Preventing Inappropriate Signals Pre- and Post-Ligand Perception by a Toggle-Switch Mechanism of ERECTA.

Dynamic control of signaling events requires swift regulation of receptors at an active state. By focusing on Arabidopsis ERECTA (ER) receptor kinase, which perceives peptide ligands to control multiple developmental processes, we report a mechanism preventing inappropriate receptor activity. The ER C-terminal tail (ER_CT) functions as an autoinhibitory domain: its removal confers higher kinase activity and hyperactivity during inflorescence and stomatal development. ER_CT is required for the binding of a receptor kinase inhibitor, BKI1, and two U-box E3 ligases PUB30 and PUB31 that inactivate activated ER. We further identify ER_CT as a phosphodomain trans-phosphorylated by the co-receptor BAK1. The phosphorylation impacts the tail structure, likely releasing from autoinhibition. The phosphonull version enhances BKI1 association, whereas the phosphomimetic version promotes PUB30/31 association. Thus, ER_CT acts as an off-on-off toggle switch, facilitating the release of BKI1 inhibition, enabling signal activation, and swiftly turning over the receptors afterwards. Our results elucidate a mechanism fine-tuning receptor signaling via a phosphoswitch module, keeping the receptor at a low basal state and ensuring the robust yet transient activation upon ligand perception. Significance: Cells perceive and process external signals through their cell-surface receptors, whose activity must be tightly maintained to prevent the spread of misinformation. How do plant cells prevent the inappropriate receptor activity? We identify a structural module within the C-terminal tail of the ERECTA (ER_CT), that inhibits the receptor pre- and post-signal activation. The ER_CT comprises of a linker and an α-helix. Before activation, ER_CT is autoinhibitory and associates with an inhibitory protein. Ligand perception triggers the transphosphorylation of ER_CT by the co-receptor, which then recruits a degradation machinery to swiftly turn over the activated receptor. Thus, we reveal an off-on-off toggle switch mechanism that finely adjusts the activity of the plant receptor, enabling the precise control over cell signaling.

Chemical genetics reveals cross-activation of plant developmental signaling by the immune peptide-receptor pathway.

Cells sense and integrate multiple signals to coordinate development and defence. A receptor-kinase signaling pathway for plant stomatal development shares components with the immunity pathway. The mechanism ensuring their signal specificities remains unclear. Using chemical genetics, here we report the identification of a small molecule, kC9, that triggers excessive stomatal differentiation by inhibiting the canonical ERECTA receptor-kinase pathway. kC9 binds to and inhibits the downstream MAP kinase MPK6, perturbing its substrate interaction. Strikingly, activation of immune signaling by a bacterial flagellin peptide nullified kC9's effects on stomatal development. This cross-activation of stomatal development by immune signaling depends on the immune receptor FLS2 and occurs even in the absence of kC9 if the ERECTA-family receptor population becomes suboptimal. Furthermore, proliferating stomatal-lineage cells are vulnerable to the immune signal penetration. Our findings suggest that the signal specificity between development and immunity can be ensured by MAP Kinase homeostasis reflecting the availability of upstream receptors, thereby providing a novel view on signal specificity.

Water wisteria genome reveals environmental adaptation and heterophylly regulation in amphibious plants.

Heterophylly is a phenomenon whereby an individual plant dramatically changes leaf shape in response to the surroundings. Hygrophila difformis (Acanthaceae; water wisteria), has recently emerged as a model plant to study heterophylly because of its striking leaf shape variation in response to various environmental factors. When submerged, H. difformis often develops complex leaves, but on land it develops simple leaves. Leaf complexity is also influenced by other factors, such as light density, humidity, and temperature. Here, we sequenced and assembled the H. difformis chromosome-level genome (scaffold N50: 60.43 Mb, genome size: 871.92 Mb), which revealed 36 099 predicted protein-coding genes distributed over 15 pseudochromosomes. H. difformis diverged from its relatives during the Oligocene climate-change period and expanded gene families related to its amphibious habit. Genes related to environmental stimuli, leaf development, and other pathways were differentially expressed in submerged and terrestrial conditions, possibly modulating morphological and physiological acclimation to changing environments. We also found that auxin plays a role in H. difformis heterophylly. Finally, we discovered candidate genes that respond to different environmental conditions and elucidated the role of LATE MERISTEM IDENTITY 1 (LMI1) in heterophylly. We established H. difformis as a model for studying interconnections between environmental adaptation and morphogenesis.

