Bacterial infection systemically suppresses stomatal density

Mitochondrial ß-Cyanoalanine Synthase Participates in flg22-Induced Stomatal Immunity

Plants regulate gas exchange with the environment and modulate transpirational water flow through guard cells, which set the aperture of the stomatal pores. External and internal stimuli are detected by guard cells and integrated into a signalling network that modulate turgor pressure and, hence, pore size. Pathogen-associated molecular patterns are among the stimuli that induce stomatal closure, to prevent pathogen entry through the pores, and this response, also referred to as stomatal immunity, is one of the hallmarks of PAMP-triggered immunity. While reactive oxygen species (ROS)-mediated signalling plays a key role in stomatal immunity, also the gasotransmitter hydrogen sulphide (H2S) interacts with key components of the guard cell signalling network to induce stomatal closure. While the role of H2S, produced by the main cytosolic source L-cysteine desulfhydrase 1, has been already investigated, there are additional enzymatic sources that synthesize H2S in different subcellular compartments. Their function has remained enigmatic, however. In this work, we elucidate the involvement of the mitochondrial H2S source, β-cyanoalanine synthase CAS-C1, on stomatal immunity induced by the bacterial PAMP flagellin (flg22). We show that cas-c1 plants are impaired to induce flg22-triggered stomatal closure and apoplastic ROS production, while they are more susceptible to bacterial surface inoculation. Moreover, mitochondrial H2S donor AP39 induced stomatal closure in an RBOHD-dependent manner, while depletion of endogenous H2S, impaired RBOHD-mediated apoplastic ROS production. In addition, pharmacological disruption of mitochondrial electron transport chain activity, affected stomatal closure produced by flg22, indicating its participation in the stomatal immunity response. Our findings add evidence to the emerging realization that intracellular organelles play a decisive role in orchestrating stomatal signalling and immune responses and suggest that mitochondrial-derived H2S is an important player of the stomatal immunity signalling network.

Fighting for Survival at the Stomatal Gate

Stomata serve as the battleground between plants and plant pathogens. Plants can perceive pathogens, inducing closure of the stomatal pore, while pathogens can overcome this immune response with their phytotoxins and elicitors. In this review, we summarize new discoveries in stomata-pathogen interactions. Recent studies have shown that stomatal movement continues to occur in a close-open-close-open pattern during bacterium infection, bringing a new understanding of stomatal immunity. Furthermore, the canonical pattern-triggered immunity pathway and ion channel activities seem to be common to plant-pathogen interactions outside of the well-studied Arabidopsis-Pseudomonas pathosystem. These developments can be useful to aid in the goal of crop improvement. New technologies to study intact leaves and advances in available omics data sets provide new methods for understanding the fight at the stomatal gate. Future studies should aim to further investigate the defense-growth trade-off in relation to stomatal immunity, as little is known at this time.

Spongospora subterranea f. sp. subterranea Affects Plant Susceptibility to Subsequent Pathogen Infections Under Controlled-Environment Conditions

Spongospora subterranea f. sp. subterranea (Sss) is a soilborne potato pathogen responsible for causing powdery scab on tubers and galls on roots, reducing root water uptake through colonizing root hairs, and vectoring of Potato mop-top virus (PMTV). However, effects of Sss on overall plant susceptibilities against subsequent infections of potato pathogens above ground have not been previously reported. This study aimed to investigate the effects of Sss on root and tuber disease expression, yield, and susceptibilities to subsequent late blight and white mold infections across six potato varieties. Sss-infected Silverton plants had 28.3% less total tuber yield and 29% fewer tubers compared to noninfected Silverton plants. We did not find a correlation across the varieties between root colonization and root gall formation. Sss-infected Silverton plants were more susceptible to hemibiotrophic late blight and less susceptible to necrotrophic white mold. Sss infection also increased susceptibilities of Goldrush and Atlantic plants to white mold. We also evaluated prevalence of asymptomatic Sss infections across the six varieties. Between 50 and 92% of the asymptomatic tubers tested positive for Sss DNA, depending on the variety. Further research is required to understand the possibility and extent of these asymptomatic infections to the spread of Sss in the field. These findings highlight the complexity of Sss-host interactions and give precedence that the lack of disease expression does not necessarily indicate resistance of a variety to Sss.

Optimizing Crop Plant Stomatal Density to Mitigate and Adapt to Climate Change

Plants take up carbon dioxide, and lose water, through pores on their leaf surfaces called stomata. We have a good understanding of the biochemical signals that control the production of stomata, and over the past decade, these have been manipulated to produce crops with fewer stomata. Crops with abnormally low stomatal densities require less water to produce the same yield and have enhanced drought tolerance. These "water-saver" crops also have improved salinity tolerance and are expected to have increased resistance to some diseases. We calculate that the widespread adoption of water-saver crops could lead to reductions in greenhouse gas emissions equivalent to a maximum of 0.5 GtCO2/yr and thus could help to mitigate the impacts of climate change on agriculture and food security through protecting yields in stressful environments and requiring fewer inputs.

