Novel cell wall architecture of isoxaben-habituated Arabidopsis suspension-cultured cells: global transcript profiling and cellular analysis

Cell wall-mediated root development is targeted by a soil-borne bacterial pathogen to promote infection

Plant pathogens manipulate host development, facilitating colonization and proliferation. Ralstonia solanacearum is a soil-borne bacterial pathogen that penetrates roots and colonizes plants through the vascular system, causing wilting and death. Here, we find that RipAC, an effector protein from R. solanacearum, alters root development in Arabidopsis, promoting the formation of lateral roots and root hairs. RipAC interacts with CELLULOSE SYNTHASE (CESA)-INTERACTIVE PROTEIN 1 (CSI1), which regulates the activity of CESA complexes at the plasma membrane. RipAC disrupts CESA-CSI1 interaction, leading to a reduction in cellulose content, root developmental alterations, and a promotion of bacterial pathogenicity. We find that CSI1 also associates with the receptor kinase FERONIA, forming a complex that negatively regulates immunity in roots; this interaction, however, is not affected by RipAC. Our work reveals a bacterial virulence strategy that selectively affects the activities of a host target, promoting anatomical alterations that facilitate infection without causing activation of immunity.

The plant cell wall-dynamic, strong, and adaptable-is a natural shapeshifter

Mythology is replete with good and evil shapeshifters, who, by definition, display great adaptability and assume many different forms-with several even turning themselves into trees. Cell walls certainly fit this definition as they can undergo subtle or dramatic changes in structure, assume many shapes, and perform many functions. In this review, we cover the evolution of knowledge of the structures, biosynthesis, and functions of the 5 major cell wall polymer types that range from deceptively simple to fiendishly complex. Along the way, we recognize some of the colorful historical figures who shaped cell wall research over the past 100 years. The shapeshifter analogy emerges more clearly as we examine the evolving proposals for how cell walls are constructed to allow growth while remaining strong, the complex signaling involved in maintaining cell wall integrity and defense against disease, and the ways cell walls adapt as they progress from birth, through growth to maturation, and in the end, often function long after cell death. We predict the next century of progress will include deciphering cell type-specific wall polymers; regulation at all levels of polymer production, crosslinks, and architecture; and how walls respond to developmental and environmental signals to drive plant success in diverse environments.

Combining extracellular matrix proteome and phosphoproteome of chickpea and meta-analysis reveal novel proteoforms and evolutionary significance of clade-specific wall-associated events in plant

Extracellular matrix (ECM) plays central roles in cell architecture, innate defense and cell wall integrity (CWI) signaling. During transition to multicellularity, modular domain structures of ECM proteins and proteoforms have evolved due to continuous adaptation across taxonomic clades under different ecological niche. Although this incredible diversity has to some extent been investigated at protein level, extracellular phosphorylation events and molecular evolution of ECM proteoform families remains unexplored. We developed matrisome proteoform atlas in a grain legume, chickpea and performed meta-analyses of 74 plant matrisomes. MS/MS analysis identified 1,424 proteins and 315 phosphoproteins involved in diverse functions. Cross-species ECM protein network identified proteoforms associated with CWI maintenance system. Phylogenetic characterization of eighteen matrix protein families highlighted the role of taxon-specific paralogs and orthologs. Novel information was acquired on gene expansion and loss, co-divergence, sub functionalization and neofunctionalization during evolution. Modular networks of matrix protein families and hub proteins showed higher diversity across taxonomic clades than among organs. Furthermore, protein families differ in nonsynonymous to synonymous substitution rates. Our study pointed towards the matrix proteoform functionality, sequence divergence variation, interactions between wall remodelers and molecular evolution using a phylogenetic framework. This is the first report on comprehensive matrisome proteoform network illustrating presence of CWI signaling proteins in land plants.

SET Domain Group 703 Regulates Planthopper Resistance by Suppressing the Expression of Defense-Related Genes

Plant defense responses against insect pests are intricately regulated by highly complex regulatory networks. Post-translational modifications (PTMs) of histones modulate the expression of genes involved in various biological processes. However, the role of PTMs in conferring insect resistance remains unclear. Through the screening of a T-DNA insertion activation-tagged mutant collection in rice, we identified the mutant planthopper susceptible 1 (phs1), which exhibits heightened expression of SET domain group 703 (SDG703). This overexpression is associated with increased susceptibility to the small brown planthopper (SBPH), an economically significant insect pest affecting rice crops. SDG703 is constitutively expressed in multiple tissues and shows substantial upregulation in response to SBPH feeding. SDG703 demonstrates the activity of histone H3K9 methyltransferase. Transcriptomic analysis revealed the downregulation of genes involved in effector-triggered immunity (ETI) and pattern-triggered immunity (PTI) in plants overexpressing SDG703. Among the downregulated genes, the overexpression of SDG703 in plants resulted in a higher level of histone H3K9 methylation compared to control plants. Collectively, these findings indicate that SDG703 suppresses the expression of defense-related genes through the promotion of histone methylation, consequently leading to reduced resistance against SBPH. The defense-related genes regulated by histone methylation present valuable targets for developing effective pest management strategies in future studies. Furthermore, our study provides novel insight into the epigenetic regulation involved in plant-insect resistance.

Increased gene expression variability hinders the formation of regional mechanical conflicts leading to reduced organ shape robustness

To relate gene networks and organ shape, one needs to address two wicked problems: i) Gene expression is often variable locally, and shape is reproducible globally; ii) gene expression can have cascading effects on tissue mechanics, with possibly counterintuitive consequences for the final organ shape. Here, we address such wicked problems, taking advantage of simpler plant organ development where shape only emerges from cell division and elongation. We confirm that mutation in VERNALIZATION INDEPENDENCE 3 (VIP3), a subunit of the conserved polymerase-associated factor 1 complex (Paf1C), increases gene expression variability in Arabidopsis. Then, we focused on the Arabidopsis sepal, which exhibits a reproducible shape and stereotypical regional growth patterns. In vip3 sepals, we measured higher growth heterogeneity between adjacent cells. This even culminated in the presence of negatively growing cells in specific growth conditions. Interestingly, such increased local noise interfered with the stereotypical regional pattern of growth. We previously showed that regional differential growth at the wild-type sepal tip triggers a mechanical conflict, to which cells resist by reinforcing their walls, leading to growth arrest. In vip3, the disturbed regional growth pattern delayed organ growth arrest and increased final organ shape variability. Altogether, we propose that gene expression variability is managed by Paf1C to ensure organ robustness by building up mechanical conflicts at the regional scale, instead of the local scale.

