidopsis thaliana) DOMAIN OF UNKNOWN FUNCTION (DUF581) protein DUF581-9/FCS-like zinc finger 3 binds to the catalytic SnRK1.1 α subunit (KIN10) to inhibit its activation by geminivirus rep-interacting kinase (GRIK)–dependent T-loop phosphorylation. Overexpression of DUF581-9 in Arabidopsis dampens SnRK1 signaling and interferes with adaptation to dark-induced starvation. The presence of DUF581-9 significantly reduced SnRK1 activity in protoplasts and in vitro. This was accompanied by a reduction in T175 T-loop phosphorylation and also diminished KIN10 auto-phosphorylation. Furthermore, DUF581-9 reduced binding of the upstream activating kinase GRIK2 to KIN10, explaining the reduced KIN10 T-loop phosphorylation. Ectopically expressed DUF581-9 protein was rapidly turned over by the proteasome when Arabidopsis plants were subjected to starvation treatment, likely releasing its inhibitory activity on the SnRK1 complex. Taken together, our results support a model in which DUF581-9 negatively regulates SnRK1 activity under energy sufficient conditions. Turnover of the protein provides a rapid way for SnRK1 activation under energy deficit without the need of de novo protein synthesis.

The ontogenetic regulation of shoot branching allows plants to adjust their architecture in accordance with the environment. This process is due to the regulation of axillary bud outgrowth into branches, which can be induced by increasing sugar availability to the buds through decapitation of the shoot tip. Different sugar signalling components have been identified in the induction of shoot branching. However, the molecular components that maintain bud dormancy in response to sugar starvation remain largely unknown. Here, we show at the genetic level that basic leucine zipper 11 (bZIP11), a transcription factor that plays important roles in response to sugar starvation in plants, inhibits shoot branching in Arabidopsis thaliana . Physiology experiments demonstrated that bZIP11 protein levels are decreased by decapitation. Molecular and genetic evidence suggests that bZIP11 acts in a negative feedback loop with trehalose 6-phosphate (Tre6P), a sugar signal that promotes shoot branching. Our data also suggest that the central energy sensor SUCROSE NON-FERMENTING 1 RELATED KINASE1 (SnRK1), alleviates the inhibitory effect of Tre6P on bZIP11 protein accumulation and inhibits shoot branching. Altogether, these data provide a working model that involves bZIP11, Tre6P and SnRK1 in the regulation of shoot branching.

Summary: In plants, SUCROSE NON-FERMENTING RELATED KINASE 1 (SnRK1) is a key energy-sensor that orchestrates large-scale transcriptional reprogramming to maintain cellular homeostasis under energy deficit. SnRK1 activity is under tight negative control, although the exact mechanisms leading to its activation are far from being understood. We show that the Arabidopsis DOMAIN OF UNKNOWN FUNCTION (DUF581) protein DUF581-9/FLZ3 binds to the catalytic SnRK1α subunit KIN10 to inhibit its activation by GRIK-dependent T-loop phosphorylation. Overexpression of DUF581-9 in Arabidopsis dampens SnRK1 signalling and interferes with adaptation to dark-induced starvation. The presence of DUF581-9 significantly reduced SnRK1 activity in protoplasts and in vitro . This was accompanied by a reduction in T175 T-loop phosphorylation and also diminished KIN10 auto-phosphorylation. Furthermore, DUF581-9 reduced binding of the up-stream activating kinase GRIK2 to KIN10, explaining the reduced KIN10 T-loop phosphorylation. Ectopically expressed DUF581-9 protein was rapidly turned over by the proteasome when Arabidopsis plants were subjected to starvation treatment, likely releasing its inhibitory activity on the SnRK1 complex. Taken together, our results support a model in which DUF581-9 negatively regulates SnRK1 activity under energy sufficient conditions. Turnover over of the protein provides a rapid way for SnRK1 activation under energy deficit without the need of de-novo protein synthesis.