A novel semi-dominant mutation in brassinosteroid signaling kinase1 increases stomatal density

Stomata play a pivotal role in balancing CO2 uptake for photosynthesis and water loss via transpiration. Thus, appropriate regulation of stomatal movement and its formation are crucial for plant growth and survival. Red and blue light induce phosphorylation of the C-terminal residue of the plasma membrane (PM) H+-ATPase, threonine, in guard cells, generating the driving force for stomatal opening. While significant progress has been made in understanding the regulatory mechanism of PM H+-ATPase in guard cells, the regulatory components for the phosphorylation of PM H+-ATPase have not been fully elucidated. Recently, we established a new immunohistochemical technique for detecting guard-cell PM H+-ATPase phosphorylation using leaves, which was expected to facilitate investigations with a single leaf. In this study, we applied the technique to genetic screening experiment to explore novel regulators for the phosphorylation of PM H+-ATPase in guard cells, as well as stomatal development. We successfully performed phenotyping using a single leaf. During the experiment, we identified a mutant exhibiting high stomatal density, jozetsu (jzt), named after a Japanese word meaning 'talkative'. We found that a novel semi-dominant mutation in BRASSINOSTEROID SIGNALING KINASE1 (BSK1) is responsible for the phenotype in jzt mutant. The present results demonstrate that the new immunohistochemical technique has a wide range of applications, and the novel mutation would provide genetic tool to expand our understanding of plant development mediated by brassinosteroid signaling.

Systemic stomatal responses in plants: Coordinating development, stress, and pathogen defense under a changing climate

To successfully survive, develop, grow and reproduce, multicellular organisms must coordinate their molecular, physiological, developmental and metabolic responses among their different cells and tissues. This process is mediated by cell-to-cell, vascular and/or volatile communication, and involves electric, chemical and/or hydraulic signals. Within this context, stomata serve a dual role by coordinating their responses to the environment with their neighbouring cells at the epidermis, but also with other stomata present on other parts of the plant. As stomata represent one of the most important conduits between the plant and its above-ground environment, as well as directly affect photosynthesis, respiration and the hydraulic status of the plant by controlling its gas and vapour exchange with the atmosphere, coordinating the overall response of stomata within and between different leaves and tissues plays a cardinal role in plant growth, development and reproduction. Here, we discuss different examples of local and systemic stomatal coordination, the different signalling pathways that mediate them, and the importance of systemic stomatal coordination to our food supply, ecosystems and weather patterns, under our changing climate. We further discuss the potential biotechnological implications of regulating systemic stomatal responses for enhancing agricultural productivity in a warmer and CO2 -rich environment.

Lipid peroxidation and stress-induced signalling molecules in systemic resistance mediated by azelaic acid/AZELAIC ACID INDUCED1: signal initiation and propagation

Systemic acquired resistance protects plants against a broad spectrum of secondary infections by pathogens. A crucial compound involved in the systemic spread of the threat information after primary pathogen infection is the C9 oxylipin azelaic acid (AZA), a breakdown product of unsaturated C18 fatty acids. AZA is generated during lipid peroxidation in the plastids and accumulates in response to various abiotic and biotic stresses. AZA stimulates the expression of AZELAIC ACID INDUCED1 (AZI1), and a pool of AZI1 accumulates in the plastid envelope in association with AZA. AZA and AZI1 utilize the symplastic pathway to travel through the plasmodesmata to neighbouring cells to induce systemic stress resistance responses in distal tissues. Here, we describe the synthesis, travel and function of AZA and AZI1 and discuss open questions of signal initiation and propagation.

Small holes, big impact: Stomata in plant-pathogen-climate epic trifecta

The regulation of stomatal aperture opening and closure represents an evolutionary battle between plants and pathogens, characterized by adaptive strategies that influence both plant resistance and pathogen virulence. The ongoing climate change introduces further complexity, affecting pathogen invasion and host immunity. This review delves into recent advances on our understanding of the mechanisms governing immunity-related stomatal movement and patterning with an emphasis on the regulation of stomatal opening and closure dynamics by pathogen patterns and host phytocytokines. In addition, the review explores how climate changes impact plant-pathogen interactions by modulating stomatal behavior. In light of the pressing challenges associated with food security and the unpredictable nature of climate changes, future research in this field, which includes the investigation of spatiotemporal regulation and engineering of stomatal immunity, emerges as a promising avenue for enhancing crop resilience and contributing to climate control strategies.