Effect of Triazole Fungicides Titul Duo and Vintage on the Development of Pea (Pisum sativum L.) Symbiotic Nodules

Triazole fungicides are widely used in agricultural production for plant protection, including pea (Pisum sativum L.). The use of fungicides can negatively affect the legume-Rhizobium symbiosis. In this study, the effects of triazole fungicides Vintage and Titul Duo on nodule formation and, in particular, on nodule morphology, were studied. Both fungicides at the highest concentration decreased the nodule number and dry weight of the roots 20 days after inoculation. Transmission electron microscopy revealed the following ultrastructural changes in nodules: modifications in the cell walls (their clearing and thinning), thickening of the infection thread walls with the formation of outgrowths, accumulation of poly-β-hydroxybutyrates in bacteroids, expansion of the peribacteroid space, and fusion of symbiosomes. Fungicides Vintage and Titul Duo negatively affect the composition of cell walls, leading to a decrease in the activity of synthesis of cellulose microfibrils and an increase in the number of matrix polysaccharides of cell walls. The results obtained coincide well with the data of transcriptomic analysis, which revealed an increase in the expression levels of genes that control cell wall modification and defense reactions. The data obtained indicate the need for further research on the effects of pesticides on the legume-Rhizobium symbiosis in order to optimize their use.

Signals and Their Perception for Remodelling, Adjustment and Repair of the Plant Cell Wall

The integrity of the cell wall is important for plant cells. Mechanical or chemical distortions, tension, pH changes in the apoplast, disturbance of the ion homeostasis, leakage of cell compounds into the apoplastic space or breakdown of cell wall polysaccharides activate cellular responses which often occur via plasma membrane-localized receptors. Breakdown products of the cell wall polysaccharides function as damage-associated molecular patterns and derive from cellulose (cello-oligomers), hemicelluloses (mainly xyloglucans and mixed-linkage glucans as well as glucuronoarabinoglucans in Poaceae) and pectins (oligogalacturonides). In addition, several types of channels participate in mechanosensing and convert physical into chemical signals. To establish a proper response, the cell has to integrate information about apoplastic alterations and disturbance of its wall with cell-internal programs which require modifications in the wall architecture due to growth, differentiation or cell division. We summarize recent progress in pattern recognition receptors for plant-derived oligosaccharides, with a focus on malectin domain-containing receptor kinases and their crosstalk with other perception systems and intracellular signaling events.

The Genetic Control of SEEDSTICK and LEUNIG-HOMOLOG in Seed and Fruit Development: New Insights into Cell Wall Control

Although much is known about seed and fruit development at the molecular level, many gaps remain in our understanding of how cell wall modifications can impact developmental processes in plants, as well as how biomechanical alterations influence seed and fruit growth. Mutants of Arabidopsis thaliana constitute an excellent tool to study the function of gene families devoted to cell wall biogenesis. We have characterized a collection of lines carrying mutations in representative cell wall-related genes for seed and fruit size developmental defects, as well as altered germination rates. We have linked these studies to cell wall composition and structure. Interestingly, we have found that disruption of genes involved in pectin maturation and hemicellulose deposition strongly influence germination dynamics. Finally, we focused on two transcriptional regulators, SEEDSTICK (STK) and LEUNIG-HOMOLOG (LUH), which positively regulate seed growth. Herein, we demonstrate that these factors regulate specific aspects of cell wall properties such as pectin distribution. We propose a model wherein changes in seed coat structure due to alterations in the xyloglucan-cellulose matrix deposition and pectin maturation are critical for organ growth and germination. The results demonstrate the importance of cell wall properties and remodeling of polysaccharides as major factors responsible for seed development.

Galactose induces formation of cell wall stubs and cell death in Arabidopsis roots

Arabidopsis seedlings growing on low concentration of galactose stop regular root growth. Incomplete cell division with cell wall stubs, binuclear and giant cells and lignified root tips are observed. Galactose is a sugar abundant in root cell walls of Arabidopsis. Nevertheless, we found that the germination of Arabidopsis seedlings on galactose containing media causes a strong modification of the root development, as shown by analysing the root with microscopy methods ranging from the bright field over confocal to transmission electron microscopy. At concentrations of about 1 mM, the growth of the primary root stops after a few days though stem cell markers like WOX5 are still expressed. The root tip swells and forms a slightly opaque, partially lignified structure in parts of the cortex and the central cylinder. The formation of the cell plate after mitosis is impaired, often leading to cell wall stubs and binuclear cells. Some cells in the cortex and the central cylinder degenerate, while some rhizodermal and cortical cells increase massively in size. The galactose toxicity phenotype in Arabidopsis depends on the activity of galactokinase and is completely diminished in galactokinase knock-out lines. From the comparison of the galactose toxicity phenotype with those of cytokinesis mutants and plants treated with appropriate inhibitors we speculate that the toxicity syndrome of galactose is caused by interference with intracellular vesicle transport or cell wall biogenesis.

Pectin methylesterase gene AtPMEPCRA contributes to physiological adaptation to simulated and spaceflight microgravity in Arabidopsis

Pectin is biosynthesized in a highly methylated form and is partially de-methylated by pectin methylesterase (PME) activity. Plant PMEs play a critical role in cell wall remodeling in many physiological processes. Here, we studied Arabidopsis seedlings, which had been exposed to simulated or actual microgravity. Simulated microgravity inhibited total PME activity in Arabidopsis seedlings. We identified that AtPMEPCRA expression played a major role in the microgravity-induced inhibition of PME activity. atpmepcra mutants did not exhibit the enlarged leaf area of Arabidopsis seedlings observed under spaceflight microgravity. The downregulation of AtPMEPCRA expression in response to microgravity was due, in part, to changes in methylation patterns. The sexual offspring of the plants grown during spaceflight retained the methylation changes at AtPMEPCRA locus for one generation and thus contribute to the physiological adaptation to microgravity among F₁ offspring seed generation. We conclude that AtPMEPCRA contributes to the spaceflight-induced transgenerational responses in Arabidopsis.

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Simulated microgravity inhibited total PME activity in Arabidopsis seedlings

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AtPMEPCRA played a major role in the microgravity-induced inhibition of PME activity

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atpmepcra mutants did not exhibit the enlarged seedlings leaf area observed under microgravity

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AtPMEPCRA regulates the spaceflight-induced transgenerational responses in Arabidopsis

Cell Wall Signaling in Plant Development and Defense

Plant architecture fundamentally differs from that of other multicellular organisms in that individual cells serve as osmotic bricks, defined by the equilibrium between the internal turgor pressure and the mechanical resistance of the surrounding cell wall, which constitutes the interface between plant cells and their environment. The state and integrity of the cell wall are constantly monitored by cell wall surveillance pathways, which relay information to the cell interior. A recent surge of discoveries has led to significant advances in both mechanistic and conceptual insights into a multitude of cell wall response pathways that play diverse roles in the development, defense, stress response, and maintenance of structural integrity of the cell. However, these advances have also revealed the complexity of cell wall sensing, and many more questions remain to be answered, for example, regarding the mechanisms of cell wall perception, the molecular players in this process, and how cell wall-related signals are transduced and integrated into cellular behavior. This review provides an overview of the mechanistic and conceptual insights obtained so far and highlights areas for future discoveries in this exciting area of plant biology.

