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Thorsten Hamann* Department of Biology, Høgskoleringen 5, Realfagbygget, Norwegian University of Science and Technology, 7491 Trondheim, Norway *Corresponding author: E-mail, [email protected]; Fax, +47 73596100. (Received October 3, 2014; Accepted November 10, 2014) One of the main differences between plant and animal cells are the walls surrounding plant cells providing structural support during development and protection like an adaptive armor against biotic and abiotic stress. During recent years it has become widely accepted that plant cells use a dedicated system to monitor and maintain the functional integrity of their walls. Maintenance of integrity is achieved by modifying the cell wall and cellular metabolism in order to permit tightly controlled changes in wall composition and structure. While a substantial amount of evidence supporting the existence of the mechanism has been reported, knowledge regarding its precise mode of action is still limited. The currently available evidence suggests similarities of the plant mechanism with respect to both design principles and molecular components involved to the very well characterized system active in the model organism Saccharomyces cerevisiae. There the system has been implicated in cell morphogenesis as well as response to abiotic stresses such as osmotic challenges. Here the currently available knowledge on the yeast system will be reviewed initially to provide a framework for the subsequent discussion of the plant cell wall integrity maintenance mechanism. The review will then end with a discussion on possible design principles for the cell wall integrity maintenance mechanism and the function of the plant turgor pressure in this context. Keywords: Biotic stress Cell wall integrity Mechanoperception Osmosensing Plant cell wall Turgor. Abbreviations: ACC, aminocyclopropane carboxylic acid; CRD, cysteine-rich domain; CWI, cell wall integrity; CWS, cell wall stress; DAMP, damage-associated molecular pattern; GAP, GTPase-activating protein; GEF, guanosine nucleotide exchange factor; JA, jasmonic acid; MAPK, mitogen-activated protein kinase; PKC, protein kinase C; RLK, receptor-like kinase; ROS, reactive oxygen species; SA, salicylic acid; STR, serine-threonine-rich; WAK, wall-associated kinase. Introduction Over the course of time, the perception of plant cell walls in the research community has changed dramatically. It has become generally accepted that plant cell walls are not inert objects but highly dynamic structures, which are intricately involved in biological processes such as cell morphogenesis as well as response to both biotic and abiotic stresses (Landrein and Hamant 2013, Malinovsky et al. 2014). Plant cell walls are able to perform their respective functions during different biological processes because their composition and fine structure are modified in response to different types of stimuli. While extensive research has been performed to understand cell wall metabolism, understanding of the mechanisms regulating stimulus-induced changes in wall composition and structure is still limited. These mechanisms must perceive and translate chemical or physical stimuli into quantitative chemical signals, which lead to modifications in cell wall and cellular metabolism that in turn bring about specific changes in wall structure and composition. An example of such a mechanism is the plant cell wall integrity (CWI) maintenance mechanism, which exhibits similarities to the one existing in Saccharomyces cerevisiae (Hamann and Denness 2011). The mechanism is monitoring the functional integrity of the plant cell wall and maintains integrity by inducing modifications in wall and cellular metabolism in response to cell wall stress (CWS). CWSs can be defined as all events which impair the functional integrity of the plant cell wall. Such events can occur during interaction with the environment (such as wounding, pathogen infection and exposure to abiotic stresses such as drought) and biological processes (such as rapid cell elongation). While the range of processes where CWS occurs is quite diverse, the effects on the cell wall and the plasma membrane are probably fairly specific and possibly quite local. They could involve membrane stretch/distortion, cell wall-derived ligand/fragment formation and/or cell wall epitope modifications. In yeast, such effects are perceived by different osmo-, mechano- and dedicated CWS sensing mechanisms, which activate different adaptive responses involving changes in cell wall metabolism, cytoskeletal organization, vesicle transport and cell cycle progression (Levin 2011). Since the yeast CWI maintenance mechanism was discovered quite some time ago, extensive research has been performed leading to a detailed understanding of its mode of action as well as the molecular machinery involved (Levin 2011). While evidence supporting the existence of a dedicated plant CWI maintenance mechanism has accumulated, precise knowledge of its mode of action and molecular components is limited. During recent years, different aspects of the plant CWI maintenance mechanism have been reviewed (Seifert and Plant Cell Physiol. 56(2): 215–223 (2015) doi:10.1093/pcp/pcu164, Advance Access publication on 21 November 2014, available online at www.pcp.oxfordjournals.org ! The Author 2014. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: [email protected] Special Focus Issue – Mini Review The Plant Cell Wall Integrity Maintenance Mechanism— Concepts for Organization and Mode of Action T. Hamann | Concepts for plant cell wall integrity maintenance Blaukopf 2010, Nühse 2012, Wolf et al. 2012, Engelsdorf and Hamann 2014, Malinovsky et al. 2014, Wolf and Hofte 2014). While these reviews covered particular aspects such as receptor-like kinase (RLK)-based signaling processes and cell wall metabolism in the context of the plant CWI maintenance mechanism very thoroughly, the role of turgor pressure during CWI maintenance has only been addressed to a limited extent. Therefore this review will try to assess the function of turgor pressure in CWI maintenance and discuss different design possibilities for the mechanism. Information about the S. cerevisiae CWI maintenance mechanism will be used initially as a framework for the discussion, since the currently available knowledge from plants suggests that there may be functional and design similarities between the plant and yeast mechanisms. The Cell Wall Integrity Maintenance Mechanism in Sacharomyces cerevisiae Although yeast is a single-celled organism with a cell wall exhibiting a significantly simpler design than the one found in multicellular plants, the principal problems and functional requirements