SHINE clade of ERF transcription factors: A significant player in abiotic and biotic stress tolerance in plants

SHINE (SHN) clade transcription factors (TFs) represents a subfamily of APETALA2/ethylene-responsive factor (AP2/ERF) proteins. The latter, is characterized by its responsiveness to the phytohormone ethylene and the presence of AP2 DNA-binding domain. They are involved in many biological processes and in responses to different environmental constraints. SHN TFs were among the first identified regulators of cuticle formation. Cuticle plays crucial role in plant tolerance to drought, salinity and high temperature as well as in defense against pathogens. In addition, SHN were shown to be involved in the regulation of stomatal development which influences resistance to drought and diseases. Interestingly, recent studies have also shown that SHN TFs are involved in mediating the beneficial effects of arbuscular mycorrhizal fungi (AMF) as well as disease resistance conferred by nanoparticles. To fulfill their roles, SHN TFs are controlled upstream by other TFs and they control, in their turn, different downstream genes. In this review, we highlight the role of SHN TFs in different abiotic and biotic stresses through their involvement in cuticle biosynthesis, stomatal development and molecular regulation of biochemical and physiological traits. In addition, we discuss the regulation of SHN TFs by plant hormones and their influence on hormone biosynthesis and signaling pathways. Knowledge of this complex regulation can be put into contribution to increase multiple abiotic stress tolerances through transgenesis, gene editing and classical breeding.

Pathogen-triggered changes in plant development: Virulence strategies or host defense mechanism?

Plants, as sessile organisms, are constantly exposed to pathogens in nature. Plants rely on physical barriers, constitutive chemical defenses, and sophisticated inducible immunity to fight against pathogens. The output of these defense strategies is highly associated with host development and morphology. Successful pathogens utilize various virulence strategies to colonize, retrieve nutrients, and cause disease. In addition to the overall defense-growth balance, the host-pathogen interactions often lead to changes in the development of specific tissues/organs. In this review, we focus on recent advances in understanding the molecular mechanisms of pathogen-induced changes in plants' development. We discuss that changes in host development could be a target of pathogen virulence strategies or an active defense strategy of plants. Current and ongoing research about how pathogens shape plant development to increase their virulence and causes diseases could give us novel views on plant disease control.

Lipid transfer proteins involved in plant-pathogen interactions and their molecular mechanisms

Nonspecific lipid transfer proteins (LTPs) are small, cysteine-rich proteins that play numerous functional roles in plant growth and development, including cutin wax formation, pollen tube adhesion, cell expansion, seed development, germination, and adaptation to changing environmental conditions. LTPs contain eight conserved cysteine residues and a hydrophobic cavity that provides a wide variety of lipid-binding specificities. As members of the pathogenesis-related protein 14 family (PR14), many LTPs inhibit fungal or bacterial growth, and act as positive regulators in plant disease resistance. Over the past decade, these essential immunity-related roles of LTPs in plant immune processes have been documented in a growing body of literature. In this review, we summarize the roles of LTPs in plant-pathogen interactions, emphasizing the underlying molecular mechanisms in plant immune responses and specific LTP functions.

L-Aspartate and L-Glutamine Inhibit Beta-Aminobutyric Acid-Induced Resistance in Tomatoes

Plant diseases caused by pathogens lead to economic and agricultural losses, while plant resistance is defined by robustness and timing of defence response. Exposure to microbial-associated molecular patterns or specific chemical compounds can promote plants into a primed state with more robust defence responses. β-aminobutyric acid (BABA) is an endogenous stress metabolite that induces resistance, thereby protecting various plants' diverse stresses by induction of non-canonical activity after binding into aspartyl-tRNA synthetase (AspRS). In this study, by integrating BABA-induced changes in selected metabolites and transcript data, we describe the molecular processes involved in BABA-induced resistance (BABA-IR) in tomatoes. BABA significantly restricted the growth of the pathogens P. syringae pv. tomato DC3000 and was related to the accumulation of transcripts for pathogenesis-related proteins and jasmonic acid signalling but not salicylic acid signalling in Arabidopsis. The resistance was considerably reduced by applying amino acids L-Asp and L-Gln when L-Gln prevents general amino acid inhibition in plants. Analysis of amino acid changes suggests that BABA-IR inhibition by L-Asp is due to its rapid metabolisation to L-Gln and not its competition with BABA for the aspartyl-tRNA synthetase (AspRS) binding site. Our results showed differences between the effect of BABA on tomatoes and other model plants. They highlighted the importance of comparative studies between plants of agronomic interest subjected to treatment with BABA.