Stomatal cell fate commitment via transcriptional and epigenetic control: Timing is crucial.

The formation of stomata presents a compelling model system for comprehending the initiation, proliferation, commitment and differentiation of de novo lineage-specific stem cells. Precise, timely and robust cell fate and identity decisions are crucial for the proper progression and differentiation of functional stomata. Deviations from this precise specification result in developmental abnormalities and nonfunctional stomata. However, the molecular underpinnings of timely cell fate commitment have just begun to be unravelled. In this review, we explore the key regulatory strategies governing cell fate commitment, emphasizing the distinctions between embryonic and postembryonic stomatal development. Furthermore, the interplay of transcription factors and cell cycle machineries is pivotal in specifying the transition into differentiation. We aim to synthesize recent studies utilizing single-cell as well as cell-type-specific transcriptomics, epigenomics and chromatin accessibility profiling to shed light on how master-regulatory transcription factors and epigenetic machineries mutually influence each other to drive fate commitment and maintenance.

A Small Compound, HYGIC, Promotes Hypocotyl Growth Through Ectopic Ethylene Response.

Plant seedlings adjust the growth of the hypocotyl in response to surrounding environmental changes. Genetic studies have revealed key players and pathways in hypocotyl growth, such as phytohormones and light signaling. However, because of genetic redundancy in the genome, it is expected that not-yet-revealed mechanisms can be elucidated through approaches different from genetic ones. Here, we identified a small compound, HYGIC (HG), that simultaneously induces hypocotyl elongation and thickening, accompanied by increased nuclear size and enlargement of cortex cells. HG-induced hypocotyl growth required the ethylene signaling pathway activated by endogenous ethylene, involving CONSTITUTIVE PHOTOMORPHOGENIC 1, ETHYLENE INSENSITIVE 2 (EIN2) and redundant transcription factors for ethylene responses, ETHYLENE INSENSITIVE 3 (EIN3) and EIN3 LIKE 1. By using EBS:GUS, a transcriptional reporter of ethylene responses based on an EIN3-binding-cis-element, we found that HG treatment ectopically activates ethylene responses at the epidermis and cortex of the hypocotyl. RNA-seq and subsequent gene ontology analysis revealed that a significant number of HG-induced genes are related to responses to hypoxia. Indeed, submergence, a representative environment where the hypoxia response is induced in nature, promoted ethylene-signaling-dependent hypocotyl elongation and thickening accompanied by ethylene responses at the epidermis and cortex, which resembled the HG treatment. Collectively, the identification and analysis of HG revealed that ectopic responsiveness to ethylene promotes hypocotyl growth, and this mechanism is activated under submergence.

Signaling in plant development and immunity through the lens of the stomata.

The proper development and function of stomata - turgor-driven valves for efficient gas-exchange and water control - impact plant survival and productivity. It has become apparent that various receptor kinases regulate stomatal development and immunity. Although stomatal development and immunity occur over different cellular time scales, their signaling components and regulatory modules are strikingly similar, and often shared. In this review, we survey the current knowledge of stomatal development and immunity signaling components, and provide a synthesis and perspectives on the key concepts to further understand the conservation and specificity of these two signaling pathways.

Targeted single-cell gene induction by optimizing the dually regulated CRE/loxP system by a newly defined heat-shock promoter and the steroid hormone in Arabidopsis thaliana.

Multicellular organisms rely on intercellular communication systems to organize their cellular functions. In studies focusing on intercellular communication, the key experimental techniques include the generation of chimeric tissue using transgenic DNA recombination systems represented by the CRE/loxP system. If an experimental system enables the induction of chimeras at highly targeted cell(s), it will facilitate the reproducibility and precision of experiments. However, multiple technical limitations have made this challenging. The stochastic nature of DNA recombination events, especially, hampers reproducible generation of intended chimeric patterns. Infrared laser-evoked gene operator (IR-LEGO), a microscopic system that irradiates targeted cells using an IR laser, can induce heat shock-mediated expression of transgenes, for example, CRE recombinase gene, in the cells. In this study, we developed a method that induces CRE/loxP recombination in the target cell(s) of plant roots and leaves in a highly specific manner. We combined IR-LEGO, an improved heat-shock-specific promoter, and dexamethasone-dependent regulation of CRE. The optimal IR-laser power and irradiation duration were estimated via exhaustive irradiation trials and subsequent statistical modeling. Under optimized conditions, CRE/loxP recombination was efficiently induced without cellular damage. We also found that the induction efficiency varied among tissue types and cellular sizes. The developed method offers an experimental system to generate a precisely designed chimeric tissue, and thus, will be useful for analyzing intercellular communication at high resolution in roots and leaves.

