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. 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. Plants have evolved a superfamily of receptor-like kinases (RLKs; proteins with an extracellular putative receptor domain, a single transmembrane domain, and a cytoplasmic kinase domain; Table 1) to perceive and transmit external and endogenous signals to properly regulate diverse processes, including plant development and immunity1Shiu S.-H. Bleecker A.B. Receptor-like kinases from Arabidopsis form a monophyletic gene family related to animal receptor kinases.Proc. Natl. Acad. Sci. USA. 2001; 98: 10763-10768Crossref PubMed Scopus (1088) Google Scholar,2Shiu S.H. Bleecker A.B. Expansion of the receptor-like kinase/Pelle gene family and receptor-like proteins in Arabidopsis.Plant Physiol. 2003; 132: 530-543Crossref PubMed Scopus (633) Google Scholar. How the specificity and conservation of RLK signaling are determined is a fundamental question. Based on various ectodomains, plant RLKs are classified into different subfamilies, among which the leucine-rich repeat RLKs (LRR-RLKs) comprise the largest subfamily of plant RLKs3Fischer I. Diévart A. Droc G. Dufayard J.-F. Chantret N. Evolutionary dynamics of the leucine-rich repeat receptor-like kinase (LRR-RLK) subfamily in angiosperms.Plant Physiol. 2016; 170: 1595-1610Crossref PubMed Scopus (74) Google Scholar. Here, we define those RLKs that are known to directly bind ligands as RKs. Prior studies have revealed the molecular and biochemical mechanisms of LRR-RK signal perception, activation, transduction, and attenuation in stomatal development and immunity in Arabidopsis thaliana (hereafter, Arabidopsis)4Torii K.U. Leucine-rich repeat receptor kinases in plants: structure, function, and signal transduction pathways.Int. Rev. Cytol. 2004; 234: 1-46Crossref PubMed Scopus (265) Google Scholar,5Couto D. Zipfel C. Regulation of pattern recognition receptor signalling in plants.Nat. Rev. Immunol. 2016; 16: 537-552Crossref PubMed Scopus (686) Google Scholar,6Herrmann A. Torii K.U. Shouting out loud: signaling modules in the regulation of stomatal development.Plant Physiol. 2021; 185: 765-780Crossref PubMed Google Scholar,7Torii K.U. Plant signaling: Peptide–receptor pair re-opens stomata after pathogen infection.Curr. Biol. 2022; 32: R783-R786Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar. Stomata, therefore, become an appropriate model system to compare the commonalities and differences between the RK signaling in development and immunity, and to further explore their potential signal crosstalk. Here, we first survey the latest knowledge on the signal transduction components mediating stomatal development and/or immunity (see Table 1 for information on the components). We then synthesize this knowledge to explore the concept of signal specificity and crosstalk.Table 1List of the signaling components involved in stomatal development and/or immunity described in this review. Components are listed in chronological order of the pathway.ClassNameLocusFunctionEPIDERMAL PATTERNING FACTOREPF2AT1G34245EFP2 binds to ERECTA to control asymmetric cell divisions during stomatal development.CLAVATA3/ESR-RELATED (CLE) class signaling peptidesCLE9AT1G26600CLE9/10 regulate stomatal cell lineage development.CLE10AT1G69320SMALL PHYTOCYTOKINES REGULATING DEFENSE AND WATER LOSSSCREW1AT1G06135SCREWs are pathogen-inducible phytocytokines and protect plants from infection.SCREW2AT2G31345SCREW3AT1G06137SCREW4AT2G31335LRR-RKERECTAAT2G26330ERf RLKs govern the initial decision of