Stomatal morphological variation contributes to global ecological adaptation and diversification of Brassica napus

Stomatal density and guard cell length of 274 global core germplasms of rapeseed reveal that the stomatal morphological variation contributes to global ecological adaptation and diversification of Brassica napus. Stomata are microscopic structures of plants for the regulation of CO₂ assimilation and transpiration. Stomatal morphology has changed substantially in the adaptation to the external environment during land plant evolution. Brassica napus is a major crop to produce oil, livestock feed and biofuel in the world. However, there are few studies on the regulatory genes controlling stomatal development and their interaction with environmental factors as well as the genetic mechanism of adaptive variation in B. napus. Here, we characterized stomatal density (SD) and guard cell length (GL) of 274 global core germplasms at seedling stage. It was found that among the significant phenotypic variation, European germplasms are mostly winter rapeseed with high stomatal density and small guard cell length. However, the germplasms from Asia (especially China) are semi-winter rapeseed, which is characterized by low stomatal density and large guard cell length. Through selective sweep analysis and homology comparison, we identified several candidate genes related to stomatal density and guard cell length, including Epidermal Patterning Factor2 (EPF2; BnaA09g23140D), Epidermal Patterning Factor Like4 (EPFL4; BnaC01g22890D) and Suppressor of LLP1 (SOL1 BnaC01g22810D). Haplotype and phylogenetic analysis showed that natural variation in EPF2, EPFL4 and SOL1 is closely associated with the winter, spring, and semi-winter rapeseed ecotypes. In summary, this study demonstrated for the first time the relation between stomatal phenotypic variation and ecological adaptation in rapeseed, which is useful for future molecular breeding of rapeseed in the context of evolution and domestication of key stomatal traits and global climate change.

The cell-type specific role of Arabidopsis bZIP59 transcription factor in plant immunity

Stomatal movement participates in plant immunity by directly affecting the invasion of bacteria, but the genes that regulate stomatal immunity have not been well identified. Here, we characterised the function of the bZIP59 transcription factor from Arabidopsis thaliana, which is constitutively expressed in guard cells. The bzip59 mutant is partially impaired in stomatal closure induced by Pseudomonas syringae pv. tomato strain (Pst) DC3000 and is more susceptible to Pst DC3000 infection. By contrast, the line overexpressing bZIP59 enhances resistance to Pst DC3000 infection. Furthermore, the bzip59 mutant is also partially impaired in stomatal closure induced by flagellin flg22 derived from Pst DC3000, and epistasis analysis revealed that bZIP59 acts upstream of reactive oxygen species (ROS) and nitric oxide (NO) and downstream of salicylic acid signalling in flg22-induced stomatal closure. In addition, the bzip59 mutant showed resistance and sensitivity to Sclerotinia sclerotiorum and Tobacco mosaic virus that do not invade through stomata, respectively. Collectively, our results demonstrate that bZIP59 plays an important role in the stomatal immunity and reveal that the same transcription factor can positively and negatively regulate disease resistance against different pathogens.

Protein glycosylation changes during systemic acquired resistance in Arabidopsis thaliana

N-glycosylation, an important post-translational modification of proteins in all eukaryotes, has been clearly shown to be involved in numerous diseases in mammalian systems. In contrast, little is known regarding the role of protein N-glycosylation in plant defensive responses to pathogen infection. We identified, for the first time, glycoproteins related to systemic acquired resistance (SAR) in an Arabidopsis thaliana model, using a glycoproteomics platform based on high-resolution mass spectrometry. 407 glycosylation sites corresponding to 378 glycopeptides and 273 unique glycoproteins were identified. 65 significantly changed glycoproteins with 80 N-glycosylation sites were detected in systemic leaves of SAR-induced plants, including numerous GDSL-like lipases, thioglucoside glucohydrolases, kinases, and glycosidases. Functional enrichment analysis revealed that significantly changed glycoproteins were involved mainly in N-glycan biosynthesis and degradation, phenylpropanoid biosynthesis, cutin and wax biosynthesis, and plant-pathogen interactions. Comparative analysis of glycoproteomics and proteomics data indicated that glycoproteomics analysis is an efficient method for screening proteins associated with SAR. The present findings clarify glycosylation status and sites of A. thaliana proteins, and will facilitate further research on roles of glycoproteins in SAR induction.