Arabidopsis pavement cell morphogenesis requires FERONIA binding to pectin for activation of ROP GTPase signaling

Sensing and signaling of cell wall status and dynamics regulate many processes in plants, such as cell growth and morphogenesis, but the underpinning mechanisms remain largely unknown. Here, we demonstrate that the CrRLK1L receptor kinase FERONIA (FER) binds the cell wall pectin, directly leading to the activation of the ROP6 guanosine triphosphatase (GTPase) signaling pathway that regulates the formation of the puzzle piece shape of pavement cells in Arabidopsis. The extracellular malectin domain of FER binds demethylesterified pectin in vivo and in vitro. Both loss-of-FER mutations and defects in pectin demethylesterification caused similar changes in pavement cell shape and ROP6 GTPase signaling. FER is required for the activation of ROP6 by demethylesterified pectin and physically and genetically interacts with the ROP6 activator, RopGEF14. Thus, our findings elucidate a signaling pathway that directly connects the cell wall pectin to cellular morphogenesis via the cell surface receptor FER.

FERONIA and microtubules independently contribute to mechanical integrity in the Arabidopsis shoot

To survive, cells must constantly resist mechanical stress. In plants, this involves the reinforcement of cell walls, notably through microtubule-dependent cellulose deposition. How wall sensing might contribute to this response is unknown. Here, we tested whether the microtubule response to stress acts downstream of known wall sensors. Using a multistep screen with 11 mutant lines, we identify FERONIA (FER) as the primary candidate for the cell’s response to stress in the shoot. However, this does not imply that FER acts upstream of the microtubule response to stress. In fact, when performing mechanical perturbations, we instead show that the expected microtubule response to stress does not require FER. We reveal that the feronia phenotype can be partially rescued by reducing tensile stress levels. Conversely, in the absence of both microtubules and FER, cells appear to swell and burst. Altogether, this shows that the microtubule response to stress acts as an independent pathway to resist stress, in parallel to FER. We propose that both pathways are required to maintain the mechanical integrity of plant cells.

In all living organisms, cells must resist mechanical stress to survive. This study of the model plant Arabidopsis reveals that the candidate cell wall mechanoreceptor FERONIA and microtubules independently contribute to this mechanical feedback.

The Cell Wall Proteome of Craterostigma plantagineum Cell Cultures Habituated to Dichlobenil and Isoxaben

The remarkable desiccation tolerance of the vegetative tissues in the resurrection species Craterostigma plantagineum (Hochst.) is favored by its unique cell wall folding mechanism that allows the ordered and reversible shrinking of the cells without damaging neither the cell wall nor the underlying plasma membrane. The ability to withstand extreme drought is also maintained in abscisic acid pre-treated calli, which can be cultured both on solid and in liquid culture media. Cell wall research has greatly advanced, thanks to the use of inhibitors affecting the biosynthesis of e.g., cellulose, since they allowed the identification of the compensatory mechanisms underlying habituation. Considering the innate cell wall plasticity of C. plantagineum, the goal of this investigation was to understand whether habituation to the cellulose biosynthesis inhibitors dichlobenil and isoxaben entailed or not identical mechanisms as known for non-resurrection species and to decipher the cell wall proteome of habituated cells. The results showed that exposure of C. plantagineum calli/cells triggered abnormal phenotypes, as reported in non-resurrection species. Additionally, the data demonstrated that it was possible to habituate Craterostigma cells to dichlobenil and isoxaben and that gene expression and protein abundance did not follow the same trend. Shotgun and gel-based proteomics revealed a common set of proteins induced upon habituation, but also identified candidates solely induced by habituation to one of the two inhibitors. Finally, it is hypothesized that alterations in auxin levels are responsible for the increased abundance of cell wall-related proteins upon habituation.

Impaired Cuticle Functionality and Robust Resistance to Botrytis cinerea in Arabidopsis thaliana Plants With Altered Homogalacturonan Integrity Are Dependent on the Class III Peroxidase AtPRX71

Pectin is a major cell wall component that plays important roles in plant development and response to environmental stresses. Arabidopsis thaliana plants expressing a fungal polygalacturonase (PG plants) that degrades homogalacturonan (HG), a major pectin component, as well as loss-of-function mutants for QUASIMODO2 (QUA2), encoding a putative pectin methyltransferase important for HG biosynthesis, show accumulation of reactive oxygen species (ROS), reduced growth and almost complete resistance to the fungal pathogen Botrytis cinerea. Both PG and qua2 plants show increased expression of the class III peroxidase AtPRX71 that contributes to their elevated ROS levels and reduced growth. In this work, we show that leaves of PG and qua2 plants display greatly increased cuticle permeability. Both increased cuticle permeability and resistance to B. cinerea in qua2 are suppressed by loss of AtPRX71. Increased cuticle permeability in qua2, rather than on defects in cuticle ultrastructure or cutin composition, appears to be dependent on reduced epidermal cell adhesion, which is exacerbated by AtPRX71, and is suppressed by the esmeralda1 mutation, which also reverts the adhesion defect and the resistant phenotype. Increased cuticle permeability, accumulation of ROS, and resistance to B. cinerea are also observed in mutants lacking a functional FERONIA, a receptor-like kinase thought to monitor pectin integrity. In contrast, mutants with defects in other structural components of primary cell wall do not have a defective cuticle and are normally susceptible to the fungus. Our results suggest that disrupted cuticle integrity, mediated by peroxidase-dependent ROS accumulation, plays a major role in the robust resistance to B. cinerea of plants with altered HG integrity.

The Fungicide Tetramethylthiuram Disulfide Negatively Affects Plant Cell Walls, Infection Thread Walls, and Symbiosomes in Pea (Pisum sativum L.) Symbiotic Nodules

In Russia, tetramethylthiuram disulfide (TMTD) is a fungicide widely used in the cultivation of legumes, including the pea (Pisum sativum). Application of TMTD can negatively affect nodulation; nevertheless, its effect on the histological and ultrastructural organization of nodules has not previously been investigated. In this study, the effect of TMTD at three concentrations (0.4, 4, and 8 g/kg) on nodule development in three pea genotypes (laboratory lines Sprint-2 and SGE, and cultivar 'Finale') was examined. In SGE, TMTD at 0.4 g/kg reduced the nodule number and shoot and root fresh weights. Treatment with TMTD at 8 g/kg changed the nodule color from pink to green, indicative of nodule senescence. Light and transmission electron microscopy analyses revealed negative effects of TMTD on nodule structure in each genotype. 'Finale' was the most sensitive cultivar to TMTD and Sprint-2 was the most tolerant. The negative effects of TMTD on nodules included the appearance of a senescence zone, starch accumulation, swelling of cell walls accompanied by a loss of electron density, thickening of the infection thread walls, symbiosome fusion, and bacteroid degradation. These results demonstrate how TMTD adversely affects nodules in the pea and will be useful for developing strategies to optimize fungicide use on legume crops.