encountered by both organisms are similar (Levin 2011, Free 2013). In both species, cells need to be plastic during development to allow changes in shape and size. In parallel, both require sturdy cell walls to withstand high levels of turgor pressure and environmental stresses such as drought. What both organisms also have in common is the need to maintain the functional integrity of their walls during exposure to stress and cell morphogenesis. In yeast, cell wall-damaging agents (zymolase), hypo/hyperosmotic shock, heat and endoplasmatic reticulum stress as well as reorganization of the cytoskeleton or pheromone-induced morphogenesis activate the CWI maintenance mechanism (Levin 2011). Since they are comparable with processes that plant cells experience (wounding, exposure to biotic/abiotic stress and cell elongation), they provide an indication of the large number of biological processes in which the plant CWI maintenance mechanism is potentially involved. During the different processes in yeast, the initial stimulus perceived seems to be distortion and/or displacement of the plasma membrane relative to the cell wall. Detecting this type of stimulus has several advantages such as being, on the one hand, highly sensitive, so even minor CWI impairment events can be detected, while in parallel permitting a quantitative perception of the damage. The importance of turgor pressure levels in this context is illustrated by the observation that provision of osmotic support (light hyperosmotic stress treatment) often neutralizes the effects of CWS-inducing treatments and prevents activation of CWI maintenance responses in yeast and plants (Kamada et al. 1995, de Nobel et al. 2000, Hamann et al. 2009). Important aspects which obviously complicate the situation in plants are multicellularity, higher chemical and structural complexity of plant cell walls, as well as differences in wall composition and structure between different cell types. These affect physical characteristics as well as the presence and accessibility of 216 ligands, cell wall fragments and wall epitopes, which could be involved in CWI maintenance by serving as indicators of the state of the wall. The latter possibilities are of particular interest in plants since a large number of RLK-encoding genes have been identified in plant genomes, with several of them implicated in CWI maintenance (Engelsdorf and Hamann 2014). In yeast, three different stimulus perception and signal generation mechanisms are contributing to CWI maintenance and interact to different degrees (Fig. 1; Levin 2011). Stimulus detection in the first one is mediated by a group of five plasma membrane-localized proteins (WSC1–WSC3, MTL1 and MID2), which exhibit structural but not sequence similarities (Jendretzki et al. 2011). MTL1 and MID2 also exhibit partial redundancy with each other, with loss-of-function alleles causing severe phenotypes (Rajavel et al. 1999). Whereas mutations in WSC1 cause strong phenotypic effects, mutations in WSC2 and WSC3 have only mild phenotypic consequences and double mutant strains exhibit additive phenotypes (Verna et al. 1997, Ketela et al. 1999). These observations suggest that the WSC proteins do not have identical functions, a limited degree of functional redundancy exists and CWS sensing is redundantly organized in yeast. The probably most crucial and best-characterized protein of the group is WSC1, which will be discussed here in some detail to illustrate the mode of action of a CWS sensor. The protein consists of a small cytoplasmic domain, a transmembrane domain and a highly glycosylated N-terminal domain, containing the so-called serine-threoninerich (STR) region and the cysteine-rich domain (CRD) (Rodicio and Heinisch 2010). Recent functional studies involving single molecule, atomic force microscopy have shown that combining the highly O-glycosylated N-terminal area of WSC1 with the STR domain generates sufficient mechanical stiffness to turn the protein into a functional mechanosensor (Heinisch et al. 2010). Having transmembrane and STR/CRD domains ensures that the WSC1 protein is anchored simultaneously in the plasma membrane and the cell wall, meaning that any displacement of the wall in relation to the membrane leads to conformational changes of a protein. These conformational changes represent the signal leading to activation of a signaling cascade. Fig. 1 Schematic, simplified overview of signaling cascades and genes implicated in CWI maintenance in a yeast cell. The cell wall is colored beige, the plasma membrane is represented by a green line, with the intermediate space highlighted in gray. The nucleus is colored light orange. Plant Cell Physiol. 56(2): 215–223 (2015) doi:10.1093/pcp/pcu164 Signal translation from WSC1 to ROM1, a cytoplasmic, plasma membrane-associated guanosine nucleotide exchange factor (GEF), is mediated via the small cytoplasmic domain in response to the previously mentioned conformational change of the extracellular part of WSC1 (Levin 2011). ROM1 in turn mediates the GTP/GDP exchange at the small GTPase RHO1. RHO1 is a central integrator responding to different inputs, whose activity is regulated both by GEFs (such asROM1/2 and TUS) and by GTPase-activating proteins (GAPs) (Schmelzle et al. 2002, Yoshida et al. 2009). While ROM1 and ROM2 are responsible for CWS-induced RHO1 activation, TUS regulates the GTPase in a cell cycle-dependent manner. In contrast, the GAPs are regulating RHO1 activity in a target-specific manner (Schmidt et al. 2002). RHO1 downstream targets are the transcription factor SKN7, SEC3 (a component of the exocyst complex), b-1,3 and b-1,6 glucan synthases, BNI1/BNR1 (regulators of actin cytoskeleton organization) and protein kinase C1 (PKC1) (Alberts et al. 1998, Ketela et al. 1999). SKN7 is a transcription factor, which is also regulated incidentally by the HOG1 turgor sensing system (see below for more details). Amongst the genes regulated by SKN7 are OCH1 and NCA3, which encode proteins required for cell wall biogenesis and septation (Nakayama et al. 1992, Shankarnarayan et al. 2008). The S. cerevisiae genome encodes only a single homolog of the mammalian PKC, with deletion of PKC1 causing lethality unless osmotic support is provided (Levin and BartlettHeubusch 1992, Paravicini et al. 1992). Signals from PKC1 are relayed via a mitogen-activated protein kinase (MAPK) module consisting of BCK1, MKK1/2, MPK1 and MLP1. They result in transcriptional regulation of the response mediators RLM1 and SBF (Jung et al. 2002, Bermejo et al. 2008, Kim et al. 2008). RLM1 in turn regulates the activity of at least 25 genes involved in cell wall biogenesis (such as the b-1,3 glucan synthase FKS2) or encoding proteins residing in the wall (Jung