Preharvest UV-C Hormesis Induces Key Genes Associated With Homeostasis, Growth and Defense in Lettuce Inoculated With Xanthomonas campestris pv. vitians

Preharvest application of hormetic doses of ultraviolet-C (UV-C) generates beneficial effects in plants. In this study, within 1 week, four UV-C treatments of 0.4 kJ/m2 were applied to 3-week-old lettuce seedlings. The leaves were inoculated with a virulent strain of Xanthomonas campestris pv. vitians (Xcv) 48 h after the last UV-C application. The extent of the disease was tracked over time and a transcriptomic analysis was performed on lettuce leaf samples. Samples of lettuce leaves, from both control and treated groups, were taken at two different times corresponding to T2, 48 h after the last UV-C treatment and T3, 24 h after inoculation (i.e., 72 h after the last UV-C treatment). A significant decrease in disease severity between the UV-C treated lettuce and the control was observed on days 4, 8, and 14 after pathogen inoculation. Data from the transcriptomic study revealed, that in response to the effect of UV-C alone and/or UV-C + Xcv, a total of 3828 genes were differentially regulated with fold change (|log2-FC|) > 1.5 and false discovery rate (FDR) < 0.05. Among these, of the 2270 genes of known function 1556 were upregulated and 714 were downregulated. A total of 10 candidate genes were verified by qPCR and were generally consistent with the transcriptomic results. The differentially expressed genes observed in lettuce under the conditions of the present study were associated with 14 different biological processes in the plant. These genes are involved in a series of metabolic pathways associated with the ability of lettuce treated with hormetic doses of UV-C to resume normal growth and to defend themselves against potential stressors. The results indicate that the hormetic dose of UV-C applied preharvest on lettuce in this study, can be considered as an eustress that does not interfere with the ability of the treated plants to carry on a set of key physiological processes namely: homeostasis, growth and defense.

Stomata and ROS changes during Botrytis elliptica infection in diploid and tetraploid Lilium rosthornii Diels

Polyploid plants often show improved resistance against many diseases, but whether they show increased resistance to grey mould, a devastating disease caused by Botrytis spp. fungi, is seldom reported. Stomata and reactive oxygen species (ROS) play dual roles in defence against grey mould, and it is unclear how their roles change after polyploidization. We addressed these questions in diploid and colchicine-induced Lilium rosthornii after B. elliptica infection. Tetraploids were less susceptible to grey mould, with lower morality rates in naturally infected plants. Before the stomata closed in artificially infected leaves, tetraploids, with larger stomatal apertures, were more easily invaded by the pathogen than diploids. However, the lesion area increased more slowly in tetraploids than in diploids, which may be explained by three causes based on histological and physiological characteristics. First, the pathogen required more time to penetrate the epidermis and closed stomata in tetraploids than in diploids. Second, the pathogen penetrated the reopened stomata more easily than the epidermis, and stomatal density was lower in tetraploids than in diploids. Third, tetraploids showed faster ROS accumulation, a more effective ROS-scavenging system and less malondialdehyde (MDA) accumulation than diploids. Stomatal starch and abnormal guard cell nuclei were present in the infected leaves. This phenomenon may be caused by oxalic acid, a pathogenic factor for many pathogens that promotes stomatal starch degradation and stomatal reopening in Sclerotinia spp., a pathogen closely related to Botrytis spp. This suggestion was primarily confirmed by immersing healthy leaves in oxalic acid solution.

Arabidopsis thaliana Response to Extracellular DNA: Self Versus Nonself Exposure

The inhibitory effect of extracellular DNA (exDNA) on the growth of conspecific individuals was demonstrated in different kingdoms. In plants, the inhibition has been observed on root growth and seed germination, demonstrating its role in plant-soil negative feedback. Several hypotheses have been proposed to explain the early response to exDNA and the inhibitory effect of conspecific exDNA. We here contribute with a whole-plant transcriptome profiling in the model species Arabidopsis thaliana exposed to extracellular self- (conspecific) and nonself- (heterologous) DNA. The results highlight that cells distinguish self- from nonself-DNA. Moreover, confocal microscopy analyses reveal that nonself-DNA enters root tissues and cells, while self-DNA remains outside. Specifically, exposure to self-DNA limits cell permeability, affecting chloroplast functioning and reactive oxygen species (ROS) production, eventually causing cell cycle arrest, consistently with macroscopic observations of root apex necrosis, increased root hair density and leaf chlorosis. In contrast, nonself-DNA enters the cells triggering the activation of a hypersensitive response and evolving into systemic acquired resistance. Complex and different cascades of events emerge from exposure to extracellular self- or nonself-DNA and are discussed in the context of Damage- and Pathogen-Associated Molecular Patterns (DAMP and PAMP, respectively) responses.