Callose deposition is essential for the completion of cytokinesis in the unicellular alga Penium margaritaceum

Cytokinesis in land plants involves the formation of a cell plate that develops into the new cell wall. Callose, a β-1,3 glucan, accumulates at later stages of cell plate development, presumably to stabilize this delicate membrane network during expansion. Cytokinetic callose is considered specific to multicellular plant species, because it has not been detected in unicellular algae. Here we present callose at the cytokinesis junction of the unicellular charophyte, Penium margaritaceum Callose deposition at the division plane of P. margaritaceum showed distinct, spatiotemporal patterns likely representing distinct roles of this polymer in cytokinesis. Pharmacological inhibition of callose deposition by endosidin 7 resulted in cytokinesis defects, consistent with the essential role for this polymer in P. margaritaceum cell division. Cell wall deposition at the isthmus zone was also affected by the absence of callose, demonstrating the dynamic nature of new wall assembly in P. margaritaceum The identification of candidate callose synthase genes provides molecular evidence for callose biosynthesis in P. margaritaceum The evolutionary implications of cytokinetic callose in this unicellular zygnematopycean alga is discussed in the context of the conquest of land by plants.This article has an associated First Person interview with the first author of the paper.

Shaping Organs: Shared Structural Principles Across Kingdoms

Development encapsulates the morphogenesis of an organism from a single fertilized cell to a functional adult. A critical part of development is the specification of organ forms. Beyond the molecular control of morphogenesis, shape in essence entails structural constraints and thus mechanics. Revisiting recent results in biophysics and development, and comparing animal and plant model systems, we derive key overarching principles behind the formation of organs across kingdoms. In particular, we highlight how growing organs are active rather than passive systems and how such behavior plays a role in shaping the organ. We discuss the importance of considering different scales in understanding how organs form. Such an integrative view of organ development generates new questions while calling for more cross-fertilization between scientific fields and model system communities.

Plant cell wall integrity maintenance in model plants and crop species-relevant cell wall components and underlying guiding principles

The walls surrounding the cells of all land-based plants provide mechanical support essential for growth and development as well as protection from adverse environmental conditions like biotic and abiotic stress. Composition and structure of plant cell walls can differ markedly between cell types, developmental stages and species. This implies that wall composition and structure are actively modified during biological processes and in response to specific functional requirements. Despite extensive research in the area, our understanding of the regulatory processes controlling active and adaptive modifications of cell wall composition and structure is still limited. One of these regulatory processes is the cell wall integrity maintenance mechanism, which monitors and maintains the functional integrity of the plant cell wall during development and interaction with environment. It is an important element in plant pathogen interaction and cell wall plasticity, which seems at least partially responsible for the limited success that targeted manipulation of cell wall metabolism has achieved so far. Here, we provide an overview of the cell wall polysaccharides forming the bulk of plant cell walls in both monocotyledonous and dicotyledonous plants and the effects their impairment can have. We summarize our current knowledge regarding the cell wall integrity maintenance mechanism and discuss that it could be responsible for several of the mutant phenotypes observed.

Cell Wall Polysaccharide-Mediated Cadmium Tolerance Between Two Arabidopsis thaliana Ecotypes

Cadmium (Cd) is a toxic metal element and the mechanism(s) underlying Cd tolerance in plants are still unclear. Increasingly more studies have been conducted on Cd binding to plant cell walls (CW) but most of them have focused on Cd fixation by CW pectin, and few studies have examined Cd binding to cellulose and hemicellulose. Here we found that Cd binding to CW pectin, cellulose, and hemicellulose was significantly higher in Tor-1, a Cd tolerant A. thaliana ecotype, than in Ph2-23, a sensitive ecotype, as were the concentrations of pectin, cellulose, and hemicellulose. Transcriptome analysis revealed that the genes regulating CW pectin, cellulose, and hemicellulose polysaccharide concentrations in Tor-1 differed significantly from those in Ph2-23. The expressions of most genes such as pectin methyl esterase inhibitors (PMEIs), pectin lyases, xyloglucan endotransglucosylase/hydrolase, expansins (EXPAs), and cellulose hydrolase were higher in Ph2-23, while the expressions of cellulose synthase-like glycosyltransferase 3 (CSLG3) and pectin ethyl esterase 4 (PAE4) were higher in Tor-1. The candidate genes identified here seem to regulate CW Cd fixation by polysaccharides. In conclusion, an increase in pectin demethylation activity, the higher concentration of cellulose and hemicellulose, regulated by related genes, in Tor-1 than in Ph2-23 are likely involved in enhanced Cd CW retention and reduce Cd toxicity.

Hechtian Strands Transmit Cell Wall Integrity Signals in Plant Cells

Hechtian strands are thread-like structures in plasmolyzed plant cells that connect the cell wall to the plasma membrane. Although these strands were first observed more than 100 years ago, their physiological roles are largely unknown. Here, we used intracellular laser microdissection to examine the effects of disrupting Hechtian strands on plasmolyzed tobacco BY-2 cells. When we focused femtosecond laser pulses on Hechtian strands, targeted disruptions were induced, but no visible changes in cell morphology were detected. However, the calcofluor white signals from β-glucans was detected in plasmolyzed cells with disrupted Hechtian strands, whereas no signals were detected in untreated plasmolyzed cells. These results suggest that Hechtian strands play roles in sensing cell wall integrity.

A wall with integrity: surveillance and maintenance of the plant cell wall under stress

The structural and functional integrity of the cell wall needs to be constantly monitored and fine-tuned to allow for growth while preventing mechanical failure. Many studies have advanced our understanding of the pathways that contribute to cell wall biosynthesis and how these pathways are regulated by external and internal cues. Recent evidence also supports a model in which certain aspects of the wall itself may act as growth-regulating signals. Molecular components of the signaling pathways that sense and maintain cell wall integrity have begun to be revealed, including signals arising in the wall, sensors that detect changes at the cell surface, and downstream signal transduction modules. Abiotic and biotic stress conditions provide new contexts for the study of cell wall integrity, but the nature and consequences of wall disruptions due to various stressors require further investigation. A deeper understanding of cell wall signaling will provide insights into the growth regulatory mechanisms that allow plants to survive in changing environments.

Interplay between Plant Cell Walls and Jasmonate Production

Plant cell walls are sophisticated carbohydrate-rich structures representing the immediate contact surface with the extracellular environment, often serving as the first barrier against biotic and abiotic stresses. Notably, a variety of perturbations in plant cell walls result in upregulated jasmonate (JA) production, a phytohormone with essential roles in defense and growth responses. Hence, cell wall-derived signals can initiate intracellular JA-mediated responses and the elucidation of the underlying signaling pathways could provide novel insights into cell wall maintenance and remodeling, as well as advance our understanding on how is JA biosynthesis initiated. This Mini Review will describe current knowledge about cell wall-derived damage signals and their effects on JA biosynthesis, as well as provide future perspectives.