protodermal cells to either divide proliferatively to produce pavement cells or divide asymmetrically to generate stomatal complexes.ERL1AT5G62230ERL2AT5G07180HSL1AT1G28440HSL1 is a CLE9/10 receptor that regulates stomatal cell lineage development.FLS2AT5G46330FLS2 perceives the bacterial flagellin and the cognate peptide flg22 in the immune response.NUTAT5G25930NUT is a SCREWs receptor that reopens stomata in immunity.SERK1AT1G71830SERKs are the co-receptors for a wide variety of LRR-RLKs and LRR-RLPs, including ERf, HSL1, FLS2, and NUT.SERK2AT1G34210BAK1AT4G33430SERK4AT2G13790LRR-RLPTMMAT1G80080TMM forms a constitutive receptor complex with ERf, creating a binding pocket for EPF1/2.RLCKBSK1AT4G35230BSK1 acts as part of a canonical signaling pathway downstream of various SERK-dependent receptor kinase pathways involved in plant growth, innate immunity, and abiotic stress response.BSK2AT5G46570BIK1AT2G39660BIK1 directly connects pattern recognition receptors (PRRs) to various downstream components.Protein phosphataseBSU1AT1G03445BSU1 is involved in BR signaling, stomatal development, and immunity.SNF1-related protein kinase 2 (SnRK2)SnRK2.2AT3G50500SnRK2.2/2.3/2.6 mediate the ABA-dependent suppression of stomatal production. Thereby, SnRK2.6 is involved in stomatal closure.SnRK2.3AT5G66880SnRK2.6 (OST1)AT4G33950MAPKKKYDAAT1G63700YDA mediates signal from ERf-TMM-SERKs receptor complex to regulate plant development.MAPKKK3AT1G53570MAPKKK3/5 function redundantly to activate MPK3/6 downstream of multiple RLKs, such as PRRs.MAPKKK5AT5G66850MEKK1AT4G08500MEKK1 mediates the activation of MAPK during the immune response.MAPKKMKK4AT1G51660MKK4/5 function redundantly to activate MPK3/6 downstream of MAPKKKs, such as YDA and MAPKKK3/5.MKK5AT3G21220MAPKMPK3AT3G45640MPK3/6 act downstream of MAPKKs (such as MKK4/5) to regulate PTI and stomatal development.MPK6AT2G43790MPK4AT4G01370MPK4 regulates during plant immunity.Plant U-box Protein E3 ligase (PUB)PUB12AT2G28830PUB12/13 ubiquitinate various RLKs, such as the immune receptor FLS2.PUB13AT3G46510PUB30AT3G49810PUB30/31 ubiquitinate ERECTA for eventual degradation.PUB31AT5G65920Clade-B-type-2C Protein Phosphatase (PP2C)AP2C1AT2G30020AP2C1 and PP2C5 act as a MAPK phosphatase that negatively regulates MPK3/4/6. PP2C5 also induces ectopic proliferation of epidermal cells leading to stomata development.PP2C5 (AP2C3)AT2G40180MAP kinase phosphatase (MKP)MKP1AT3G55270MKP1/2 act as a MAPK phosphatase that negatively regulates MPK3/6 during PTI.MKP2AT3G06110Protein Phosphatase 2A (PP2A)PP2A-A1AT1G25490PP2A phosphatases act as positive regulators of stomatal development, as well as negative regulators of the activation of PRR complexes by modulating the phosphostatus of BAK1.PP2A-A2AT3G25800PP2A-A3AT1G13320PP2A-C1AT1G59830PP2A-C3AT2G42500PP2A-C4AT3G58500Calcium channelOSCA1.3AT1G11960OSCA1.3/1.7 are BIK1-activated calcium-permeable channels during PTI.OSCA1.7AT4G02900Anion channelSLAC1AT1G12480The activation of SLAC1 by OST1 results in the exit of anions from the cytoplasm of the guard cells and ultimately leads to stomata closure.NADPH-oxidaseRBOHDAT5G47910RBOHD is required for pattern-induced reactive oxygen species (ROS) production.Polarity proteinBASLAT5G60880BASL is a regulator of asymmetric divisions during stomatal development.Villin (involved in actin filament bundling)VLN3AT3G57410VLN3 acts downstream of MPK3/6 and regulates actin bundle formation in the cortical actin array during stomatal immunity.Basic helix-loop-helix (bHLH) transcription factorSPCHAT5G53210SPCH acts downstream of MPK3/6 and drives stomatal initiation during stomatal development. Open table in a new tab Stomata are generated through a series of stereotypical cell-state transitions from the developing epidermis6Herrmann A. Torii K.U. Shouting out loud: signaling modules in the regulation of stomatal development.Plant Physiol. 