Overexpression of the CaHB12 transcription factor in cotton (Gossypium hirsutum) improves drought tolerance

The Coffea arabica HB12 gene (CaHB12), which encodes a transcription factor belonging to the HD-Zip I subfamily, is upregulated under drought, and its constitutive overexpression (35S:CaHB12^OX) improves the Arabidopsis thaliana tolerance to drought and salinity stresses. Herein, we generated transgenic cotton events constitutively overexpressing the CaHB12 gene, characterized these events based on their increased tolerance to water deficit, and exploited the gene expression level from the CaHB12 network. The segregating events Ev8.29.1, Ev8.90.1, and Ev23.36.1 showed higher photosynthetic yield and higher water use efficiency under severe water deficit and permanent wilting point conditions compared to wild-type plants. Under well-irrigated conditions, these three promising transformed events showed an equivalent level of Abscisic acid (ABA) and decreased Indole-3-acetic acid (IAA) accumulation, and a higher putrescine/(spermidine + spermine) ratio in leaf tissues was found in the progenies of at least two transgenic cotton events compared to non-transgenic plants. In addition, genes that are considered as modulated in the A. thaliana 35S:CaHB12^OX line were also shown to be modulated in several transgenic cotton events maintained under field capacity conditions. The upregulation of GhPP2C and GhSnRK2 in transgenic cotton events maintained under permanent wilting point conditions suggested that CaHB12 might act enhancing the ABA-dependent pathway. All these data confirmed that CaHB12 overexpression improved the tolerance to water deficit, and the transcriptional modulation of genes related to the ABA signaling pathway or downstream genes might enhance the defense responses to drought. The observed decrease in IAA levels indicates that CaHB12 overexpression can prevent leaf abscission in plants under or after stress. Thus, our findings provide new insights on CaHB12 gene and identify several promising cotton events for conducting field trials on water deficit tolerance and agronomic performance.

Acid or base? How do plants regulate the ecology of their phylloplane?

Plants interface with and modify the external environment across their surfaces, and in so doing, can control or mitigate the impacts of abiotic stresses and also mediate their interactions with other organisms. Botanically, it is known that plant roots have a multi-faceted ability to modify rhizosphere conditions like pH, a factor with a large effect on a plant's biotic interactions with microbes. But plants can also modify pH levels on the surfaces of their leaves. Plants can neutralize acid rain inputs in a period of hours, and either acidify or alkalinize the pH of neutral water droplets in minutes. The pH of the phylloplane-that is, the outermost surface of the leaf-varies across species, from incredibly acidic (carnivorous plants: as low as pH 1) to exceptionally alkaline (species in the plant family, Malvaceae, up to pH 11). However, most species mildly acidify droplets on the phylloplane by 1.5 orders of magnitude in pH. Just as rhizosphere pH helps shape the plant microbiome and is known to influence belowground interactions, so too can phylloplane pH influence aboveground interactions in plant canopies. In this review, we discuss phylloplane pH regulation from the physiological, molecular, evolutionary, and ecological perspectives and address knowledge gaps and identify future research directions.

Leaf temperature responses to ABA and dead bacteria in wheat and Arabidopsis

Stomatal densities, aperture openness and their responsiveness to environmental change determine plant water loss and regulate entry of pathogens. Stomatal responsiveness is usually assessed on restricted areas of leaves or isolated epidermal peels floated in solution. Analyzing these responses in the whole plant context could give valuable additional information, for example on the role of mesophyll in stomatal responses. We analyzed stomatal responses to the phytohormone abscisic acid (ABA) and pathogenic elicitors in intact plants by dynamic measurement of leaf temperature. We tested whether ABA-induced stomatal closure in wheat requires external nitrate and whether bacterial elicitor-induced stomatal closure can be detected by dynamic thermal imaging in intact Arabidopsis. We found that wheat was hypersensitive to all applied treatments, as even mock-treated leaves showed a strong increase in leaf temperature. Nevertheless, ABA activated stomatal closure in wheat independent of exogenous nitrate. Pathogenic elicitors triggered a fast and transient increase in leaf temperature in intact Arabidopsis, indicating short-term stomatal closure. The data suggest that the dynamics of pathogen-induced stomatal closure is different in whole plants compared to epidermal peels, where elicitor-induced stomatal closure persists longer. We propose that dynamic thermal imaging could be applied to address the effect of pathogenic elicitors on stomatal behavior in whole plants to complement detached sample assays and gain a better understanding of stomatal immunity.