The onset of plant life is characterized by a major phase transition. During early heterotrophic seedling establishment, seed storage reserves fuel metabolic demands, allowing the plant to switch to autotrophic metabolism. Although metabolic pathways leading to storage compound mobilization are well-described, the regulatory circuits remain largely unresolved. Using an inducible knockdown approach of the evolutionarily conserved energy master regulator Snf1-RELATED-PROTEIN-KINASE1 (SnRK1), phenotypic studies reveal its crucial function in Arabidopsis thaliana seedling establishment. Importantly, glucose feeding largely restores growth defects of the kinase mutant, supporting its major impact in resource mobilization. Detailed metabolite studies reveal sucrose as a primary resource early in seedling establishment, in a SnRK1-independent manner. Later, SnRK1 orchestrates catabolism of triacylglycerols and amino acids. Concurrent transcriptomic studies highlight SnRK1 functions in controlling metabolic hubs fuelling gluconeogenesis, as exemplified by cytosolic PYRUVATE ORTHOPHOSPHATE DIKINASE (cyPPDK). Here, SnRK1 establishes its function via phosphorylation of the transcription factor BASIC LEUCINE ZIPPER63 (bZIP63), which directly targets and activates the cyPPDK promoter. Taken together, our results disclose developmental and catabolic functions of SnRK1 in seed storage mobilization and describe a prototypic gene regulatory mechanism. As seedling establishment is important for plant vigor and crop yield, our findings are of agronomical importance.

Plants adjust their energy metabolism to continuous environmental fluctuations, resulting in a tremendous plasticity in their architecture. The regulatory circuits involved, however, remain largely unresolved. In Arabidopsis, moderate perturbations in photosynthetic activity, administered by short-term low light exposure or unexpected darkness, lead to increased lateral root (LR) initiation. Consistent with expression of low-energy markers, these treatments alter energy homeostasis and reduce sugar availability in roots. Here, we demonstrate that the LR response requires the metabolic stress sensor kinase Snf1-RELATED-KINASE1 (SnRK1), which phosphorylates the transcription factor BASIC LEUCINE ZIPPER63 (bZIP63) that directly binds and activates the promoter of AUXIN RESPONSE FACTOR19 (ARF19), a key regulator of LR initiation. Consistently, starvation-induced ARF19 transcription is impaired in bzip63 mutants. This study highlights a positive developmental function of SnRK1. During energy limitation, LRs are initiated and primed for outgrowth upon recovery. Hence, this study provides mechanistic insights into how energy shapes the agronomically important root system. Significance Plant architecture is highly plastic and known to respond sensitively to nutritional changes. Although of great agronomic importance, the underlying molecular mechanisms that sense and transduce these cues into plant development and growth are poorly understood. Applying diverse genetic, biochemical, and microscopic approaches, we disclosed that signaling via the central, evolutionarily conserved fuel-sensor kinase Snf1-RELATED KINASE1 (SnRK1) initiates lateral root (LR) primordia formation in response to transient metabolic perturbations. This is accomplished by SnRK1-mediated activation of a signaling cascade involving the pivotal LR regulator AUXIN RESPONSE FACTOR19 (ARF19). We propose that this developmental priming strategy represents a cost-efficient approach to ensure rapid growth recovery after stress release, providing in competitive ecosystems a clear advantage in terms of Darwinian fitness.

Verticillium longisporum is a vascular fungal pathogen leading to severe crop loss, particular in oilseed rape. Transcription factors (TF) are highly suited for genetic engineering of pathogen-resistant crops, as they control sets of functionally associated genes. Applying the AtTORF-Ex (Arabidopsis thaliana transcription factor open reading frame expression) collection, a simple and robust screen of TF-overexpressing plants was established displaying reduced fungal colonization. Distinct members of the large ethylene response factor (ERF) family, namely ERF96 and the six highly related subgroup IXb members ERF102 to ERF107, were identified. Whereas overexpression of these ERF significantly reduces fungal propagation, single loss-of-function approaches did not reveal altered susceptibility. Hence, this gain-of-function approach is particularly suited to identify redundant family members. Expression analyses disclosed distinct ERF gene activation patterns in roots and leaves, suggesting functional differences. Transcriptome studies performed on chemically induced ERF106 expression revealed an enrichment of genes involved in the biosynthesis of antimicrobial indole glucosinolates (IG), such as CYP81F2 (CYTOCHROME P450-MONOOXYGENASE 81F2), which is directly regulated by IXb-ERF via two GCC-like cis-elements. The impact of IG in restricting fungal propagation was further supported as the cyp81f2 mutant displayed significantly enhanced susceptibility. Taken together, this proof-of-concept approach provides a novel strategy to identify candidate TF that are valuable genetic resources for engineering or breeding pathogen-resistant crop plants.