and Levin 1999). This condensed overview illustrates how CWS perception is mediated via mechanosensitive plasma membrane proteins exemplified here by WSC1, highlights the cascades responsible for signal translation and, by listing the cellular processes (in)directly involved, indicates how influential CWI maintenance is in yeast metabolism and cell biology. The second mechanism capable of detecting CWS involves a stretch-activated, plasma membrane-localized channel complex consisting of CCH1 and MID1 and a signaling cascade based on changes in cytoplasmic Ca2+ concentrations to activate downstream responses (Levin 2011). The complex is involved in perception of different stresses such as cold, hyper/hypo-osmotic and oxidative stress (Matsumoto et al. 2002, Peiter et al. 2005, Popa et al. 2010). The stretch-activated MID1 channel protein is cation specific, connected to the membrane via a GPI (glycosylphosphatidylinositol) anchor, is found only in the fungal kingdom and was originally identified through a forward genetic screen aimed at identifying mutants exhibiting mating pheromone-induced cell death (Iida et al. 1994, Rispail et al. 2009). CCH1, on the other hand, is a transmembrane Ca2+ channel, which exhibits similarity to the a1 subunit of voltage-gated channels found in animals (Paidhungat and Garrett 1997). An example illustrating the links between different signaling cascades perceiving CWS is the regulation of CCH1 activity by the previously mentioned MAPK MPK1 (Rispail et al. 2009). Influx of Ca2+ ions into the cytoplasm activates yeast calmodulin, which in turn regulates the activity of calcineurin, a heterodimer that has Ca2+- and calmodulin-dependent phosphatase activity. One of the primary targets of calcineurin activity is CRZ1, a zinc finger transcription factor whose nuclear localization is dependent on dephosphorylation by calcineurin (Yoshimoto et al. 2002). Expression analysis has shown that CRZ1 regulates the activity of different P-ATPases involved in ion homeostasis and of FKS2, exemplifying how the activity of genes involved in cell wall metabolism can be regulated by both CWS and CCH1–MID1mediated mechanoperception (Yoshimoto et al. 2002). Hypo-osmotic stress perception is mediated by both the CCH1–MID1 channel complex and the hybrid-histidine kinase SLN1, with the kinase apparently detecting changes in turgor pressure levels and hypo-osmotic shock (Batiza et al. 1996, Reiser et al. 2003). SLN1 forms an essential part of one branch of the HOG1 system, which represents the main system mediating osmoperception in yeast. It consists of two branches, which detect hypo- (SLN1) and hyper- (SHO1) osmotic stress, with the resulting signals being relayed via the MAPK kinase PBS2 to HOG1 kinase, after which the system is named (Levin 2011). The SLN1 branch involves a histidine-phosphotransfer protein (YPD1) and two downstream regulators (SSK1 and SKN7) (Fassler and West 2010). While previous work has shown that changes in turgor levels/hypo-osmotic shock initiate a phospho-transfer from a histidine in the kinase domain to an aspartate within the receiver domain of SLN1, the specific biophysical stimulus responsible still remains to be understood (Saito and Posas 2012). The second transfer occurs then between the SLN1 receiver domain and YPD1, which permits phospho-transfers to SSK1 and SKN7. While SSK1 is inactivated through this modification, the transcription factor SKN7 is activated and induces expression of target genes such as OCH1 required for cell wall metabolism. In contrast, under hyperosmotic conditions, the SLN1 signaling activity is reduced, thus allowing enhanced signaling via the SHO1 branch (Kaserer et al. 2009). Perception of hyperosmotic stress requires plasma membrane-localized SHO1 as well as HKR1, MSB2 and OPY2 (Saito and Posas 2012). HKR1 and MSB2 encode mucin-like proteins, which contain STR and transmembrane domains like WSC1 in addition to the approximately 200 amino acid long HKR1–MSB2 domain. Results from genetic analysis suggest that the STR domain has an inhibitory activity while HKR1–MSB2 is activating (Cullen et al. 2004, Tatebayashi et al. 2007). Recently it was shown that the actin cytoskeleton is an essential component for signal generation by MSB2, which suggests that the actin cytoskeleton could contribute to turgor monitoring by acting as a highly efficient sensor (Tanaka et al. 2014). Signals generated by the different sensors are translated via a MAPK module involving STE11/PBS2/HOG1 and lead to different types of responses (Saito and Posas 2012). Responses range from very fast changes (seconds to minutes) in ion and glycerol transport activity as well as metabolic and translational changes to slower (nuclear) 217 T. Hamann | Concepts for plant cell wall integrity maintenance effects allowing long-term modulated adaptations including changes in gene expression (around 500 transcript levels change within 10 min after initial stress perception) and inhibition of cell cycle progression (Capaldi et al. 2008). Interestingly, responses to hypo-osmotic stress lead primarily to transcript level changes in genes required for cell wall metabolism, while hyperosmotic stress primarily affects genes required for glycerol and energy metabolism, hinting at qualitatively different responses required to adapt to hypo- vs. hyperosmotic stress. To summarize, the available data from research into the yeast CWI maintenance mechanism show that three different sensor systems monitor the state of the wall. By combining inputs from osmo-, mechano- and CWS perception, the yeast cell constantly has very detailed information about the state of its cell wall and can make adaptive changes in composition and structure by modifying cell wall and/or cellular metabolism to maintain CWI. This can involve direct regulation of cell wall biosynthetic genes or cytoskeletal reorganization. In parallel, CWI signaling is also influencing fundamental cellular processes such as cell cycle progression, highlighting the importance of CWI during developmental processes. Plant Cell Wall Metabolism Involved in Cell Wall Integrity Maintenance In recent years, data supporting the existence of a dedicated plant CWI mechanism have increased but have not yet led to the same level of understanding as in yeast. However, enough information has been generated to allow a discussion of possible organizational structures for the plant CWI maintenance mechanism. Initially in this section there will be a brief introduction of plant cell wall components required for discussing plant CWI maintenance. The second part will focus on experiments using manipulation of cellulose biosynthesis as a tool to impair CWI and the data generated will be used to outline the