Rice Stomatal Mega-Papillae Restrict Water Loss and Pathogen Entry

Rice (Oryza sativa) is a water-intensive crop, and like other plants uses stomata to balance CO₂ uptake with water-loss. To identify agronomic traits related to rice stomatal complexes, an anatomical screen of 64 Thai and 100 global rice cultivars was undertaken. Epidermal outgrowths called papillae were identified on the stomatal subsidiary cells of all cultivars. These were also detected on eight other species of the Oryza genus but not on the stomata of any other plant species we surveyed. Our rice screen identified two cultivars that had "mega-papillae" that were so large or abundant that their stomatal pores were partially occluded; Kalubala Vee had extra-large papillae, and Dharia had approximately twice the normal number of papillae. These were most accentuated on the flag leaves, but mega-papillae were also detectable on earlier forming leaves. Energy dispersive X-Ray spectrometry revealed that silicon is the major component of stomatal papillae. We studied the potential function(s) of mega-papillae by assessing gas exchange and pathogen infection rates. Under saturating light conditions, mega-papillae bearing cultivars had reduced stomatal conductance and their stomata were slower to close and re-open, but photosynthetic assimilation was not significantly affected. Assessment of an F₃ hybrid population treated with Xanthomonas oryzae pv. oryzicola indicated that subsidiary cell mega-papillae may aid in preventing bacterial leaf streak infection. Our results highlight stomatal mega-papillae as a novel rice trait that influences gas exchange, stomatal dynamics, and defense against stomatal pathogens which we propose could benefit the performance of future rice crops.

Protein Phosphorylation Changes During Systemic Acquired Resistance in Arabidopsis thaliana

Systemic acquired resistance (SAR) in plants is a defense response that provides resistance against a wide range of pathogens at the whole-plant level following primary infection. Although the molecular mechanisms of SAR have been extensively studied in recent years, the role of phosphorylation that occurs in systemic leaves of SAR-induced plants is poorly understood. We used a data-independent acquisition (DIA) phosphoproteomics platform based on high-resolution mass spectrometry in an Arabidopsis thaliana model to identify phosphoproteins related to SAR establishment. A total of 8011 phosphorylation sites from 3234 proteins were identified in systemic leaves of Pseudomonas syringae pv. maculicola ES4326 (Psm ES4326) and mock locally inoculated plants. A total of 859 significantly changed phosphoproteins from 1119 significantly changed phosphopeptides were detected in systemic leaves of Psm ES4326 locally inoculated plants, including numerous transcription factors and kinases. A variety of defense response-related proteins were found to be differentially phosphorylated in systemic leaves of Psm ES4326 locally inoculated leaves, suggesting that these proteins may be functionally involved in SAR through phosphorylation or dephosphorylation. Significantly changed phosphoproteins were enriched mainly in categories related to response to abscisic acid, regulation of stomatal movement, plant-pathogen interaction, MAPK signaling pathway, purine metabolism, photosynthesis-antenna proteins, and flavonoid biosynthesis. A total of 28 proteins were regulated at both protein and phosphorylation levels during SAR. RT-qPCR analysis revealed that changes in phosphorylation levels of proteins during SAR did not result from changes in transcript abundance. This study provides comprehensive details of key phosphoproteins associated with SAR, which will facilitate further research on the molecular mechanisms of SAR.

A Stomatal Model of Anatomical Tradeoffs Between Gas Exchange and Pathogen Colonization

Stomatal pores control leaf gas exchange and are one route for infection of internal plant tissues by many foliar pathogens, setting up the potential for tradeoffs between photosynthesis and pathogen colonization. Anatomical shifts to lower stomatal density and/or size may also limit pathogen colonization, but such developmental changes could permanently reduce the gas exchange capacity for the life of the leaf. I developed and analyzed a spatially explicit model of pathogen colonization on the leaf as a function of stomatal size and density, anatomical traits which partially determine maximum rates of gas exchange. The model predicts greater stomatal size or density increases the probability of colonization, but the effect is most pronounced when the fraction of leaf surface covered by stomata is low. I also derived scaling relationships between stomatal size and density that preserves a given probability of colonization. These scaling relationships set up a potential anatomical conflict between limiting pathogen colonization and minimizing the fraction of leaf surface covered by stomata. Although a connection between gas exchange and pathogen defense has been suggested empirically, this is the first mathematical model connecting gas exchange and pathogen defense via stomatal anatomy. A limitation of the model is that it does not include variation in innate immunity and stomatal closure in response to pathogens. Nevertheless, the model makes predictions that can be tested with experiments and may explain variation in stomatal size and density among plants. The model is generalizable to many types of pathogens, but lacks significant biological realism that may be needed for precise predictions.

Global Dynamic Molecular Profiling of Stomatal Lineage Cell Development by Single-Cell RNA Sequencing

The regulation of stomatal lineage cell development has been extensively investigated. However, a comprehensive characterization of this biological process based on single-cell transcriptome analysis has not yet been reported. In this study, we performed RNA sequencing on 12 844 individual cells from the cotyledons of 5-day-old Arabidopsis seedlings. We identified 11 cell clusters corresponding mostly to cells at specific stomatal developmental stages using a series of marker genes. Comparative analysis of genes with the highest variable expression among these cell clusters revealed transcriptional networks that regulate development from meristemoid mother cells to guard mother cells. Examination of the developmental dynamics of marker genes via pseudo-time analysis revealed potential interactions between these genes. Collectively, our study opens the door for understanding how the identified novel marker genes participate in the regulation of stomatal lineage cell development.

