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.

Plants and Small Molecules: An Up-and-Coming Synergy

The emergence of Arabidopsis thaliana as a model system has led to a rapid and wide improvement in molecular genetics techniques for studying gene function and regulation. However, there are still several drawbacks that cannot be easily solved with molecular genetic approaches, such as the study of unfriendly species, which are of increasing agronomic interest but are not easily transformed, thus are not prone to many molecular techniques. Chemical genetics represents a methodology able to fill this gap. Chemical genetics lies between chemistry and biology and relies on small molecules to phenocopy genetic mutations addressing specific targets. Advances in recent decades have greatly improved both target specificity and activity, expanding the application of this approach to any biological process. As for classical genetics, chemical genetics also proceeds with a forward or reverse approach depending on the nature of the study. In this review, we addressed this topic in the study of plant photomorphogenesis, stress responses and epigenetic processes. We have dealt with some cases of repurposing compounds whose activity has been previously proven in human cells and, conversely, studies where plants have been a tool for the characterization of small molecules. In addition, we delved into the chemical synthesis and improvement of some of the compounds described.

Identification of stomatal-regulating molecules from de novo arylamine collection through aromatic C-H amination

Stomata—small pores generally found on the leaves of plants—control gas exchange between plant and the atmosphere. Elucidating the mechanism that underlies such control through the regulation of stomatal opening/closing is important to understand how plants regulate photosynthesis and tolerate against drought. However, up-to-date, molecular components and their function involved in stomatal regulation are not fully understood. We challenged such problem through a chemical genetic approach by isolating and characterizing synthetic molecules that influence stomatal movement. Here, we describe that a small chemical collection, prepared during the development of C–H amination reactions, lead to the discovery of a Stomata Influencing Molecule (SIM); namely, a sulfonimidated oxazole that inhibits stomatal opening. The starting molecule SIM1 was initially isolated from screening of compounds that inhibit light induced opening of dayflower stomata. A range of SIM molecules were rapidly accessed using our state-of-the-art C–H amination technologies. This enabled an efficient structure–activity relationship (SAR) study, culminating in the discovery of a sulfonamidated oxazole derivative (SIM*) having higher activity and enhanced specificity against stomatal regulation. Biological assay results have shed some light on the mode of action of SIM molecules within the cell, which may ultimately lead to drought tolerance-conferring agrochemicals through the control of stomatal movement.

The Cell Differentiation of Idioblast Myrosin Cells: Similarities With Vascular and Guard Cells

Idioblasts are defined by abnormal shapes, sizes, and contents that are different from neighboring cells. Myrosin cells are Brassicales-specific idioblasts and accumulate a large amount of thioglucoside glucohydrolases (TGGs, also known as myrosinases) in their vacuoles. Myrosinases convert their substrates, glucosinolates, into toxic compounds when herbivories and pests attack plants. In this review, we highlight the similarities and differences between myrosin cells and vascular cells/guard cells (GCs) because myrosin cells are distributed along vascular cells, especially the phloem parenchyma, and myrosin cells share the master transcription factor FAMA with GCs for their cell differentiation. In addition, we analyzed the overlap of cell type-specific genes between myrosin cells and GCs by using published single-cell transcriptomics (scRNA-seq) data, suggesting significant similarities in the gene expression patterns of these two specialized cells.

Identification of a Devernalization Inducer by Chemical Screening Approaches in Arabidopsis thaliana

Vernalization is the promotion of flowering after prolonged exposure to cold. In Arabidopsis thaliana, vernalization induces epigenetic silencing of the floral repressor gene FLOWERING LOCUS C (FLC). The repressive epigenetic mark trimethylation of lysine 27 on histone H3 proteins (H3K27me3) is a critical contributor to the epigenetic silencing of FLC. Interestingly, the deposited H3K27me3 in the FLC locus can be erased by short-term high-temperature treatment. This is referred to as devernalization. In this study, we identified a novel chemical compound, 4-Isoxazolecarboxylic acid, 3,5-dimethyl-2-(4-fluorophenyl)-4-isoxazole carboxylic acid 1-methyl-2-oxoethyl ester named as DEVERNALIZER01 (DVR01), which induces devernalization in Arabidopsis seedlings, by an FLC-luciferase reporter-based high-throughput screening assay. DVR01 decreased the amount of H3K27me3 in the FLC locus in vernalized plants, resulting in the upregulation of FLC in the whole plant, including the vasculature and meristem, where FLC represses floral induction genes. We also showed that a 2-week treatment with DVR01 reverted plants with a vernalized status back to a fully non-vernalized status. Collectively, this study provides a novel structure of DVR01, which modulates devernalization via demethylation of H3K27me3 in the FLC locus.