possible mode of action of the plant CWI maintenance mechanism. This will involve discussions of our current knowledge of possible stimuli responsible for activating the response mechanism and whether turgor pressure is a passive element in plant CWI maintenance or is actively participating. The plant cell wall performs many different functions during cell differentiation and in response to a changing environment. This is possible because of the large number of different cell wall polysaccharides and proteins in cell wall metabolism, which enable it to adapt its characteristics to different requirements. Here only cell wall components and processes directly relevant for discussing the plant CWI maintenance mechanism (based on the currently available data) will be covered. For in-depth coverage of plant cell wall metabolism, several recently published, comprehensive reviews are recommended (Liepman et al. 2010, McFarlane et al. 2014, Rennie and Scheller 2014, Sénéchal et al. 2014). Primary (elastic) cell walls are formed directly after cell division, whereas mechanically reinforced walls (called secondary walls) can be formed later on during differentiation. The main load-bearing element of plant cell walls is cellulose, which consists of strands of b-1,4-linked 218 glucose units organized in microfibrils that are produced by plasma membrane-localized rosette complexes (McFarlane et al. 2014). The composition of the rosette complexes active during primary cell wall formation differs from that of those active during secondary cell wall formation, which also results in different sensitivities to cellulose biosynthesis inhibitors such as isoxaben (Heim et al. 1990). Isoxaben is a frequently used tool in plant CWI maintenance research to cause CWS (Manfield et al. 2004, Hematy et al. 2007, Hamann et al. 2009, Tsang et al. 2011). It inhibits cellulose production during primary cell wall formation by blocking the activity of CELLULOSE SYNTHASE A (CESA) 3 and 6 based on data from mutations causing resistance to the inhibitor (Heim et al. 1989, Scheible et al. 2001, Desprez et al. 2002). Thus, isoxaben treatment probably causes CWS by weakening the load-bearing element of the wall while (high) turgor levels remain unchanged. Combining generation of CWS in a highly specific way with availability of mutants causing resistance makes isoxaben a convenient tool to study CWI maintenance in a controlled manner. To illustrate the effects of isoxaben treatment on a single-cell level, Fig. 2A–D shows cells in the epidermis from mock-, sorbitol- and/or isoxaben-treated primary root tips of Arabidopsis seedlings expressing a plasma membrane marker (M. Veerabagu and T. Hamann, unpublished). While mock- (Fig. 2A) and sorbitol-treated (Fig. 2B) epidermal cells do not exhibit dramatic distortion, cells in roots treated with isoxaben (Fig. 2C) are bloated. Intriguingly, epidermal cells treated with a combination of isoxaben and sorbitol exhibit shapes more similar to the mock control than the isoxaben-treated roots (compare Fig. 2D and B). While these observations illustrate the effects of weakened cell walls vs. constant turgor pressure, the specific nature of the initial stimulus indicative of CWI impairment remains to be determined and will be discussed below in more detail. Cellulose microfibrils are cross-linked with xyloglucans and the resulting mesh is embedded in a matrix consisting of pectins in primary cell walls. Pectins consist mainly of galacturonic acid and form some of the most complex wall polysaccharides (Sénéchal et al. 2014). While the backbones of the pectic polysaccharides are fairly conserved and involve distinct domains such as homogalacturonan, xylogalacturonan and rhamnogalacturonan I or II, the side chains exhibit significant diversity with respect to the neutral cell wall sugars attached. In contrast to cellulose, xyloglucans and pectins are synthesized in the Golgi before being transported to the wall for integration. During the integration process, pectins can also be de-esterified or methylated, which influences wall characteristics such as adhesion, porosity and rheological properties (Sénéchal et al. 2014). These modifications are mediated by members of large gene families such as pectin methyl esterases or acetylesterases (Wolf et al. 2009). This generates a highly redundantly organized system and provides a large number of options for dynamic modifications and cross-linking in a highly controlled manner. These in turn will affect the availability of epitopes for proteinaceous binding partners as well as physical and biological characteristics of the wall (Bethke et al. 2014). One of the main differences between primary and secondary cell walls is the deposition of lignin in secondary cell walls. Lignin is one of Plant Cell Physiol. 56(2): 215–223 (2015) doi:10.1093/pcp/pcu164 Fig. 2 Effects of cellulose biosynthesis inhibition and sorbitol treatments on cell shape in Arabidopsis seedling root tips. Seedling root tips were mock (A), 300 mM sorbitol (B), 600 nM isoxaben (C) or sorbitol/isoxaben treated for 2 h. Expression of the WAVE131– yellow fluorescent protein (YFP) plasma membrane marker was detected using a Leica SP5 laser confocal microscope. Cells of interest are highlighted with white arrows. the most abundant cell wall polymers and has different functions such as water proofing the walls of xylem cells or reinforcing cell walls in response to pathogen infection (Moura et al. 2010, Wang et al. 2013). Monolignols such as p-coumaryl, coniferyl and sinapyl-alcohols give rise to the main lignin units (phydroxyphenyl, guaiacyl and syringyl). After their synthesis, monolignols are transported to the cell wall and transformed into monolignol radicals through the activities of laccases and peroxidases. These radicals form random cross-links, giving rise to three-dimensional structures in which other cell wall components such as cellulose microfibrils are embedded. Callose is another cell wall component consisting of b-1,3-linked glucose units, which is frequently formed at the wall by callose synthases in response to biotic stress or as a wound response to reinforce a damaged wall (Hardham et al. 2007). Plant Cell Wall Integrity Maintenance Responses The first two reports postulating the existence of a plant CWI maintenance mechanism elegantly illustrate its involvement in plant development and defense. Cano-Delgado and colleagues aimed to isolate genes required for cell morphogenesis during Arabidopsis thaliana primary root development (Cano-Delgado et al. 2000, Cano-Delgado et al. 2003). They isolated a novel allele (ectopic lignification) for CESA3. While the mutation caused, on the one hand, a reduction in cellulose production in elongating cells, in