Cotton CSLD3 restores cell elongation and cell wall integrity mainly by enhancing primary cellulose production in the Arabidopsis cesa6 mutant

Overexpression of cotton cellulose synthase like D3 (GhCSLD3) gene partially rescued growth defect of atcesa6 mutant with restored cell elongation and cell wall integrity mainly by enhancing primary cellulose production. Among cellulose synthase like (CSL) family proteins, CSLDs share the highest sequence similarity to cellulose synthase (CESA) proteins. Although CSLD proteins have been implicated to participate in the synthesis of carbohydrate-based polymers (cellulose, pectins and hemicelluloses), and therefore plant cell wall formation, the exact biochemical function of CSLD proteins remains controversial and the function of the remaining CSLD genes in other species have not been determined. In this study, we attempted to illustrate the function of CSLD proteins by overexpressing Arabidopsis AtCSLD2, -3, -5 and cotton GhCSLD3 genes in the atcesa6 mutant, which has a background that is defective for primary cell wall cellulose synthesis in Arabidopsis. We found that GhCSLD3 overexpression partially rescued the growth defect of the atcesa6 mutant during early vegetative growth. Despite the atceas6 mutant having significantly reduced cellulose contents, the defected cell walls and lower dry mass, GhCSLD3 overexpression largely restored cell wall integrity (CWI) and improved the biomass yield. Our result suggests that overexpression of the GhCSLD protein enhances primary cell wall synthesis and compensates for the loss of CESAs, which is required for cellulose production, therefore rescuing defects in cell elongation and CWI.

Functional characterization of genes mediating cell wall metabolism and responses to plant cell wall integrity impairment

BACKGROUND

Plant cell walls participate in all plant-environment interactions. Maintaining cell wall integrity (CWI) during these interactions is essential. This realization led to increased interest in CWI and resulted in knowledge regarding early perception and signalling mechanisms active during CWI maintenance. By contrast, knowledge regarding processes mediating changes in cell wall metabolism upon CWI impairment is very limited.

RESULTS

To identify genes involved and to investigate their contributions to the processes we selected 23 genes with altered expression in response to CWI impairment and characterized the impact of T-DNA insertions in these genes on cell wall composition using Fourier-Transform Infrared Spectroscopy (FTIR) in Arabidopsis thaliana seedlings. Insertions in 14 genes led to cell wall phenotypes detectable by FTIR. A detailed analysis of four genes found that their altered expression upon CWI impairment is dependent on THE1 activity, a key component of CWI maintenance. Phenotypic characterizations of insertion lines suggest that the four genes are required for particular aspects of CWI maintenance, cell wall composition or resistance to Plectosphaerella cucumerina infection in adult plants.

CONCLUSION

Taken together, the results implicate the genes in responses to CWI impairment, cell wall metabolism and/or pathogen defence, thus identifying new molecular components and processes relevant for CWI maintenance.

Are microtubules tension sensors?

Mechanical signals play many roles in cell and developmental biology. Several mechanotransduction pathways have been uncovered, but the mechanisms identified so far only address the perception of stress intensity. Mechanical stresses are tensorial in nature, and thus provide dual mechanical information: stress magnitude and direction. Here we propose a parsimonious mechanism for the perception of the principal stress direction. In vitro experiments show that microtubules are stabilized under tension. Based on these results, we explore the possibility that such microtubule stabilization operates in vivo, most notably in plant cells where turgor-driven tensile stresses exceed greatly those observed in animal cells.

Cellular mechanical stress is a key determinant of cell shape and function, but how the cell senses stress direction is unclear. In this Perspective the authors propose that microtubules autonomously sense stress directions in plant cells, where tensile stresses are higher than in animal cells.

Cell Wall Enzymes in Zygnema circumcarinatum UTEX 1559 Respond to Osmotic Stress in a Plant-Like Fashion

Previous analysis of charophyte green algal (CGA) genomes and transcriptomes for specific protein families revealed that numerous land plant characteristics had already evolved in CGA. In this study, we have sequenced and assembled the transcriptome of Zygnema circumcarinatum UTEX 1559, and combined its predicted protein sequences with those of 13 additional species [five embryophytes (Emb), eight charophytes (Cha), and two chlorophytes (Chl) as the outgroup] for a comprehensive comparative genomics analysis. In total 25,485 orthologous gene clusters (OGCs, equivalent to protein families) of the 14 species were classified into nine OGC groups. For example, the Cha+Emb group contains 4,174 OGCs found in both Cha and Emb but not Chl species, representing protein families that have evolved in the common ancestor of Cha and Emb. Different OGC groups were subjected to a Gene Ontology (GO) enrichment analysis with the Chl+Cha+Emb group (including 5,031 OGCs found in Chl and Cha and Emb) as the control. Interestingly, nine of the 20 top enriched GO terms in the Cha+Emb group are cell wall-related, such as biological processes involving celluloses, pectins, lignins, and xyloglucans. Furthermore, three glycosyltransferase families (GT2, 8, 43) were selected for in-depth phylogenetic analyses, which confirmed their presence in UTEX 1559. More importantly, of different CGA groups, only Zygnematophyceae has land plant cellulose synthase (CesA) orthologs, while other charophyte CesAs form a CGA-specific CesA-like (Csl) subfamily (likely also carries cellulose synthesis activity). Quantitative real-time-PCR experiments were performed on selected GT family genes in UTEX 1559. After osmotic stress treatment, significantly elevated expression was found for GT2 family genes ZcCesA, ZcCslC and ZcCslA-like (possibly mannan and xyloglucan synthases, respectively), as well as for GT8 family genes (possibly pectin synthases). All these suggest that the UTEX 1559 cell wall polysaccharide synthesis-related genes respond to osmotic stress in a manner that is similar to land plants.

COI1-dependent jasmonate signalling affects growth, metabolite production and cell wall protein composition in arabidopsis

Background and Aims

Cultured cell suspensions have been the preferred model to study the apoplast as well as to monitor metabolic and cell cycle-related changes. Previous work showed that methyl jasmonate (MeJA) inhibits leaf growth in a CORONATINE INSENSITIVE 1 (COI1)-dependent manner, with COI1 being the jasmonate (JA) receptor. Here, the effect of COI1 overexpression on the growth of stably transformed arabidopsis cell cultures is described.

Methods

Time-course experiments were carried out to analyse gene expression, and protein and metabolite levels.

Key Results

Both MeJA treatment and the overexpression of COI1 modify growth, by altering cell proliferation and expansion. DNA content as well as transcript patterns of cell cycle and cell wall remodelling markers were altered. COI1 overexpression also increases the protein levels of OLIGOGALACTURONIDE OXIDASE 1, BETA-GLUCOSIDASE/ENDOGLUCANASES and POLYGALACTURONASE INHIBITING PROTEIN2, reinforcing the role of COI1 in mediating defence responses and highlighting a link between cell wall loosening and growth regulation. Moreover, changes in the levels of the primary metabolites alanine, serine and succinic acid of MeJA-treated Arabidopsis cell cultures were observed. In addition, COI1 overexpression positively affects the availability of metabolites such as β-alanine, threonic acid, putrescine, glucose and myo-inositol, thereby providing a connection between JA-inhibited growth and stress responses.