2021; 185: 765-780Crossref PubMed Google Scholar,8Torii K.U. Mix-and-match: ligand–receptor pairs in stomatal development and beyond.Trends Plant Sci. 2012; 17: 711-719Abstract Full Text Full Text PDF PubMed Scopus (88) Google Scholar. ERECTA-family (ERf) LRR-RKs, ERECTA (ER), ERECTA-LIKE 1 (ERL1), and ERL2, and the LRR-receptor like protein (RLP; membrane-localized proteins similar to RLKs, but without the cytoplasmic kinase domain) TOO MANY MOUTHS (TMM) form heterodimers to enforce proper stomatal spacing and patterning9Lee J.S. Hnilova M. Maes M. Lin Y.-C.L. Putarjunan A. Han S.-K. Avila J. Torii K.U. Competitive binding of antagonistic peptides fine-tunes stomatal patterning.Nature. 2015; 522: 439-443Crossref PubMed Scopus (179) Google Scholar,10Lin G. Zhang L. Han Z. Yang X. Liu W. Li E. Chang J. Qi Y. Shpak E.D. Chai J. A receptor-like protein acts as a specificity switch for the regulation of stomatal development.Genes Dev. 2017; 31: 927-938Crossref PubMed Scopus (77) Google Scholar. The ERf–TMM receptor complexes perceive a family of secreted cysteine-rich peptides, EPIDERMAL PATTERNING FACTOR (EPF)/EPF-LIKE (EPFL)8Torii K.U. Mix-and-match: ligand–receptor pairs in stomatal development and beyond.Trends Plant Sci. 2012; 17: 711-719Abstract Full Text Full Text PDF PubMed Scopus (88) Google Scholar,11Hara K. Kajita R. Torii K.U. Bergmann D.C. Kakimoto T. The secretory peptide gene EPF1 enforces the stomatal one-cell-spacing rule.Genes Dev. 2007; 21: 1720-1725Crossref PubMed Scopus (379) Google Scholar,12Hara K. Yokoo T. Kajita R. Onishi T. Yahata S. Peterson K.M. Torii K.U. Kakimoto T. Epidermal cell density is autoregulated via a secretory peptide, EPIDERMAL PATTERNING FACTOR 2 in Arabidopsis leaves.Plant Cell Physiol. 2009; 50: 1019-1031Crossref PubMed Scopus (267) Google Scholar,13Sugano S.S. Shimada T. Imai Y. Okawa K. Tamai A. Mori M. Hara-Nishimura I. Stomagen positively regulates stomatal density in Arabidopsis.Nature. 2010; 463: 241-244Crossref PubMed Scopus (338) Google Scholar (Figure 1). Among them, EPF2 negatively regulates stomatal development by acting as a ligand to activate ERfs at the early proliferating stage of stomatal cell lineages, whereas STOMAGEN acts as a positive regulator by competing with EPF2 for the receptor binding9Lee J.S. Hnilova M. Maes M. Lin Y.-C.L. Putarjunan A. Han S.-K. Avila J. Torii K.U. Competitive binding of antagonistic peptides fine-tunes stomatal patterning.Nature. 2015; 522: 439-443Crossref PubMed Scopus (179) Google Scholar,10Lin G. Zhang L. Han Z. Yang X. Liu W. Li E. Chang J. Qi Y. Shpak E.D. Chai J. A receptor-like protein acts as a specificity switch for the regulation of stomatal development.Genes Dev. 2017; 31: 927-938Crossref PubMed Scopus (77) Google Scholar. Recently, another ligand–receptor module, which consists of CLAVATA3/ENDOSPERM SURROUNDING REGION-RELATED 9/10 (CLE9/10) and HAESA-LIKE1 (HSL1), was shown to be also involved in stomatal development. Notably, the CLE9/10–HSL1 and EPFs–ERf modules act independently of each other14Qian P. Song W. Yokoo T. Minobe A. Wang G. Ishida T. Sawa S. Chai J. Kakimoto T. The CLE9/10 secretory peptide regulates stomatal and vascular development through distinct receptors.Nat. Plants. 