seedling roots simultaneous ectopic production of lignin was observed. The authors showed that the same effect can be achieved by using isoxaben and suggested that a compensatory reaction was taking place where a missing load-bearing element was replaced with another one. In parallel, Ellis and colleagues were interested in isolating novel signaling components required for jasmonic acid- (JA) mediated responses to pathogen infection (Ellis and Turner 2001, Ellis et al. 2002). They performed a reporter-based forward genetic screen using the JA-inducible VEGETATIVE STORAGE PROTEIN1 (VSP1) promoter to drive expression of a luciferase reporter gene. They also isolated a mutant allele of CESA3 they named constitutive expression of VSP1 (cev1), because it caused activation of the JA-sensitive reporter construct. Their phenotypic characterization of the mutant plants detected increased JA (lignin) production and resistance to Erisyphe orontii as well as induction of defense gene expression. In the following years, other groups used genetic and inhibitor-based methods to assess the impacts of both short (minutes up to 36 h) and long-term (days to weeks) cellulose biosynthesis inhibition in Arabidopsis seedlings and callus cultures (Manfield et al. 2004, Duval et al. 2005, Paredez et al. 2006, Duval and Beaudoin, 2009, Hamann et al. 2009, Denness et al. 2011, Tsang et al. 2011, Wormit et al. 2012). The results of these studies have provided a global overview of the impact of CWS (here caused by isoxaben treatment) on cell signaling, as well as cellular and cell wall metabolism. Previous work showed that cellulose biosynthesis inhibition affects cytoskeleton organization within 15 min after the start of treatment, reduces elongation of root epidermal cells after 1 h, causes ectopic production of JA and reactive oxygen species (ROS) after 3–4 h, lignification of cells in the root elongation zone after 6 h, transient redistribution of soluble sugars after 8–10 h, inhibition of photosynthetic activity, callose deposition and necrosis in cotyledons after 18 h and later on changes in levels of cell wall components such as arabinose, uronic acid and galactose. Interestingly, isoxabeninduced lignin, callose and JA production, necrosis and redistribution of soluble sugars can be suppressed by provision of osmotic support similarly to the results obtained in yeast (Hamann et al. 2009, Wormit et al. 2012). These observations illustrate the wide range of responses and highlight the similarities to phenotypes observed when plants are exposed to biotic or abiotic stress. All the studies employed cellulose biosynthesis inhibition as a tool, which has the advantage of causing a specific, standardized CWS and making results comparable. However, this is only one possible type of CWS, so it remains to be determined how generally applicable the results are since the responses probably differ dependent on the type of CWS. For example, mutations in CESA4, 7 and 8 required for secondary cell wall formation activate ABA-mediated signaling cascades and affect 219 T. Hamann | Concepts for plant cell wall integrity maintenance resistance to different pathogens (Ralstonia solanacearum and Plectosphaerella cucumerina) compared with mutations in the genes required for primary cell wall formation (HernandezBlanco et al. 2007). This suggests that, for example, the pathogen infection mechanisms or the CWS responses could differ between elongating and differentiated cells and highlights the need for additional, detailed studies. Stimuli Indicating Cell Wall Integrity Impairment Currently both the specific nature and whether one or several qualitatively different types of stimuli are responsible for activating the CWI maintenance mechanism remain to be determined. Here three different options will be discussed to illustrate possible scenarios taking place in planta upon exposure to CWI impairment. In the first scenario, the stimuli could consist of fragments/ligands released, increased accessibility of cell wall epitopes or a combination of these. These stimuli could occur during cell wall degradation by invading pathogens, wounding or cell elongation, and could activate the CWI maintenance mechanism via RLKs, which would bind to ligands/ fragments/epitopes becoming available. This consideration also highlights the possibility that the CWI maintenance mechanism could form an essential component of plant immunity such as responses to damage-associated molecular patterns (DAMPs), since cell wall-derived fragments can be considered to be DAMPs. Different RLKs have been implicated in CWI maintenance. The available evidence suggests that THESEUS1, which belongs to the CrRLK1L family of RLKs, has a key role in the response to isoxaben-induced CWI impairment since it suppresses cellulose deficiency phenotypes and THESEUS1overexpressing lines exhibit increased lignin deposition (Hematy et al. 2007, Denness et al. 2011). An important question remaining to be addressed is whether THESEUS1 binds a ligand released during CWI impairment or an epitope residing in the cell wall that becomes (in)accessible. Wall-associated kinases (WAKs) are also regularly discussed in this context, since they have been shown to bind pectin and pectin-derived epitopes as well as being implicated in turgor-sensitive processes with their signals being relayed via MAPKs to downstream response targets (Brutus et al. 2010, Kohorn and Kohorn, 2012, Kohorn et al. 2014). Since other candidate RLKs such as PEPR1/2, FERONIA, HERKULES1/2 and FEI1/2 have been discussed extensively in recent reviews they will not be covered here again (Lindner et al. 2012, Engelsdorf and Hamann 2014, Wolf and Hofte 2014). Signals generated by these RLKs could be translated through established signaling cascades involving ROS, JA, salicylic acid (SA), aminocyclopropane carboxylic acid (ACC) and ABA, leading to downstream responses such as modification/adaptation of cell wall metabolism (Malinovsky et al. 2014, Miedes et al. 2014). A second scenario involves turgor pressure pushing outwards against a cell wall weakened due to active cell elongation, metabolic defects or biotic/abiotic stress. All these processes could cause distortion of the wall or displacement of the 220 plasma membrane in relation to the wall. From a turgor point of view, this could correspond to a hypo-osmotic shock situation, with the available data supporting this view since mild hyperosmotic shocks (support) suppress CWI impairment in Arabidopsis seedlings and yeast (Fig. 2; Hamann et al. 2009, Levin 2011). In this scenario, stimuli would be perceived by (plasma membrane-localized) proteins, which are able to sense displacement of the membrane vs. the wall (mechanosensitive) and/or