Conclusions

This study contributes to the understanding of the regulation of growth and the production of metabolic resources by JAs and COI1. This will have important implications in dissecting the complex relationships between hormonal and cell wall signalling in plants. The work also provides tools to uncover novel mechanisms co-ordinating cell division and post-mitotic cell expansion in the absence of organ developmental control.

Hitting the Wall-Sensing and Signaling Pathways Involved in Plant Cell Wall Remodeling in Response to Abiotic Stress

Plant cells are surrounded by highly dynamic cell walls that play important roles regulating aspects of plant development. Recent advances in visualization and measurement of cell wall properties have enabled accumulation of new data about wall architecture and biomechanics. This has resulted in greater understanding of the dynamics of cell wall deposition and remodeling. The cell wall is the first line of defense against different adverse abiotic and biotic environmental influences. Different abiotic stress conditions such as salinity, drought, and frost trigger production of Reactive Oxygen Species (ROS) which act as important signaling molecules in stress activated cellular responses. Detection of ROS by still-elusive receptors triggers numerous signaling events that result in production of different protective compounds or even cell death, but most notably in stress-induced cell wall remodeling. This is mediated by different plant hormones, of which the most studied are jasmonic acid and brassinosteroids. In this review we highlight key factors involved in sensing, signal transduction, and response(s) to abiotic stress and how these mechanisms are related to cell wall-associated stress acclimatization. ROS, plant hormones, cell wall remodeling enzymes and different wall mechanosensors act coordinately during abiotic stress, resulting in abiotic stress wall acclimatization, enabling plants to survive adverse environmental conditions.

Outside-in control - does plant cell wall integrity regulate cell cycle progression?

During recent years it has become accepted that plant cell walls are not inert objects surrounding all plant cells but are instead highly dynamic, plastic structures. They are involved in a large number of cell biological processes and contribute actively to plant growth, development and interaction with environment. Therefore, it is not surprising that cellular processes can control plant cell wall integrity (CWI) while, simultaneously, CWI can influence cellular processes. In yeast and animal cells such a bidirectional relationship also exists between the yeast/animal extracellular matrices and the cell cycle. In yeast, the CWI maintenance mechanism and a dedicated plasma membrane integrity checkpoint are mediating this relationship. Recent research has yielded insights into the mechanism controlling plant cell wall metabolism during cytokinesis. However, the knowledge regarding putative regulatory pathways controlling adaptive modifications in plant cell cycle activity in response to changes in the state of the plant cell wall are not yet identified. In this review, we summarize similarities and differences in regulatory mechanisms coordinating extracellular matrices and cell cycle activity in animal and yeast cells, discuss the available evidence supporting the existence of such a mechanism in plants and suggest that the plant CWI maintenance mechanism might also control cell cycle activity in plant cells.

The contribution of mechanosensing to epidermal cell fate specification

In land plants, the aerial epidermis is essential for growth control, protection and environmental interactions. Epidermal cell fate is specified early during embryogenesis and maintained throughout plant life. Molecular actors of epidermal specification have been characterized, but how epidermal fate is maintained during growth remains unclear. DEFECTIVE KERNEL 1 (DEK1) is required for epidermal cell fate maintenance during both embryonic and post-embryonic plant development. The activation of a mechanosensitive Ca²⁺ channel was recently shown to depend on DEK1, suggesting that the interpretation of mechanical cues could be critical for maintaining epidermal cell fate. Here, we integrate these findings into the epidermal specification network and propose a model explaining why epidermal specification may depend upon the sensing of epidermal tension.

Gain-of-function mutation of AtDICE1, encoding a putative endoplasmic reticulum-localized membrane protein, causes defects in anisotropic cell elongation by disturbing cell wall integrity in Arabidopsis

Background and Aims

Anisotropic cell elongation depends on cell wall relaxation and cellulose microfibril arrangement. The aim of this study was to characterize the molecular function of AtDICE1 encoding a novel transmembrane protein involved in anisotropic cell elongation in Arabidopsis.

Methods

Phenotypic characterizations of transgenic Arabidopsis plants mis-regulating AtDICE1 expression with different pharmacological treatments were made, and biochemical, cell biological and transcriptome analyses were performed.

Key Results

Upregulation of AtDICE1 in Arabidopsis (35S::AtDICE1) resulted in severe dwarfism, probably caused by defects in anisotropic cell elongation. Epidermal cell swelling was evident in all tissues, and abnormal secondary wall thickenings were observed in pith cells of stems. These phenotypes were reproduced not only by inducible expression of AtDICE1 but also by overexpression of its poplar homologue in Arabidopsis. RNA interference suppression lines of AtDICE1 resulted in no observable phenotypic changes. Interestingly, wild-type plants treated with isoxaben, a cellulose biosynthesis inhibitor, phenocopied the 35S::AtDICE1 plants, suggesting that cellulose biosynthesis was compromised in the 35S::AtDICE1 plants. Indeed, disturbed cortical microtubule arrangements in 35S::AtDICE1/GFP-TuA6 plants were observed, and the cellulose content was significantly reduced in 35S::AtDICE1 plants. A promoter::GUS analysis showed that AtDICE1 is mainly expressed in vascular tissue, and transient expression of GFP:AtDICE1 in tobacco suggests that AtDICE1 is probably localized in the endoplasmic reticulum (ER). In addition, the external N-terminal conserved domain of AtDICE1 was found to be necessary for AtDICE1 function. Whole transcriptome analyses of 35S::AtDICE1 revealed that many genes involved in cell wall modification and stress/defence responses were mis-regulated.

Conclusions

AtDICE1, a novel ER-localized transmembrane protein, may contribute to anisotropic cell elongation in the formation of vascular tissue by affecting cellulose biosynthesis.

The plant cell wall integrity maintenance and immune signaling systems cooperate to control stress responses in Arabidopsis thaliana

Cell walls surround all plant cells, and their composition and structure are modified in a tightly controlled, adaptive manner to meet sometimes opposing functional requirements during growth and development. The plant cell wall integrity (CWI) maintenance mechanism controls these functional modifications, as well as responses to cell wall damage (CWD). We investigated how the CWI system mediates responses to CWD in Arabidopsis thaliana CWD induced by cell wall-degrading enzymes or an inhibitor of cellulose biosynthesis elicited similar, turgor-sensitive stress responses. Phenotypic clustering with 27 genotypes identified a core group of receptor-like kinases (RLKs) and ion channels required for the activation of CWD responses. A genetic analysis showed that the RLK FEI2 and the plasma membrane-localized mechanosensitive Ca²⁺ channel MCA1 functioned downstream of the RLK THE1 in CWD perception. In contrast, pattern-triggered immunity (PTI) signaling components, including the receptors for plant elicitor peptides (AtPeps) PEPR1 and PEPR2, repressed responses to CWD. CWD induced the expression of PROPEP1 and PROPEP3, which encode the precursors of AtPep1 and AtPep3, and the release of PROPEP3 into the growth medium. Application of AtPep1 and AtPep3 repressed CWD-induced phytohormone accumulation in a concentration-dependent manner. These results suggest that AtPep-mediated signaling suppresses CWD-induced defense responses controlled by the CWI mechanism. This suppression was alleviated when PTI signaling downstream of PEPR1 and PEPR2 was impaired. Defense responses controlled by the CWI maintenance mechanism might thus compensate to some extent for the loss of PTI signaling elements.