2018; 4: 1071-1081Crossref PubMed Scopus (83) Google Scholar,15Roman A.-O. Jimenez-Sandoval P. Augustin S. Broyart C. Hothorn L.A. Santiago J. HSL1 and BAM1/2 impact epidermal cell development by sensing distinct signaling peptides.Nat. Commun. 2022; 13: 876Crossref PubMed Scopus (11) Google Scholar. Once fully differentiated, mature stomata adjust the apertures for efficient gas exchange and transpiration. They are also exploited by pathogens as entry gates into inner leaf tissues. As a coping strategy, plants have evolved immune responses that include stomatal closure triggered by pathogen-associated molecular patterns (PAMPs), as well as stomatal reopening to inhibit pathogen colonization by enhancing leaf transpiration16Melotto M. Underwood W. Koczan J. Nomura K. He S.Y. Plant stomata function in innate immunity against bacterial invasion.Cell. 2006; 126: 969-980Abstract Full Text Full Text PDF PubMed Scopus (1325) Google Scholar,17Liu Z. Hou S. Rodrigues O. Wang P. Luo D. Munemasa S. Lei J. Liu J. Ortiz-Morea F.A. Wang X. et al.Phytocytokine signalling reopens stomata in plant immunity and water loss.Nature. 2022; 605: 332-339Crossref PubMed Scopus (22) Google Scholar. These behaviors of stomatal guard cells in response to pathogens are collectively termed ‘stomatal immunity’16Melotto M. Underwood W. Koczan J. Nomura K. He S.Y. Plant stomata function in innate immunity against bacterial invasion.Cell. 2006; 126: 969-980Abstract Full Text Full Text PDF PubMed Scopus (1325) Google Scholar,18Zhang J. Coaker G. Zhou J.-M. Dong X. Plant immune mechanisms: From reductionistic to holistic points of view.Mol. Plant. 2020; 13: 1358-1378Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar. The LRR-RK pattern recognition receptor (PRR) FLAGELLIN SENSING 2 (FLS2) perceives a conserved 22-amino acid peptide (flg22) from bacterial flagellin19Gómez-Gómez L. Boller T. FLS2: An LRR Receptor–like kinase involved in the perception of the bacterial elicitor flagellin in Arabidopsis.Mol. Cell. 2000; 5: 1003-1011Abstract Full Text Full Text PDF PubMed Google Scholar (Figure 2). The perception of PAMPs (e.g., flg22) triggers stomatal closure, which serves as the first layer of stomatal immunity16Melotto M. Underwood W. Koczan J. Nomura K. He S.Y. Plant stomata function in innate immunity against bacterial invasion.Cell. 2006; 126: 969-980Abstract Full Text Full Text PDF PubMed Scopus (1325) Google Scholar,20Li L. Li M. Yu L. Zhou Z. Liang X. Liu Z. Cai G. Gao L. Zhang X. Wang Y. et al.The FLS2-associated kinase BIK1 directly phosphorylates the NADPH oxidase RbohD to control plant immunity.Cell Host Microbe. 2014; 15: 329-338Abstract Full Text Full Text PDF PubMed Scopus (486) Google Scholar,21Thor K. Jiang S. Michard E. George J. Scherzer S. Huang S. Dindas J. Derbyshire P. Leitão N. DeFalco T.A. et al.The calcium-permeable channel OSCA1.3 regulates plant stomatal immunity.Nature. 2020; 585: 569-573Crossref PubMed Scopus (146) Google Scholar,22Kadota Y. Sklenar J. Derbyshire P. Stransfeld L. Asai S. Ntoukakis V. Jones J.D. Shirasu K. Menke F. Jones A. Zipfel C. Direct regulation of the NADPH oxidase RBOHD by the PRR-associated kinase BIK1 during plant immunity.Mol. Cell. 2014; 54: 43-55Abstract Full Text Full Text PDF PubMed Scopus (566) Google Scholar. As a counteracting strategy, pathogens deploy phytotoxins and effectors to reopen stomata to enter through the apoplast23Melotto M. Zhang L. Oblessuc P.R. He S.Y. Stomatal defense a decade later.Plant Physiol. 