Light Regulation of Stomatal Development and Patterning: Shifting the Paradigm from Arabidopsis to Grasses

The stomatal pores of plant leaves control gas exchange with the environment. Stomatal development is prevised regulated by both internal genetic programs and environmental cues. Among various environmental factors, light regulation of stomata formation has been extensively studied in Arabidopsis. In this review, we summarize recent advances in the genetic control of stomata development and its regulation by light. We also present a comparative analysis of the conserved and diverged stomatal regulatory networks between Arabidopsis and cereal grasses. Lastly, we provide our perspectives on manipulation of the stomata density on plant leaves for the purpose of breeding crops that are better adapted to the adverse environment and high-density planting conditions.

Stomatal Development and Perspectives toward Agricultural Improvement

Stomata are small pores on the surface of land plants that facilitate gas exchange-acquiring CO₂ from surrounding atmosphere and releasing water vapor. In adverse conditions, such as drought, stomata close to minimize water loss. The activities of stomata are vital for plant growth and survival. In the last two decades, key players for stomatal development have been discovered thanks to the model plant Arabidopsis thaliana Our knowledge about the formation of stomata and their response to environmental changes are accumulating. In this review, we summarize the genetic and molecular mechanisms of stomatal development, with specific emphasis on recent findings and potential applications toward enhancing the sustainability of agriculture.

Identification and Characterization of Compounds that Affect Stomatal Movements

Regulation of stomatal aperture is essential for plant growth and survival in response to environmental stimuli. Opening of stomata induces uptake of CO2 for photosynthesis and transpiration, which enhances uptake of nutrients from roots. Light is the most important stimulus for stomatal opening. Under drought stress, the plant hormone ABA induces stomatal closure to prevent water loss. However, the molecular mechanisms of stomatal movements are not fully understood. In this study, we screened chemical libraries to identify compounds that affect stomatal movements in Commelina benghalensis and characterize the underlying molecular mechanisms. We identified nine stomatal closing compounds (SCL1-SCL9) that suppress light-induced stomatal opening by >50%, and two compounds (temsirolimus and CP-100356) that induce stomatal opening in the dark. Further investigations revealed that SCL1 and SCL2 had no effect on autophosphorylation of phototropin or the activity of the inward-rectifying plasma membrane (PM) K+ channel, KAT1, but suppressed blue light-induced phosphorylation of the penultimate residue, threonine, in PM H+-ATPase, which is a key enzyme for stomatal opening. SCL1 and SCL2 had no effect on ABA-dependent responses, including seed germination and expression of ABA-induced genes. These results suggest that SCL1 and SCL2 suppress light-induced stomatal opening at least in part by inhibiting blue light-induced activation of PM H+-ATPase, but not by the ABA signaling pathway. Interestingly, spraying leaves onto dicot and monocot plants with SCL1 suppressed wilting of leaves, indicating that inhibition of stomatal opening by these compounds confers tolerance to drought stress in plants.

Specialized Vacuoles of Myrosin Cells: Chemical Defense Strategy in Brassicales Plants

Plant vacuoles display many versatile functions. Vacuoles in vegetative tissues are generally involved in protein degradation, and are called lytic vacuoles. However, vegetative vacuoles in specialized cells can accumulate large concentrations of proteins, such as those in idioblast myrosin cells along veins in the order Brassicales, which store large amounts of myrosinases (thioglucoside glucohydrolase and thioglucoside glucohydrolase). Myrosinases cleave the bond between sulfur and glucose in sulfur-rich compounds (glucosinolates) to produce toxic compounds (isothiocyanates) when plants are damaged by pests. This defense strategy is called the myrosinase-glucosinolate system. Recent studies identified atypical myrosinases, PENETRATION 2 (PEN2) and PYK10, along with key components for development of myrosin cells. In this review, we discuss three topics in the myrosinase-glucosinolate system. First, we summarize the complexity and importance of the myrosinase-glucosinolate system, including classical myrosinases, atypical myrosinases and the system that counteracts the myrosinase-glucosinolate system. Secondly, we describe molecular machineries underlying myrosin cell development, including specific reporters, cell lineage, cell differentiation and cell fate determination. The master regulators for myrosin cell differentiation, FAMA and SCREAM, are key transcription factors involved in guard cell differentiation. This indicates that myrosin cells and guard cells share similar transcriptional networks. Finally, we hypothesize that the myrosinase-glucosinolate system may have originated in stomata of ancestral Brassicales plants and, after that, plants co-opted this defense strategy into idioblasts near veins at inner tissue layers.

Discovery of synthetic small molecules that enhance the number of stomata: C–H functionalization chemistry for plant biology

The first-in-class synthetic small molecules enhancing the number of stomata in Arabidopsis thaliana have been discovered.