Stomata and Sporophytes of the Model Moss Physcomitrium patens

Mosses are an ancient land plant lineage and are therefore important in studying the evolution of plant developmental processes. Here, we describe stomatal development in the model moss species Physcomitrium patens (previously known as Physcomitrella patens) over the duration of sporophyte development. We dissect the molecular mechanisms guiding cell division and fate and highlight how stomatal function might vary under different environmental conditions. In contrast to the asymmetric entry divisions described in Arabidopsis thaliana, moss protodermal cells can enter the stomatal lineage directly by expanding into an oval shaped guard mother cell (GMC). We observed that when two early stage P. patens GMCs form adjacently, a spacing division can occur, leading to separation of the GMCs by an intervening epidermal spacer cell. We investigated whether orthologs of Arabidopsis stomatal development regulators are required for this spacing division. Our results indicated that bHLH transcription factors PpSMF1 and PpSCRM1 are required for GMC formation. Moreover, the ligand and receptor components PpEPF1 and PpTMM are also required for orientating cell divisions and preventing single or clustered early GMCs from developing adjacent to one another. The identification of GMC spacing divisions in P. patens raises the possibility that the ability to space stomatal lineage cells could have evolved before mosses diverged from the ancestral lineage. This would have enabled plants to integrate stomatal development with sporophyte growth and could underpin the adoption of multiple bHLH transcription factors and EPF ligands to more precisely control stomatal patterning in later diverging plant lineages. We also observed that when P. patens sporophyte capsules mature in wet conditions, stomata are typically plugged whereas under drier conditions this is not the case; instead, mucilage drying leads to hollow sub-stomatal cavities. This appears to aid capsule drying and provides further evidence for early land plant stomata contributing to capsule rupture and spore release.

Guard Cell Metabolism and Stomatal Function

The control of gaseous exchange between the leaf and external atmosphere is governed by stomatal conductance (g_s); therefore, stomata play a critical role in photosynthesis and transpiration and overall plant productivity. Stomatal conductance is determined by both anatomical features and behavioral characteristics. Here we review some of the osmoregulatory pathways in guard cell metabolism, genes and signals that determine stomatal function and patterning, and the recent work that explores coordination between g_s and carbon assimilation (A) and the influence of spatial distribution of functional stomata on underlying mesophyll anatomy. We also evaluate the current literature on mesophyll-driven signals that may coordinate stomatal behavior with mesophyll carbon assimilation and explore stomatal kinetics as a possible target to improve A and water use efficiency. By understanding these processes, we can start to provide insight into manipulation of these regulatory pathways to improve stomatal behavior and identify novel unexploited targets for altering stomatal behavior and improving crop plant productivity.

Regulation of stomatal development by stomatal lineage miRNAs

Stomatal cell fate and patterning, which are regulated by key transcriptional factors and intercellular communications, are critical for plant growth and survival. The known regulators of stomatal development do not appear to have microRNAs (miRNAs) regulating them. Thus, it remains elusive as to whether and how miRNAs are involved in stomatal development. This study identifies stomatal lineage miRNAs including developmental stage-specific miRNAs. Genetic analysis shows that stomatal lineage miRNAs positively or negatively regulate stomatal formation and patterning. Moreover, biological processes modulated by stomatal lineage miRNAs reveal previously unknown regulatory pathways in stomatal development, indicating that miRNAs function as a critical element of stomatal development. These results provide a resource for guiding the study of stomatal development.

Stomata in the plant epidermis play a critical role in growth and survival by controlling gas exchange, transpiration, and immunity to pathogens. Plants modulate stomatal cell fate and patterning through key transcriptional factors and signaling pathways. MicroRNAs (miRNAs) are known to contribute to developmental plasticity in multicellular organisms; however, no miRNAs appear to target the known regulators of stomatal development. It remains unclear as to whether miRNAs are involved in stomatal development. Here, we report highly dynamic, developmentally stage-specific miRNA expression profiles from stomatal lineage cells. We demonstrate that stomatal lineage miRNAs positively and negatively regulate stomatal formation and patterning to avoid clustered stomata. Target prediction of stomatal lineage miRNAs implicates potential cellular processes in stomatal development. We show that miR399-mediated PHO2 regulation, involved in phosphate homeostasis, contributes to the control of stomatal development. Our study demonstrates that miRNAs constitute a critical component in the regulatory mechanisms controlling stomatal development.