2017; 174: 561-571Crossref PubMed Scopus (173) Google Scholar. Once the pathogens enter, a moist apoplastic environment promotes their propagation and further infection. It is therefore beneficial for plants to reopen stomata after infection to reduce the moisture in internal tissues. The perception of PAMPs by PRRs swiftly activates the expression of phytocytokines, which are plant endogenous peptides released into the apoplast to modulate plant immunity24Hou S. Liu D. He P. Phytocytokines function as immunological modulators of plant immunity.Stress Biol. 2021; 1: 8Crossref Scopus (10) Google Scholar. A phytocytokine–receptor pair, SMALL PHYTOCYTOKINES REGULATING DEFENSE AND WATER LOSS (SCREWs, also named CTNIPs) and the LRR-RK PLANT SCREW UNRESPONSIVE RECEPTOR (NUT, also named HAESA-LIKE3, HSL3), is activated, which re-opens stomata17Liu Z. Hou S. Rodrigues O. Wang P. Luo D. Munemasa S. Lei J. Liu J. Ortiz-Morea F.A. Wang X. et al.Phytocytokine signalling reopens stomata in plant immunity and water loss.Nature. 2022; 605: 332-339Crossref PubMed Scopus (22) Google Scholar,25Liu X.-S. Liang C.-C. Hou S.-G. Wang X. Chen D.-H. Shen J.-L. Zhang W. Wang M. The LRR-RLK protein HSL3 regulates stomatal closure and the drought stress response by modulating hydrogen peroxide homeostasis.Front. Plant Sci. 2020; 11: 548034Crossref Scopus (17) Google Scholar,26Rhodes J. Roman A.-O. Bjornson M. Brandt B. Derbyshire P. Wyler M. Schmid M.W. Menke F.L.H. Santiago J. Zipfel C. Perception of a conserved family of plant signalling peptides by the receptor kinase HSL3.eLife. 2022; 11e74687Crossref Scopus (5) Google Scholar (Figure 3). This phytocytokine-induced stomatal re-opening limits pathogen multiplication, serving as a second layer of stomatal immunity. Overall, at the level of ligand-sensing receptors, each RK (or each member of the closely-related RK family) detects specific signal inputs. It has been reported that ERECTA is involved in pathogen resistance27Godiard L. Sauviac L. Torii K.U. Grenon O. Mangin B. Grimsley N.H. Marco Y. ERECTA, an LRR receptor-like kinase protein controlling development pleiotropically affects resistance to bacterial wilt.Plant J. 2003; 36: 353-365Crossref PubMed Scopus (191) Google Scholar,28Sánchez-Rodríguez C. Estévez J.M. Llorente F. Hernández-Blanco C. Jordá L. Pagán I. Berrocal M. Marco Y. Somerville S. Molina A. The ERECTA receptor-like kinase regulates cell wall–mediated resistance to pathogens in Arabidopsis thaliana.Mol. Plant. Microbe Interact. 2009; 22: 953-963Crossref PubMed Scopus (77) Google Scholar. Given that ERfs are not highly expressed after the cell-fate transition from guard mother cell (GMC) to guard cells29Horst R.J. Fujita H. Lee J.S. Rychel A.L. Garrick J.M. Kawaguchi M. Peterson K.M. Torii K.U. Molecular framework of a regulatory circuit initiating two-dimensional spatial patterning of stomatal lineage.PLoS Genet. 2015; 11e1005374Crossref Scopus (63) Google Scholar, it is less likely that they direct stomatal immunity in mature guard cells. Upon ligand perception, the central co-receptors, BRI1-ASSOCIATED RECEPTOR KINASE 1/SOMATIC EMBRYOGENESIS RECEPTOR KINASE (BAK1/SERK) family LRR-RKs, are recruited to ER/ERL–TMM heterodimers and HSL1, respectively14Qian P. Song W. Yokoo T. Minobe A. Wang G. Ishida T. Sawa S. Chai J. Kakimoto T. The CLE9/10 secretory peptide regulates stomatal and vascular development through distinct receptors.Nat. Plants. 