Identification of a pluripotency-inducing small compound, PLU, that induces callus formation via Heat Shock Protein 90-mediated activation of auxin signaling.

Plants retain the ability to generate a pluripotent tissue called callus by dedifferentiating somatic cells. A pluripotent callus can also be artificially induced by culturing explants with hormone mixtures of auxin and cytokinin, and an entire body can then be regenerated from the callus. Here we identified a pluripotency-inducing small compound, PLU, that induces the formation of callus with tissue regeneration potency without the external application of either auxin or cytokinin. The PLU-induced callus expressed several marker genes related to pluripotency acquisition via lateral root initiation processes. PLU-induced callus formation required activation of the auxin signaling pathway though the amount of active auxin was reduced by PLU treatment. RNA-seq analysis and subsequent experiments revealed that Heat Shock Protein 90 (HSP90) mediates a significant part of the PLU-initiated early events. We also showed that HSP90-dependent induction of TRANSPORT INHIBITOR RESPONSE 1, an auxin receptor gene, is required for the callus formation by PLU. Collectively, this study provides a new tool for manipulating and investigating the induction of plant pluripotency from a different angle from the conventional method with the external application of hormone mixtures.

Rewiring of hormones and light response pathways underlies the inhibition of stomatal development in an amphibious plant Rorippa aquatica underwater.

Land plants have evolved the ability to cope with submergence. Amphibious plants are adapted to both aerial and aquatic environments through phenotypic plasticity in leaf form and function, known as heterophylly. In general, underwater leaves of amphibious plants are devoid of stomata, yet their molecular regulatory mechanisms remain elusive. Using the emerging model of the Brassicaceae amphibious species Rorippa aquatica, we lay the foundation for the molecular physiological basis of the submergence-triggered inhibition of stomatal development. A series of temperature shift experiments showed that submergence-induced inhibition of stomatal development is largely uncoupled from morphological heterophylly and likely regulated by independent pathways. Submergence-responsive transcriptome analysis revealed rapid reprogramming of gene expression, exemplified by the suppression of RaSPEECHLESS and RaMUTE within 1 h and the involvement of light and hormones in the developmental switch from terrestrial to submerged leaves. Further physiological studies place ethylene as a central regulator of the submergence-triggered inhibition of stomatal development. Surprisingly, red and blue light have opposing functions in this process: blue light promotes, whereas red light inhibits stomatal development, through influencing the ethylene pathway. Finally, jasmonic acid counteracts the inhibition of stomatal development, which can be attenuated by the red light. The actions and interactions of light and hormone pathways in regulating stomatal development in R. aquatica are different from those in the terrestrial species, Arabidopsis thaliana. Thus, our work suggests that extensive rewiring events of red light to ethylene signaling might underlie the evolutionary adaption to water environment in Brassicaceae.

Cell Cycle Dynamics during Stomatal Development: Window of MUTE Action and Ramification of Its Loss-of-Function on an Uncommitted Precursor.

Plants develop in the absence of cell migration. As such, cell division and differentiation need to be coordinated for functional tissue formation. Cellular valves on the plant epidermis, stomata, are generated through a stereotypical sequence of cell division and differentiation events. In Arabidopsis, three master regulatory transcription factors, SPEECHLESS (SPCH), MUTE and FAMA, sequentially drive initiation, proliferation and differentiation of stomata. Among them, MUTE switches the cell cycle mode from proliferative asymmetric division to terminal symmetric division and orchestrates the execution of the single symmetric division event. However, it remains unclear to what extent MUTE regulates the expression of cell cycle genes through the symmetric division and whether MUTE accumulation itself is gated by the cell cycle. Here, we show that MUTE directly upregulates the expression of cell cycle components throughout the terminal cell cycle phases of a stomatal precursor, not only core cell cycle engines but also check-point regulators. Time-lapse live imaging using the multicolor Plant Cell Cycle Indicator revealed that MUTE accumulates up to the early G2 phase, whereas its successor and direct target, FAMA, accumulate at late G2 through terminal mitosis. In the absence of MUTE, meristemoids fail to differentiate and their G1 phase elongates as they reiterate asymmetric divisions. Together, our work provides the framework of cell cycle and master regulatory transcription factors to coordinate a single symmetric cell division and suggests a mechanism for the eventual cell cycle arrest of an uncommitted stem-cell-like precursor at the G1 phase.