changes in turgor pressure levels. Stimulus perception could lead to conformational changes (analogous to WSC1) or opening of membrane channels (MID1–CCH1), which initiate signaling cascades. In recent years, different gene families encoding mechano- (MCA) and turgor-sensitive (MSL, OSCA and AHK) proteins have been discussed in this context, with functional evidence implicating MID1 COMPLEMENTING ACTIVITY1 (MCA1), Arabidopsis histidine kinase 4 (AHK4/ CRE1) and calcium-based signaling cascades in the response to CWD caused by isoxaben (Denness et al. 2011, Wormit et al. 2012, Kurusu et al. 2013, Monshausen and Haswell, 2013, Yuan et al. 2014). While the available evidence supports both scenarios just described, a third scenario combining them is theoretically possible as well. It is reasonable to assume that stimuli indicative of CWI impairment encompass both chemical (ligands/epitopes) and physical (displacement) signals. By detecting and integrating both types of signals, the plant cell would receive detailed qualitative and quantitative information regarding the (functional) integrity of its wall, enabling it to modulate/ adapt the responses specifically to particular functional requirements. Such a system design has the additional benefit of generating a highly redundantly organized mechanism where even if one type of sensor is inactivated the other ones would still be able to detect CWI impairment. Since previous work has implicated at least one RLK (THESEUS1) and a putative mechanosensor (MCA1) in plant CWI maintenance, it should be possible to test this hypothesis by combining relevant genotypes. With respect to the function of turgor pressure in plant CWI maintenance, the available evidence is ambiguous. Data from yeast suggest that turgor sensing is involved in CWI maintenance and regulates to some extent the activity of the same target genes such as the WSC1 cell wall sensor and the MID1–CCH1 complex. In Arabidopsis, turgor manipulation suppresses all responses to isoxaben-induced CWS similar to yeast (Hamann et al. 2009). In parallel, AHK4, which can complement the yeast turgor sensor (SLN1), is required to mediate osmosensitive, isoxaben-induced metabolic changes (Wormit et al. 2012). However, AHK4 is thought to function as a cytokinin receptor in plants, which does not help to clarify the function of turgor sensing in CWI maintenance (Stolz et al. 2011). Bearing in mind the limited experimental evidence available, the logical next step has to be a systematic functional analysis of genes possibly capable of detecting changes in turgor pressure levels using standardized assays such as isoxaben-induced lignin and JA production to determine their respective contributions and make them comparable. Candidate gene families of particular interest in this context are the AHKs, Plant Cell Physiol. 56(2): 215–223 (2015) doi:10.1093/pcp/pcu164 MSLs and OSCA (Wilson et al. 2013, Kumar and Verslues 2014, Yuan et al. 2014). To summarize, currently it is an exciting time to work on the plant CWI maintenance mechanism since enough evidence has accumulated to support the notion that it is an important component of plant developmental, stress response and cell wall metabolic processes. Additionally the available functional data provide sufficient guidance for targeted, hypothesis-driven experiments to dissect its mode of action. In parallel it has been suggested that understanding the mechanism may generate novel options to overcome successfully the plasticity of plant cell walls, which has slowed down previous attempts to manipulate biomass quality in a knowledge driven way to facilitate bioenergy production from ligno-cellulosic feed stocks (Burton and Fincher 2014). Funding Work in the author’s research group is supported by the DFG, the Sather Centre, EMBO and the Norwegian University of Science and Technology. Acknowledgments The author would like to thank the anonymous referees for their comments and apologize to colleagues whose work could not be discussed due to space limitations. Disclosures The authors have no conflicts of interest to declare. References Alberts, A.S., Bouquin, N., Johnston, L.H. and Treisman, R. (1998) Analysis of RhoA-binding proteins reveals an interaction domain conserved in heterotrimeric G protein beta subunits and the yeast response regulator protein Skn7. J. Biol. Chem. 273: 8616–8622. Batiza, A.F., Schulz, T. and Masson, P.H. (1996) Yeast respond to hypotonic shock with a calcium pulse. J. Biol. Chem. 271: 23357–23362. Bermejo, C., Rodriguez, E., Garcia, R., Rodriguez-Pena, J.M., Rodriguez de la Concepcion, M.L., Rivas, C. et al. (2008) The sequential activation of the yeast HOG and SLT2 pathways is required for cell survival to cell wall stress. Mol. Biol. Cell 19: 1113–1124. Bethke, G., Grundman, R.E., Sreekanta, S., Truman, W., Katagiri, F. and Glazebrook, J. (2014) Arabidopsis PECTIN METHYLESTERASEs contribute to immunity against Pseudomonas syringae. Plant Physiol. 164: 1093–1107. Brutus, A., Sicilia, F., Macone, A., Cervone, F. and De Lorenzo, G. (2010) A domain swap approach reveals a role of the plant wall-associated kinase 1 (WAK1) as a receptor of oligogalacturonides. Proc. Natl Acad. Sci. USA 107: 9452–9457. Burton, R.A. and Fincher, G.B. (2014) Plant cell wall engineering: applications in biofuel production and improved human health. Curr. Opin. Biotechnol. 26: 79–84. Cano-Delgado, A., Penfield, S., Smith, C., Catley, M. and Bevan, M. (2003) Reduced cellulose synthesis invokes lignification and defense responses in Arabidopsis thaliana. Plant J. 34: 351–362. Cano-Delgado, A.I., Metzlaff, K. and Bevan, M.W. (2000) The eli1 mutation reveals a link between cell expansion and secondary cell wall formation in Arabidopsis thaliana. Development 127: 3395–3405. Capaldi, A.P., Kaplan, T., Liu, Y., Habib, N., Regev, A., Friedman, N. and O’Shea, E.K. (2008) Structure and function of a transcriptional network activated by the MAPK Hog1. Nat. Genet. 40: 1300–1306. Cullen, P.J., Sabbagh, W., Graham, E., Irick, M.M., van Olden, E.K., Neal, C. et al. (2004) A signaling mucin at the head of the Cdc42- and MAPKdependent filamentous growth pathway in yeast. Genes Dev. 18: 1695–1708. Denness, L., McKenna, J.F., Segonzac, C., Wormit, A., Madhou, P., Bennett, M. et al. (2011) Cell wall damage-induced lignin biosynthesis is regulated by a reactive oxygen species- and jasmonic acid-dependent process in Arabidopsis. Plant Physiol. 156: 1364–1374. Desprez, T., Vernhettes, S., Fagard, M., Refregier, G., Desnos, T., Aletti, E. et al. (2002) Resistance against herbicide isoxaben and cellulose deficiency caused by distinct mutations in same cellulose synthase isoform CESA6. Plant Physiol. 