2018; 4: 1071-1081Crossref PubMed Scopus (83) Google Scholar,30Meng X. Chen X. Mang H. Liu C. Yu X. Gao X. Torii K.U. He P. Shan L. Differential function of Arabidopsis SERK family receptor-like kinases in stomatal patterning.Curr. Biol. 2015; 25: 2361-2372Abstract Full Text Full Text PDF PubMed Scopus (176) Google Scholar (Figure 1). Similarly, perception of flg22 by FLS2 triggers the interaction between FLS2 with the co-receptors BAK1 (also known as SERK3), leading to rapid phosphorylation of both kinases31Sun Y. Li L. Macho A.P. Han Z. Hu Z. Zipfel C. Zhou J.M. Chai J. Structural basis for flg22-induced activation of the Arabidopsis FLS2-BAK1 immune complex.Science. 2013; 342: 624-628Crossref PubMed Scopus (480) Google Scholar (Figure 2). Notably, all four functional SERKs are essential for stomatal development, with unequal functional redundancy. On the other hand, only BAK1/SERK3 and BAK1-LIKE1/SERK4, but not other SERKs, regulate FLS2-dependent immune pathways32Chinchilla D. Bauer Z. Regenass M. Boller T. Felix G. The Arabidopsis receptor kinase FLS2 binds flg22 and determines the specificity of flagellin perception.Plant Cell. 2006; 18: 465-476Crossref PubMed Scopus (588) Google Scholar,33Chinchilla D. Zipfel C. Robatzek S. Kemmerling B. Nürnberger T. Jones J.D.G. Felix G. Boller T. A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence.Nature. 2007; 448: 497-500Crossref PubMed Scopus (1318) Google Scholar,34Roux M. Schwessinger B. Albrecht C. Chinchilla D. Jones A. Holton N. Malinovsky F.G. Tör M. de Vries S. Zipfel C. The Arabidopsis leucine-rich repeat receptor-like kinases BAK1/SERK3 and BKK1/SERK4 are required for innate immunity to hemibiotrophic and biotrophic pathogens.Plant Cell. 2011; 23: 2440-2455Crossref PubMed Scopus (486) Google Scholar. Thus, BAK1/SERK-family members may exhibit functional specificity in stomatal development versus immunity. This could be due to their differential binding affinities with the cognate receptors, or due to differential expression patterns. Alternatively, the signal specificity may lie in the BAK1/SERK phosphorylation codes. Indeed, in vivo and in vitro phosphorylation analysis of BAK1 uncovered specific phosphosites that are required for immunity, but dispensable for brassinosteroid-mediated plant growth35Perraki A. DeFalco T.A. Derbyshire P. Avila J. Séré D. Sklenar J. Qi X. Stransfeld L. Schwessinger B. Kadota Y. et al.Phosphocode-dependent functional dichotomy of a common co-receptor in plant signalling.Nature. 2018; 561: 248-252Crossref PubMed Scopus (76) Google Scholar. It would be interesting to address whether specific BAK1 phosphosites are required for stomatal development, or whether having TMM in the receptor complex alters BAK1/SERK phosphorylation patterns. Notably, clustering of receptors within the plasma membrane could provide functional specificity via modulating intermolecular interactions36Bray D. Levin M.D. Morton-Firth C.J. Receptor clustering as a cellular mechanism to control sensitivity.Nature. 1998; 393: 85-88Crossref PubMed Scopus (523) Google Scholar,37Abulrob A. Lu Z. Baumann E. Vobornik D. Taylor R. Stanimirovic D. Johnston L.J. Nanoscale imaging of epidermal growth factor receptor clustering: effects of inhibitors.J. Biol. Chem. 2010; 285: 3145-3156Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar,38Bücherl C.A. Jarsch I.K. Schudoma C. Segonzac C. Mbengue M. Robatzek S. MacLean D. Ott T. Zipfel C. Plant immune and growth receptors share common signalling components but localise to distinct plasma membrane nanodomains.eLife. 