EPFL peptide signalling ensures robust self-pollination success under cool temperature stress by aligning the length of the stamen and pistil.

Successful sexual reproduction of plants requires temperature-sensitive processes, and temperature stress sometimes causes developmental asynchrony between male and female reproductive tissues. In Arabidopsis thaliana, self-pollination occurs when the stamen and pistil lengths are aligned in a single flower so that pollens at the stamen tip are delivered to the stigma at the pistil tip. Although intercellular signalling acts in several reproduction steps, how signalling molecules, including secreted peptides, contribute to the synchronous growth of reproductive tissues remains limited. Here, we show that the mutant of the secreted peptide EPIDERMAL PATTERNING FACTOR LIKE 6 (EPFL6), which shows no phenotypes at a moderate temperature, fails in fruit production at a cool temperature due to insufficient elongation of stamens. EPFL6 is expressed in stamen filaments and promotes filament elongation to achieve the alignment of stamen and pistil lengths at a cool temperature. We also found that, at a moderate temperature, all EPFL6-subfamily genes are required for stamen elongation. Furthermore, we showed that ERECTA (ER), known as a common receptor for EPFL-family peptides, mediates the stamen-pistil growth coordination. Lastly, we provided evidence that modulation of ER activity rescues the reproduction failure caused by insufficient stamen elongation by realigning the stamen and pistil lengths.

Direct attenuation of Arabidopsis ERECTA signalling by a pair of U-box E3 ligases.

Plants sense a myriad of signals through cell-surface receptors to coordinate their development and environmental response. The Arabidopsis ERECTA receptor kinase regulates diverse developmental processes via perceiving multiple EPIDERMAL PATTERNING FACTOR (EPF)/EPF-LIKE peptide ligands. How the activated ERECTA protein is turned over is unknown. Here we identify two closely related plant U-box ubiquitin E3 ligases, PUB30 and PUB31, as key attenuators of ERECTA signalling for two developmental processes: inflorescence/pedicel growth and stomatal development. Loss-of-function pub30 pub31 mutant plants exhibit extreme inflorescence/pedicel elongation and reduced stomatal numbers owing to excessive ERECTA protein accumulation. Ligand activation of ERECTA leads to phosphorylation of PUB30/31 via BRI1-ASSOCIATED KINASE1 (BAK1), which acts as a coreceptor kinase and a scaffold to promote PUB30/31 to associate with and ubiquitinate ERECTA for eventual degradation. Our work highlights PUB30 and PUB31 as integral components of the ERECTA regulatory circuit that ensure optimal signalling outputs, thereby defining the role for PUB proteins in developmental signalling.

Voicing the story behind the cover.

In this selection, we celebrate the art of science by highlighting some of the submitted cover images from the past year. In this collection, our authors share the stories behind their inspiration for how to portray their science to captivate a broader audience.

Chemical synthesis of the EPF-family of plant cysteine-rich proteins and late-stage dye attachment by chemoselective amide-forming ligations.

Chemical protein synthesis can provide well-defined modified proteins. Herein, we report the chemical synthesis of plant-derived cysteine-rich secretory proteins and late-stage derivatization of the synthetic proteins. The syntheses were achieved with distinct chemoselective amide bond forming reactions - EPF2 by native chemical ligation (NCL), epidermal patterning factor (EPF) 1 by the α-ketoacid-hydroxylamine (KAHA) ligation, and fluorescent functionalization of their folded variants by potassium acyltrifluoroborate (KAT) ligation. The chemically synthesized EPFs exhibit bioactivity on stomatal development in Arabidopsis thaliana. Comprehensive synthesis of EPF derivatives allowed us to identify suitable fluorescent variants for bioimaging of the subcellar localization of EPFs.