Grass cell walls have a role in the inherent tolerance of grasses to the cellulose biosynthesis inhibitor isoxaben

BACKGROUND

Cellulose biosynthesis inhibitors (CBIs) are pre-emergence herbicides that inhibit anisotropic cell expansion resulting in a severely swollen and stunted growth phenotype. Resistance to group 21 CBIs, such as isoxaben, is conferred by missense mutations in CELLOSE SYNTHASE A (CesA) genes required for primary cell wall synthesis, concluding that this is their in vivo target.

RESULTS

Herein, we show that grasses exhibit tolerance to group 21 CBIs and explore the mechanism of tolerance to isoxaben in the grass Brachypodium distachyon (L.). Comparative genomics failed to identify synonymous point mutations that have been found to confer isoxaben resistance in the dicot Arabidopsis thaliana (L.). Brachypodium did not metabolize ¹⁴ C-isoxaben. We next explored the role of grass-specific non-cellulosic cell wall components, specifically the hemicellulose polysaccharide mix linkage glucans (MLG), as a potential tolerance mechanism by compensating for the loss of cellulose during cell elongation. A partial-transcriptional knockdown T-DNA insertion was found in a key MLG synthesis gene, Cellulose synthase-like F6 (CslF6) and this mutant was found to be 2.1 times more sensitive to isoxaben than wild-type plants.

CONCLUSION

These data suggest that the composition and compensatory response of grass cell walls may be a factor in conferring tolerance to group 21 CBIs. © 2017 Society of Chemical Industry.

Plant cell wall-mediated immunity: cell wall changes trigger disease resistance responses

Plants have evolved a repertoire of monitoring systems to sense plant morphogenesis and to face environmental changes and threats caused by different attackers. These systems integrate different signals into overreaching triggering pathways which coordinate developmental and defence-associated responses. The plant cell wall, a dynamic and complex structure surrounding every plant cell, has emerged recently as an essential component of plant monitoring systems, thus expanding its function as a passive defensive barrier. Plants have a dedicated mechanism for maintaining cell wall integrity (CWI) which comprises a diverse set of plasma membrane-resident sensors and pattern recognition receptors (PRRs). The PRRs perceive plant-derived ligands, such as peptides or wall glycans, known as damage-associated molecular patterns (DAMPs). These DAMPs function as 'danger' alert signals activating DAMP-triggered immunity (DTI), which shares signalling components and responses with the immune pathways triggered by non-self microbe-associated molecular patterns that mediate disease resistance. Alteration of CWI by impairment of the expression or activity of proteins involved in cell wall biosynthesis and/or remodelling, as occurs in some plant cell wall mutants, or by wall damage due to colonization by pathogens/pests, activates specific defensive and growth responses. Our current understanding of how these alterations of CWI are perceived by the wall monitoring systems is scarce and few plant sensors/PRRs and DAMPs have been characterized. The identification of these CWI sensors and PRR-DAMP pairs will help us to understand the immune functions of the wall monitoring system, and might allow the breeding of crop varieties and the design of agricultural strategies that would enhance crop disease resistance.

Pectin and cellulose cell wall composition enables different strategies to leaf water uptake in plants from tropical fog mountain

Leaf water uptake (LWU) has been observed in plants of different ecosystems and this process is distinct among different species. Four plant species from the Brazilian fog mountain fields were evaluated in order to detect if leaf water uptake capacity is related to the cell wall composition of leaf epidermis. LWU measurements and their relation to anatomical and biochemical traits were analyzed. Cell wall composition was verified through immunocytochemistry using monoclonal antibodies recognizing pectin compounds, and histochemistry with calcofluor white to track cellulose. Differences in LWU among the four species were clearly revealed. Two species presented higher maximum leaf water content and the lowest values of water absorption speed. The other two species presented opposite behavior, namely, low leaf water uptake and the highest values of water absorption speed. The anatomical traits associated with the cell wall composition corroborated the data on the different LWU strategies. The species with abundant detection of cellulose in their epidermal cell walls absorbed more water, but more slowly, while those with abundant detection of pectins absorbed water at a higher speed. These results indicate that cell wall composition regarding pectin and cellulose are significant for water uptake by the leaf epidermis. Pectin provides greater porosity and absorption speed, while cellulose provides greater hydrophilicity and greater water uptake capacity. Current data indicate that the composition of epidermal cell walls is a relevant trait for leaf water uptake.

The impact of silicon on cell wall composition and enzymatic saccharification of Brachypodium distachyon

BACKGROUND

Plants and in particular grasses benefit from a high uptake of silicon (Si) which improves their growth and productivity by alleviating adverse effects of biotic and abiotic stress. However, the silicon present in plant tissues may have a negative impact on the processing and degradation of lignocellulosic biomass. Solutions to reduce the silicon content either by biomass engineering or development of downstream separation methods are therefore targeted. Different cell wall components have been proposed to interact with the silica pool in plant shoots, but the understanding of the underlying processes is still limited.

RESULTS

In the present study, we have characterized silicon deposition and cell wall composition in Brachypodium distachyon wild-type and low-silicon 1 (Bdlsi1-1) mutant plants. Our analyses included different organs and plant developmental stages. In the mutant defective in silicon uptake, low silicon availability favoured deposition of this element in the amorphous form or bound to cell wall polymers rather than as silicified structures. Several alterations in non-cellulosic polysaccharides and lignin were recorded in the mutant plants, indicating differences in the types of linkages and in the three-dimensional organization of the cell wall network. Enzymatic saccharification assays showed that straw from mutant plants was marginally more degradable following a 190 °C hydrothermal pretreatment, while there were no differences without or after a 120 °C hydrothermal pretreatment.

CONCLUSIONS

We conclude that silicon affects the composition of plant cell walls, mostly by altering linkages of non-cellulosic polymers and lignin. The modifications of the cell wall network and the reduced silicon concentration appear to have little or no implications on biomass recalcitrance to enzymatic saccharification.