2017; 6e25114Crossref PubMed Scopus (147) Google Scholar. Distinct localization of receptors to specific membrane domains may contribute to specificity between distinct plant signaling pathways38Bücherl C.A. Jarsch I.K. Schudoma C. Segonzac C. Mbengue M. Robatzek S. MacLean D. Ott T. Zipfel C. Plant immune and growth receptors share common signalling components but localise to distinct plasma membrane nanodomains.eLife. 2017; 6e25114Crossref PubMed Scopus (147) Google Scholar. Whether differential signal transduction of common co-receptors during stomatal development and immunity results from spatially separated signaling complexes remains to be analyzed in future studies. The BRASSINOSTEROID SIGNALING KINASE (BSK) family receptor-like cytoplasmic kinases (RLCKs) have been described to be substrates of the receptor kinase BRASSINOSTEROID INSENSITIVE 1 (BRI1) in the context of brassinosteroid signaling39Tang W. Kim T.W. Oses-Prieto J.A. Sun Y. Deng Z. Zhu S. Wang R. Burlingame A.L. Wang Z.Y. BSKs mediate signal transduction from the receptor kinase BRI1 in Arabidopsis.Science. 2008; 321: 557-560Crossref PubMed Scopus (507) Google Scholar. Genetic evidence suggests that BSK1 and BSK2 may act as bridges between ERf–TMM–SERKs and Mitogen Activated Protein Kinase (MAPK) cascade that inhibits stomatal development40Neu A. Eilbert E. Asseck L.Y. Slane D. Henschen A. Wang K. Bürgel P. Hildebrandt M. Musielak T.J. Kolb M. et al.Constitutive signaling activity of a receptor-associated protein links fertilization with embryonic patterning in Arabidopsis thaliana.Proc. Natl. Acad. Sci. USA. 2019; 116: 5795-5804Crossref PubMed Scopus (27) Google Scholar (Figure 1). Biochemical studies on the interaction of BSK1/2 and the ERf–TMM–SERK ternary receptor complex are needed to confirm whether these two RLCKs are the missing link. Interestingly, BSK1 also associates with FLS2 and BAK1 in the context of immunity responses41Shi H. Shen Q. Qi Y. Yan H. Nie H. Chen Y. Zhao T. Katagiri F. Tang D. BR-SIGNALING KINASE1 physically associates with FLAGELLIN SENSING2 and regulates plant innate immunity in Arabidopsis.Plant Cell. 2013; 25: 1143-1157Crossref PubMed Scopus (179) Google Scholar. BSK1 transduces the immune signal from the PRR complex by phosphorylating MAPKKK5, a MAPKKK in immunity signaling that further interacts with multiple MAPKKs (e.g., MKK4/5)42Yan H. Zhao Y. Shi H. Li J. Wang Y. Tang D. BRASSINOSTEROID-SIGNALING KINASE1 phosphorylates MAPKKK5 to regulate immunity in Arabidopsis.Plant Physiol. 2018; 176: 2991-3002Crossref PubMed Scopus (80) Google Scholar (Figure 2). Taken together, BSK family RLCKs relay signaling in both stomatal development and immunity. Another RLCK, BOTRYTIS-INDUCED KINASE1 (BIK1), plays a major role in the immune signal transduction. Within minutes, pattern recognition triggers considerable cellular events via BIK1, including production of extracellular reactive oxygen species (ROS), ion-flux changes at the plasma membrane, and the increase of cytosolic calcium levels20Li L. Li M. Yu L. Zhou Z. Liang X. Liu Z. Cai G. Gao L. Zhang X. Wang Y. et al.The FLS2-associated kinase BIK1 directly phosphorylates the NADPH oxidase RbohD to control plant immunity.Cell Host Microbe. 2014; 15: 329-338Abstract Full Text Full Text PDF PubMed Scopus (486) Google Scholar,21Thor K. Jiang S. Michard E. George J. Scherzer S. Huang S. Dindas J. Derbyshire P. Leitão N. DeFalco T.A. et al.The calcium-permeable channel OSCA1.3 regulates plant stomatal immunity.Nature. 