128: 482–490. Duval, I. and Beaudoin, N. (2009) Transcriptional profiling in response to inhibition of cellulose synthesis by thaxtomin A and isoxaben in Arabidopsis thaliana suspension cells. Plant Cell Rep. 28: 811–830. Duval, I., Brochu, V., Simard, M., Beaulieu, C. and Beaudoin, N. (2005) Thaxtomin A induces programmed cell death in Arabidopsis thaliana suspension-cultured cells. Planta 222: 820–831. Ellis, C., Karafyllidis, I., Wasternack, C. and Turner, J.G. (2002) The Arabidopsis mutant cev1 links cell wall signaling to jasmonate and ethylene responses. Plant Cell 14: 1557–1566. Ellis, C. and Turner, J.G. (2001) The Arabidopsis mutant cev1 has constitutively active jasmonate and ethylene signal pathways and enhanced resistance to pathogens. Plant Cell 13: 1025–1033. Engelsdorf, T. and Hamann, T. (2014) An update on receptor-like kinase involvement in the maintenance of plant cell wall integrity. Ann. Bot. 114: 1339–1347. Fassler, J.S. and West, A.H. (2010) Genetic and biochemical analysis of the SLN1 pathway in Saccharomyces cerevisiae. Methods Enzymol. 471: 291–317. Free, S.J. (2013) Fungal cell wall organization and biosynthesis. Adv. Genet. 81: 33–82. Hamann, T., Bennett, M., Mansfield, J. and Somerville, C. (2009) Identification of cell-wall stress as a hexose-dependent and osmosensitive regulator of plant responses. Plant J. 57: 1015–1026. Hamann, T. and Denness, L. (2011) Cell wall integrity maintenance in plants: lessons to be learned from yeast?. Plant Signal. Behav. 6: 1–5. Hardham, A.R., Jones, D.A. and Takemoto, D. (2007) Cytoskeleton and cell wall function in penetration resistance. Curr. Opin. Plant Biol. 10: 342–348. Heim, D.R., Roberts, J.L., Pike, P.D. and Larrinua, I.M. (1989) Mutation of a locus of Arabidopsis thaliana confers resistance to the herbicide isoxaben. Plant Physiol. 90: 146–150. Heim, D.R., Skomp, J.R., Tschabold, E.E. and Larrinua, I.M. (1990) Isoxaben inhibits the synthesis of acid insoluble cell wall materials in Arabidopsis thaliana. Plant Physiol. 93: 695–700. Heinisch, J.J., Dupres, V., Alsteens, D. and Dufrene, Y.F. (2010) Measurement of the mechanical behavior of yeast membrane sensors using single-molecule atomic force microscopy. Nat. Protoc. 5: 670–677. Hematy, K., Sado, P.E., Van Tuinen, A., Rochange, S., Desnos, T., Balzergue, S. et al. (2007) A receptor-like kinase mediates the response of Arabidopsis cells to the inhibition of cellulose synthesis. Curr. Biol. 17: 922–931. Hernandez-Blanco, C., Feng, D.X., Hu, J., Sanchez-Vallet, A., Deslandes, L., Llorente, F. et al. (2007) Impairment of cellulose synthases required for Arabidopsis secondary cell wall formation enhances disease resistance. Plant Cell 19: 890–903. Iida, H., Nakamura, H., Ono, T., Okumura, M.S. and Anraku, Y. (1994) Mid1, a novel Saccharomyces cerevisiae gene encoding a plasma-membrane 221 T. Hamann | Concepts for plant cell wall integrity maintenance protein, is required for Ca2+ influx and mating. Mol. Cell. Biol. 14: 8259–8271. Jendretzki, A., Wittland, J., Wilk, S., Straede, A. and Heinisch, J.J. (2011) How do I begin? Sensing extracellular stress to maintain yeast cell wall integrity. Eur. J. Cell Biol. 90: 740–744. Jung, U.S. and Levin, D.E. (1999) Genome-wide analysis of gene expression regulated by the yeast cell wall integrity signalling pathway. Mol. Microbiol. 34: 1049–1057. Jung, U.S., Sobering, A.K., Romeo, M.J. and Levin, D.E. (2002) Regulation of the yeast Rlm1 transcription factor by the Mpk1 cell wall integrity MAP kinase. Mol. Microbiol. 46: 781–789. Kamada, Y., Jung, U.S., Piotrowski, J. and Levin, D.E. (1995) The protein kinase C-activated MAP kinase pathway of Saccharomyces cerevisiae mediates a novel aspect of the heat shock response. Genes Dev. 9: 1559–1571. Kaserer, A.O., Andi, B., Cook, P.F. and West, A.H. (2009) Effects of osmolytes on the SLN1–YPD1–SSK1 phosphorelay system from Saccharomyces cerevisiae. Biochemistry 48: 8044–8050. Ketela, T., Green, R. and Bussey, H. (1999) Saccharomyces cerevisiae mid2p is a potential cell wall stress sensor and upstream activator of the PKC1–MPK1 cell integrity pathway. J. Bacteriol. 181: 3330–3340. Kim, K.Y., Truman, A.W. and Levin, D.E. (2008) Yeast Mpk1 mitogenactivated protein kinase activates transcription through Swi4/Swi6 by a noncatalytic mechanism that requires upstream signal. Mol. Cell. Biol. 28: 2579–2589. Kohorn, B.D. and Kohorn, S.L. (2012) The cell wall-associated kinases, WAKs, as pectin receptors. Front. Plant Sci. 3: 88. Kohorn, B.D., Kohorn, S.L., Saba, N.J. and Martinez, V.M. (2014) Requirement for pectin methyl esterase and preference for fragmented over native pectins for wall-associated kinase-activated, EDS1/PAD4dependent stress response in Arabidopsis. J. Biol. Chem. 289: 18978–18986. Kumar, M.N. and Verslues, P.E. (2014) Stress physiology functions of the Arabidopsis histidine kinase (AHK) cytokinin receptors. Physiol. Plant. (in press). Kurusu, T., Kuchitsu, K., Nakano, M., Nakayama, Y. and Iida, H. (2013) Plant mechanosensing and Ca2+ transport. Trends Plant Sci. 18: 227–233. Landrein, B. and Hamant, O. (2013) How mechanical stress controls microtubule behavior and morphogenesis in plants: history, experiments and revisited theories. Plant J. 75: 324–338. Levin, D.E. (2011) Regulation of cell wall biogenesis in Saccharomyces cerevisiae: the cell wall integrity signaling pathway. Genetics 189: 1145–1175. Levin, D.E. and Bartlett-Heubusch, E. (1992) Mutants in the S. cerevisiae PKC1 gene display a cell cycle-specific osmotic stability defect. The Journal of Cell Biology 116: 1221–1229. Liepman, A.H., Wightman, R., Geshi, N., Turner, S.R. and Scheller, H.V. (2010) Arabidopsis—a powerful model system for plant cell wall research. Plant J. 61: 1107–1121. Lindner, H., Müller, L.M., Boisson-Dernier, A. and Grossniklaus, U. (2012) CrRLK1L receptor-like kinases: not just another brick in the wall. Curr. Opin. Plant Biol. 15: 659–669. Malinovsky, F.G., Fangel, J.U. and Willats, W.G.T. (2014) The role of the cell wall in plant immunity. Front. Plant Sci. 5: 178. Manfield, I.W., Orfila, C., McCartney, L., Harholt, J., Bernal, A.J., Scheller, H.V. et al. (2004) Novel cell wall architecture of isoxabenhabituated Arabidopsis suspension-cultured cells: global transcript profiling and cellular analysis. Plant J. 40: 260–275. Matsumoto, T.K., Ellsmore, A.J., Cessna, S.G., Low, P.S., Pardo, J.M., Bressan, R.A. et al. (2002) An osmotically induced cytosolic Ca2+ transient activates calcineurin signaling to mediate ion homeostasis and salt tolerance of Saccharomyces cerevisiae. J. Biol. Chem. 277: 33075–33080. McFarlane, H.E., Döring, A. and Persson, S. (2014) The cell biology of cellulose synthesis. Annu. Rev. Plant Biol. 65: 69–94. 