Dynamic chromatin accessibility deploys heterotypic cis/trans-acting factors driving stomatal cell-fate commitment.

Chromatin architecture and transcription factor (TF) binding underpin cell-fate specification during development, but their mutual regulatory relationships remain unclear. Here we report an atlas of dynamic chromatin landscapes during stomatal cell-lineage progression, in which sequential cell-state transitions are governed by lineage-specific bHLH TFs. Major reprogramming of chromatin accessibility occurs at the proliferation-to-differentiation transition. We discover novel co-cis regulatory elements (CREs) signifying the early precursor stage, BBR/BPC (GAGA) and bHLH (E-box) motifs, where master-regulatory bHLH TFs, SPEECHLESS and MUTE, consecutively bind to initiate and terminate the proliferative state, respectively. BPC TFs complex with MUTE to repress SPEECHLESS expression through a local deposition of repressive histone marks. We elucidate the mechanism by which cell-state-specific heterotypic TF complexes facilitate cell-fate commitment by recruiting chromatin modifiers via key co-CREs.

Plant signaling: Peptide-receptor pair re-opens stomata after pathogen infection.

Stomata - cellular valves in the epidermis of land plants - close their apertures to prevent water loss or pathogen entry. A new study now reports that the plant immune response induces the expression of a peptide ligand-receptor pair that re-opens stomata to resume gas exchange and transpiration after pathogen infection.

Intragenic suppressors unravel the role of the SCREAM ACT-like domain for bHLH partner selectivity in stomatal development.

Multicellular organisms develop specialized cell types to achieve complex functions of tissues and organs. The basic helix-loop-helix (bHLH) proteins act as master regulatory transcription factors of such specialized cell types. Plant stomata are cellular valves in the aerial epidermis for efficient gas exchange and water control. Stomatal differentiation is governed by sequential actions of three lineage-specific bHLH proteins, SPEECHLESS (SPCH), MUTE, and FAMA, specifying initiation and proliferation, commitment, and terminal differentiation, respectively. A broadly expressed bHLH, SCREAM (SCRM), heterodimerizes with SPCH/MUTE/FAMA and drives stomatal differentiation via switching its partners. Yet nothing is known about its heterodimerization properties or partner preference. Here, we report the role of the SCRM C-terminal ACT-like (ACTL) domain for heterodimerization selectivity. Our intragenic suppressor screen of a dominant scrm-D mutant identified the ACTL domain as a mutation hotspot. Removal of this domain or loss of its structural integrity abolishes heterodimerization with MUTE, but not with SPCH or FAMA, and selectively abrogates the MUTE direct target gene expression. Consequently, the scrm-D ACTL mutants confer massive clusters of arrested stomatal precursor cells that cannot commit to differentiation when redundancy is removed. Structural and biophysical studies further show that SPCH, MUTE, and FAMA also possess the C-terminal ACTL domain, and that ACTL•ACTL heterodimerization is sufficient for partner selectivity. Our work elucidates a role for the SCRM ACTL domain in the MUTE-governed proliferation-differentiation switch and suggests mechanistic insight into the biological function of the ACTL domain, a module uniquely associated with plant bHLH proteins, as a heterodimeric partner selectivity interface.

Deceleration of the cell cycle underpins a switch from proliferative to terminal divisions in plant stomatal lineage.

Differentiation of specialized cell types requires precise cell-cycle control. Plant stomata are generated through asymmetric divisions of a stem-cell-like precursor followed by a single symmetric division that creates paired guard cells surrounding a pore. The stomatal-lineage-specific transcription factor MUTE terminates the asymmetric divisions and commits to differentiation. However, the role of cell-cycle machineries in this transition remains unknown. We discover that the symmetric division is slower than the asymmetric division in Arabidopsis. We identify a plant-specific cyclin-dependent kinase inhibitor, SIAMESE-RELATED4 (SMR4), as a MUTE-induced molecular brake that decelerates the cell cycle. SMR4 physically and functionally associates with CYCD3;1 and extends the G1 phase of asymmetric divisions. By contrast, SMR4 fails to interact with CYCD5;1, a MUTE-induced G1 cyclin, and permits the symmetric division. Our work unravels a molecular framework of the proliferation-to-differentiation switch within the stomatal lineage and suggests that a timely proliferative cell cycle is critical for stomatal-lineage identity.