2020; 585: 569-573Crossref PubMed Scopus (146) Google Scholar,22Kadota Y. Sklenar J. Derbyshire P. Stransfeld L. Asai S. Ntoukakis V. Jones J.D. Shirasu K. Menke F. Jones A. Zipfel C. Direct regulation of the NADPH oxidase RBOHD by the PRR-associated kinase BIK1 during plant immunity.Mol. Cell. 2014; 54: 43-55Abstract Full Text Full Text PDF PubMed Scopus (566) Google Scholar,43Lu D. Wu S. Gao X. Zhang Y. Shan L. He P. A receptor-like cytoplasmic kinase, BIK1, associates with a flagellin receptor complex to initiate plant innate immunity.Proc. Natl. Acad. Sci. USA. 2010; 107: 496-501Crossref PubMed Scopus (558) Google Scholar,44Zhang J. Li W. Xiang T. Liu Z. Laluk K. Ding X. Zou Y. Gao M. Zhang X. Chen S. et al.Receptor-like cytoplasmic kinases integrate signaling from multiple plant immune receptors and are targeted by a Pseudomonas syringae effector.Cell Host Microbe. 2010; 7: 290-301Abstract Full Text Full Text PDF PubMed Scopus (571) Google Scholar,45Tian W. Hou C. Ren Z. Wang C. Zhao F. Dahlbeck D. Hu S. Zhang L. Niu Q. Li L. et al.A calmodulin-gated calcium channel links pathogen patterns to plant immunity.Nature. 2019; 572: 131-135Crossref PubMed Scopus (227) Google Scholar (Figure 2). In addition, BIK1 and its phosphorylation substrate BRI1-SUPPRESSOR1 (BSU1) phosphatase mediate flagellin-induced MAP kinase activation and immunity, suggesting indirect regulation of MAPK cascade by BIK146Park C.H. Bi Y. Youn J.-H. Kim S.-H. Kim J.-G. Xu N.Y. Shrestha R. Burlingame A.L. Xu S.-L. Mudgett M.B. et al.Deconvoluting signals downstream of growth and immune receptor kinases by phosphocodes of the BSU1 family phosphatases.Nat. Plants. 2022; 8: 646-655Crossref Scopus (5) Google Scholar (Figure 2). One recent study claimed that BIK1 interacts with ERf LRR-RKs and plays an opposite role with ERECTA in leaf morphogenesis and inflorescence architecture47Chen S. Liu J. Liu Y. Chen L. Sun T. Yao N. Wang H.B. Liu B. BIK1 and ERECTA play opposing roles in both leaf and inflorescence development in Arabidopsis.Front. Plant Sci. 2019; 10: 1480Crossref PubMed Scopus (6) Google Scholar. Since ERf LRR-RKs play a critical role in stomatal formation and patterning, it will be of interest to explore whether and how BIK1 is also involved in stomatal development. The formation of ERf–TMM–SERKs ternary receptor complexes relays the peptide signals to a MAPK cascade consisting of YODA (YDA)–MKK4/5–MPK3/6, which negatively regulates the stability of the core transcription factors (e.g., SPEECHLESS (SPCH)), which is necessary and sufficient for initiating the stomatal cell lineages) to enforce stomatal development14Qian P. Song W. Yokoo T. Minobe A. Wang G. Ishida T. Sawa S. Chai J. Kakimoto T. The CLE9/10 secretory peptide regulates stomatal and vascular development through distinct receptors.Nat. Plants. 2018; 4: 1071-1081Crossref PubMed Scopus (83) Google Scholar,29Horst R.J. Fujita H. Lee J.S. Rychel A.L. Garrick J.M. Kawaguchi M. Peterson K.M. Torii K.U. Molecular framework of a regulatory circuit initiating two-dimensional spatial patterning of stomatal lineage.PLoS Genet. 2015; 11e1005374Crossref Scopus (63) Google Scholar,48Wang H. Ngwenyama N. Liu Y. Walker J.C. Zhang S. Stomatal development and patterning are regulated by environmentally responsive mitogen-activated protein kinases in Arabidopsis.Plant Cell. 2007; 19: 63-73Crossref PubMed Scopus (638) Google Scholar,49Lampard G.R. Macalister C.A. Bergmann D.C. Arabidopsis stomatal initiation is controlled by MAPK-mediated regulation of the bHLH SPEECHLESS.Science. 2008; 322: 1113-1116Crossref PubMed Scopus (328) Google Scholar (Figure 1). In stomatal immunity, MAPK cascades play an important role downstream of RLCKs in response to flg22. In particular, PAMP-triggered activation of the MKK4/5–MPK3/6 module can lead to stomatal closure in