222 Miedes, E., Vanholme, R., Boerjan, W. and Molina, A. (2014) The role of the secondary cell wall in plant resistance to pathogens. Front. Plant Sci. 5: 1–13. Monshausen, G.B. and Haswell, E.S. (2013) A force of nature: molecular mechanisms of mechanoperception in plants. J. Exp. Bot. 64: 4663–4680. Moura, J.C.M.S., Bonine, C.A.V., Viana, J.D.F., Dornelas, M.C. and Mazzafera, P. (2010) Abiotic and biotic stresses and changes in the lignin content and composition in plants. J. Integr. Plant Biol. 52: 360–376. Nakayama, K., Nagasu, T., Shimma, Y., Kuromitsu, J. and Jigami, Y. (1992) OCH1 encodes a novel membrane bound mannosyltransferase: outer chain elongation of asparagine-linked oligosaccharides. EMBO J. 11: 2511–2519. De Nobel, H., Ruiz, C., Martin, H., Morris, W., Brul, S., Molina, M. et al. (2000) Cell wall perturbation in yeast results in dual phosphorylation of the Slt2/Mpk1 MAP kinase and in an Slt2-mediated increase in FKS2lacZ expression, glucanase resistance and thermotolerance. Microbiology 146: 2121–2132. Nühse, T.S. (2012) Cell wall integrity signaling and innate immunity in plants. Front. Plant Sci. 3: 280. Paidhungat, M. and Garrett, S. (1997) A homolog of mammalian, voltagegated calcium channels mediates yeast pheromone-stimulated Ca2+ uptake and exacerbates the cdc1(Ts) growth defect. Mol. Cell. Biol. 17: 6339–6347. Paravicini, G., Cooper, M., Friedli, L., Smith, D.J., Carpentier, J.L., Klig, L.S. and Payton, M.A. (1992) The osmotic integrity of the yeast cell requires a functional PKC1 gene product. Molecular and Cellular Biology 12: 4896–4905. Paredez, A.R., Somerville, C.R. and Ehrhardt, D.W. (2006) Visualization of cellulose synthase demonstrates functional association with microtubules. Science 312: 1491–1495. Peiter, E., Fischer, M., Sidaway, K., Roberts, S.K. and Sanders, D. (2005) The Saccharomyces cerevisiae Ca2+ channel Cch1pMid1p is essential for tolerance to cold stress and iron toxicity. FEBS Lett. 579: 5697–5703. Popa, C.-V., Dumitru, I., Ruta, L.L., Danet, A.F. and Farcasanu, I.C. (2010) Exogenous oxidative stress induces Ca2+ release in the yeast Saccharomyces cerevisiae. FEBS J. 277: 4027–4038. Rajavel, M., Philip, B., Buehrer, B.M., Errede, B. and Levin, D.E. (1999) Mid2 is a putative sensor for cell integrity signaling in Saccharomyces cerevisiae. Mol. Cell. Biol. 19: 3969–3976. Reiser, V., Raitt, D.C. and Saito, H. (2003) Yeast osmosensor Sln1 and plant cytokinin receptor Cre1 respond to changes in turgor pressure. J. Cell Biol. 161: 1035–1040. Rennie, E.A. and Scheller, H.V. (2014) Xylan biosynthesis. Curr. Opin. Biotechnol. 26: 100–107. Rispail, N., Soanes, D.M., Ant, C., Czajkowski, R., Grünler, A., Huguet, R. et al. (2009) Comparative genomics of MAP kinase and calcium– calcineurin signalling components in plant and human pathogenic fungi. Fungal Genet. Biol. 46: 287–298. Rodicio, R. and Heinisch, J.J. (2010) Together we are strong—cell wall integrity sensors in yeasts. Yeast 27: 531–540. Saito, H. and Posas, F. (2012) Response to hyperosmotic stress. Genetics 192: 289–318. Scheible, W.R., Eshed, R., Richmond, T., Delmer, D. and Somerville, C. (2001) Modifications of cellulose synthase confer resistance to isoxaben and thiazolidinone herbicides in Arabidopsis Ixr1 mutants. Proc. Natl Acad. Sci. USA 98: 10079–10084. Schmelzle, T., Helliwell, S.B. and Hall, M.N. (2002) Yeast protein kinases and the RHO1 exchange factor TUS1 are novel components of the cell integrity pathway in yeast. Mol. Cell. Biol. 22: 1329–1339. Schmidt, A., Schmelzle, T. and Hall, M.N. (2002) The RHO1-GAPs SAC7, BEM2 and BAG7 control distinct RHO1 functions in Saccharomyces cerevisiae. Mol. Microbiol. 45: 1433–1441. Plant Cell Physiol. 56(2): 215–223 (2015) doi:10.1093/pcp/pcu164 Seifert, G.J. and Blaukopf, C. (2010) Irritable walls: the plant extracellular matrix and signaling. Plant Physiol. 153: 467–478. Sénéchal, F., Wattier, C., Rustérucci, C. and Pelloux, J. (2014) Homogalacturonan-modifying enzymes: structure, expression, and roles in plants. J. Exp. Bot. 65: 5125–5160. Shankarnarayan, S., Narang, S.S., Malone, C.L., Deschenes, R.J. and Fassler, J.S. (2008) Modulation of yeast Sln1 kinase activity by the CCW12 cell wall protein. J. Biol. Chem. 283: 1962–1973. Stolz, A., Riefler, M., Lomin, S.N., Achazi, K., Romanov, G.A. and Schmulling, T. (2011) The specificity of cytokinin signalling in Arabidopsis thaliana is mediated by differing ligand affinities and expression profiles of the receptors. Plant J. 67: 157–168. Tanaka, K., Tatebayashi, K., Nishimura, A., Yamamoto, K., Yang., H.-Y. and Saito, H. (2014) Yeast osmosensors Hkr1 and Msb2 activate the Hog1 MAPK cascade by different mechanisms. Sci. Signal. 7: ra21. Tatebayashi, K., Tanaka, K., Yang, H.-Y., Yamamoto, K., Matsushita, Y., Tomida, T. et al. (2007) Transmembrane mucins Hkr1 and Msb2 are putative osmosensors in the SHO1 branch of yeast HOG pathway. EMBO J. 26: 3521–3533. Tsang, D.L., Edmond, C., Harrington, J.L. and Nuhse, T.S. (2011) Cell wall integrity controls root elongation via a general 1-aminocyclopropane-1-carboxylic acid-dependent, ethylene-independent pathway. Plant Physiol. 156: 596–604. Verna, J., Lodder, A., Lee, K., Vagts, A. and Ballester, R. (1997) A family of genes required for maintenance of cell wall integrity and for the stress response in Saccharomyces cerevisiae. Proc. Natl Acad. Sci. USA 94: 13804–13809. Wang, Y., Chantreau, M., Sibout, R. and Hawkins, S. (2013) Plant cell wall lignification and monolignol metabolism. Front. Plant Sci. 4: 220. Wilson, M.E., Maksaev, G. and Haswell, E.S. (2013) MscS-like mechanosensitive channels in plants and microbes. Biochemistry 52: 5708–5722. Wolf, S., Hématy, K. and Höfte, H. (2012) Growth control and cell wall signaling in plants. Annu. Rev. Plant Biol. 63: 381–407. Wolf, S. and Hofte, H. (2014) Growth control: a saga of cell walls, ROS, and peptide receptors. Plant Cell 26: 1848–1856. Wolf, S., Rausch, T. and Greiner, S. (2009) The N-terminal pro region mediates retention of unprocessed type-I PME in the Golgi apparatus. Plant J. 58: 361–375. Wormit, A., Butt, S., Chairam, I., McKenna, J.F., Nunes-Nesi, A., Kjaer, L. et al. (2012) Osmosensitive changes of carbohydrate metabolism in response to cellulose biosynthesis inhibition. Plant Physiol. 159: 105–117. Yoshida, S., Bartolini, S. and Pellman, D. (2009) Mechanisms for concentrating Rho1 during cytokinesis. Genes Dev. 23: 810–823. Yoshimoto, H., Saltsman, K., Gasch, A.P., Li, H.X., Ogawa, N., Botstein, D. et al. (2002) Genome-wide analysis of gene expression regulated by the calcineurin/Crz1p signaling pathway in Saccharomyces cerevisiae. J. Biol. Chem. 277: 31079–31088. Yuan, F., Yang, H., Xue, Y., Kong, D., Ye, R., Li, C. et al. (2014) OSCA1 mediates osmotic-stress-evoked Ca2+ increases vital for osmosensing in Arabidopsis. Nature 514: 367–371. 223