Oxygen sensing coordinates photomorphogenesis to facilitate seedling survival

Redox regulation in shoot growth, SAM maintenance and flowering

Reactive oxygen species (ROS) and associated reduction/oxidation (redox) controls involving glutathione, glutaredoxins and thioredoxins play key roles in the regulation of plant growth and development. While many questions remain concerning redox functions in the shoot apical meristem (SAM), accumulating evidence suggests that redox master switches integrate major hormone signals and transcriptional networks in the SAM, and so regulate organ growth, polarity and floral development. Auxin-induced activation of plasma-membrane located NADPH-oxidases and mitochondrial respiratory bioenergetics are likely regulators of the ROS bursts that drive the cell cycle in proliferating regions, with other hormones such as jasmonic acid playing propagating or antagonistic roles in gene regulation. Moreover, the activation of oxygen production by photosynthesis and oxygen-dependent N-end rule controls are linked to the transition from cell proliferation to cell expansion and differentiation. While much remains to be understood, the nexus of available redox controls provides a key underpinning mechanism linking hormonal controls, energy metabolism and bioenergetics to plant growth and development.

Activation of HLS1 by Mechanical Stress via Ethylene-Stabilized EIN3 Is Crucial for Seedling Soil Emergence

The seeds of terrestrial flowering plants often start their life cycle in subterranean darkness. To protect the fragile apical meristematic tissues and cotyledons from mechanical injuries during soil penetration, dicotyledonous seedlings form an elegant apical hook at the top of the hypocotyl. The apical hook has been considered as an adaption structure to the subterranean environment. However, the role of the apical hook in seedling emergence and the molecular mechanism of apical hook formation under real-life conditions remain highly speculative. Here, we find that HOOKLESS 1 (HLS1), a critical gene in apical hook formation in Arabidopsis thaliana, is required for seedling emergence from the soil. When grown under soil, hls1 mutant exhibits severe emergence defects. By contrast, HLS1 overexpression in the hls1 background fully restores emergence defects and displays better emergence capacity than that of WT. Our results indicate that HLS1 transcription is stimulated in response to the mechanical stress of soil cover, which is dependent on the function of the transcription factors ETHYLENE INSENSITIVE 3 (EIN3) and EIN3-LIKE 1 (EIL1). Soil-conferred mechanical stress activates the ethylene signaling pathway to stabilize EIN3 by repressing the activity of the F-box proteins EBF1 and EBF2. These combined results reveal a signaling pathway in which plant seedlings transduce the mechanical pressure of soil cover to correctly modulate apical hook formation during soil emergence.

Redundant ERF-VII Transcription Factors Bind to an Evolutionarily Conserved cis-Motif to Regulate Hypoxia-Responsive Gene Expression in Arabidopsis

The response of Arabidopsis thaliana to low-oxygen stress (hypoxia), such as during shoot submergence or root waterlogging, includes increasing the levels of ∼50 hypoxia-responsive gene transcripts, many of which encode enzymes associated with anaerobic metabolism. Upregulation of over half of these mRNAs involves stabilization of five group VII ethylene response factor (ERF-VII) transcription factors, which are routinely degraded via the N-end rule pathway of proteolysis in an oxygen- and nitric oxide-dependent manner. Despite their importance, neither the quantitative contribution of individual ERF-VIIs nor the cis-regulatory elements they govern are well understood. Here, using single- and double-null mutants, the constitutively synthesized ERF-VIIs RELATED TO APETALA2.2 (RAP2.2) and RAP2.12 are shown to act redundantly as principle activators of hypoxia-responsive genes; constitutively expressed RAP2.3 contributes to this redundancy, whereas the hypoxia-induced HYPOXIA RESPONSIVE ERF1 (HRE1) and HRE2 play minor roles. An evolutionarily conserved 12-bp cis-regulatory motif that binds to and is sufficient for activation by RAP2.2 and RAP2.12 is identified through a comparative phylogenetic motif search, promoter dissection, yeast one-hybrid assays, and chromatin immunopurification. This motif, designated the hypoxia-responsive promoter element, is enriched in promoters of hypoxia-responsive genes in multiple species.

Seedlings Transduce the Depth and Mechanical Pressure of Covering Soil Using COP1 and Ethylene to Regulate EBF1/EBF2 for Soil Emergence

The survival of seed plants in natural environments requires the successful emergence from the soil. In this process, the ethylene signaling pathway is utilized by plants to sense and respond to the mechanical resistance of the soil. Here, we report that constitutive photomorphogenesis 1 (COP1), a central repressor of light signaling, is a key component required for seedlings to sense the depth of soil overlay. Mutation in COP1 causes severe defects in penetrating soil, due to decreased level of EIN3, a master transcription factor in ethylene pathway that mediates seedling emergence. We show that COP1 directly targets the F box proteins EBF1 and EBF2 for ubiquitination and degradation, thus stabilizing EIN3. As seedlings grow toward the surface, the depth of soil overlay decreases, resulting in a gradual increase of light fluences. COP1 channels the light signals, while ethylene transduces the information on soil mechanical conditions, which cooperatively control EIN3 protein levels to promote seedling emergence from the soil. The COP1-EBF1/2-EIN3 module reveals a mechanism by which plants sense the depth to surface and uncovers a novel regulatory paradigm of an ubiquitin E3 ligase cascade.

Group VII Ethylene Response Factors Coordinate Oxygen and Nitric Oxide Signal Transduction and Stress Responses in Plants

The group VII ethylene response factors (ERFVIIs) are plant-specific transcription factors that have emerged as important regulators of abiotic and biotic stress responses, in particular, low-oxygen stress. A defining feature of ERFVIIs is their conserved N-terminal domain, which renders them oxygen- and nitric oxide (NO)-dependent substrates of the N-end rule pathway of targeted proteolysis. In the presence of these gases, ERFVIIs are destabilized, whereas an absence of either permits their accumulation; ERFVIIs therefore coordinate plant homeostatic responses to oxygen availability and control a wide range of NO-mediated processes. ERFVIIs have a variety of context-specific protein and gene interaction partners, and also modulate gibberellin and abscisic acid signaling to regulate diverse developmental processes and stress responses. This update discusses recent advances in our understanding of ERFVII regulation and function, highlighting their role as central regulators of gaseous signal transduction at the interface of ethylene, oxygen, and NO signaling.

Seedling Germination: Seedlings Follow Sunshine and Fresh Air

The journey from seedling to plant requires guidance in the dark to establish which directions the roots and shoots should grow. A new study shows that, after germinating in darkness, plant seedlings sense the oxygen content of the surrounding airspace to guide further development.