Cell wall integrity modulates Arabidopsis thaliana cell cycle gene expression in a cytokinin- and nitrate reductase-dependent manner

During plant growth and defense, cell cycle activity needs to be coordinated with cell wall integrity. Little is known about how coordination is achieved. Here we investigated coordination in Arabidopsis thaliana seedlings by studying the impact of cell wall damage (CWD, caused by cellulose biosynthesis inhibition) on cytokinin homeostasis, cell cycle gene expression and shape in root tips. CWD inhibited cell cycle gene expression and increased transition zone cell width in an osmo-sensitive manner. These results were correlated with CWD-induced, osmo-sensitive changes in cytokinin homeostasis. Expression of CYTOKININ OXIDASE/DEHYDROGENASE2 and 3 (CKX2, CKX3), encoding cytokinin-degrading enzymes was induced by CWD and reduced by osmoticum treatment. In nitrate reductase1 nitrate reductase2 (nia1 nia2) seedlings, neither CKX2 and CKX3 transcript levels were increased nor cell cycle gene expression repressed by CWD. Moreover, established CWD-induced responses like jasmonic acid, salicylic acid and lignin production, were also absent, implying a central role of NIA1- and NIA2-mediated processes in regulation of CWD responses. These results suggest that CWD enhances cytokinin degradation rates through a NIA1 and NIA2-mediated process, subsequently attenuating cell cycle gene expression.

Plant cell wall signalling and receptor-like kinases

Communication between the extracellular matrix and the cell interior is essential for all organisms as intrinsic and extrinsic cues have to be integrated to co-ordinate development, growth, and behaviour. This applies in particular to plants, the growth and shape of which is governed by deposition and remodelling of the cell wall, a rigid, yet dynamic, extracellular network. It is thus generally assumed that cell wall surveillance pathways exist to monitor the state of the wall and, if needed, elicit compensatory responses such as altered expression of cell wall remodelling and biosynthesis genes. Here, I highlight recent advances in the field of cell wall signalling in plants, with emphasis on the role of plasma membrane receptor-like kinase complexes. In addition, possible roles for cell wall-mediated signalling beyond the maintenance of cell wall integrity are discussed.

The Arabidopsis leucine-rich repeat receptor kinase MIK2/LRR-KISS connects cell wall integrity sensing, root growth and response to abiotic and biotic stresses

Plants actively perceive and respond to perturbations in their cell walls which arise during growth, biotic and abiotic stresses. However, few components involved in plant cell wall integrity sensing have been described to date. Using a reverse-genetic approach, we identified the Arabidopsis thaliana leucine-rich repeat receptor kinase MIK2 as an important regulator of cell wall damage responses triggered upon cellulose biosynthesis inhibition. Indeed, loss-of-function mik2 alleles are strongly affected in immune marker gene expression, jasmonic acid production and lignin deposition. MIK2 has both overlapping and distinct functions with THE1, a malectin-like receptor kinase previously proposed as cell wall integrity sensor. In addition, mik2 mutant plants exhibit enhanced leftward root skewing when grown on vertical plates. Notably, natural variation in MIK2 (also named LRR-KISS) has been correlated recently to mild salt stress tolerance, which we could confirm using our insertional alleles. Strikingly, both the increased root skewing and salt stress sensitivity phenotypes observed in the mik2 mutant are dependent on THE1. Finally, we found that MIK2 is required for resistance to the fungal root pathogen Fusarium oxysporum. Together, our data identify MIK2 as a novel component in cell wall integrity sensing and suggest that MIK2 is a nexus linking cell wall integrity sensing to growth and environmental cues.

Plants are constantly exposed to external stresses of biotic and abiotic nature, as well as internal stresses, resulting from growth and mechanical tension. Feedback information about the integrity of the cell wall can enable the plant to perceive such stresses, and respond adequately. Plants are known to perceive signals from their environment through receptor kinases at the plant cell surface. Here, we reveal that the Arabidopsis thaliana receptor kinase MIK2 regulates responses to cell wall perturbation. Moreover, we find that MIK2 controls root growth angle, modulates cell wall structure in the root tip, contributes to salt stress tolerance, and is required for resistance against a root-infecting pathogen. Our data suggest that MIK2 is involved in sensing cell wall perturbations in plants, whereby it allows the plant to cope with a diverse range of environmental stresses. These data provide an important step forward in our understanding of the mechanisms plants deploy to sense internal and external danger.

A cell wall reference profile for Miscanthus bioenergy crops highlights compositional and structural variations associated with development and organ origin

Miscanthus spp. are promising lignocellulosic energy crops, but cell wall recalcitrance to deconstruction still hinders their widespread use as bioenergy and biomaterial feedstocks. Identification of cell wall characteristics desirable for biorefining applications is crucial for lignocellulosic biomass improvement. However, the task of scoring biomass quality is often complicated by the lack of a reference for a given feedstock. A multidimensional cell wall analysis was performed to generate a reference profile for leaf and stem biomass from several miscanthus genotypes harvested at three developmentally distinct time points. A comprehensive suite of 155 monoclonal antibodies was used to monitor changes in distribution, structure and extractability of noncellulosic cell wall matrix glycans. Glycan microarrays complemented with immunohistochemistry elucidated the nature of compositional variation, and in situ distribution of carbohydrate epitopes. Key observations demonstrated that there are crucial differences in miscanthus cell wall glycomes, which may impact biomass amenability to deconstruction. For the first time, variations in miscanthus cell wall glycan components were comprehensively characterized across different harvests, organs and genotypes, to generate a representative reference profile for miscanthus cell wall biomass. Ultimately, this portrait of the miscanthus cell wall will help to steer breeding and genetic engineering strategies for the development of superior energy crops.

Genetic and biochemical analysis reveals linked QTLs determining natural variation for fruit post-harvest water loss in pepper (Capsicum)

Molecular markers linked to QTLs controlling post-harvest fruit water loss in pepper may be utilized to accelerate breeding for improved shelf life and inhibit over-ripening before harvest. Bell pepper (Capsicum annuum L.) is an important vegetable crop world-wide. However, marketing is limited by the relatively short shelf life of the fruit due to water loss and decay that occur during prolonged storage. Towards breeding pepper with reduced fruit post-harvest water loss (PWL), we studied the genetic, physiological and biochemical basis for natural variation of PWL. We performed quantitative trait locus (QTL) mapping of fruit PWL in multiple generations of an interspecific cross of pepper, which resulted in the identification of two linked QTLs on chromosome 10 that control the trait. We further developed near-isogenic lines (NILs) for characterization of the QTL effects. Transcriptome analysis of the NILs allowed the identification of candidate genes associated with fruit PWL-associated traits such as cuticle biosynthesis, cell wall metabolism and fruit ripening. Significant differences in PWL between the NILs in the immature fruit stage, differentially expressed cuticle-associated genes and differences in the content of specific chemical constituents of the fruit cuticle, indicated a likely influence of cuticle composition on the trait. Reduced PWL in the NILs was associated with delayed over-ripening before harvest, low total soluble solids before storage, and reduced fruit softening after storage. Our study enabled a better understanding of the genetic and biological processes controlling natural variation in fruit PWL in pepper. Furthermore, the genetic materials and molecular markers developed in this study may be utilized to breed peppers with improved shelf life and inhibited over-ripening before harvest.

Tuning of pectin methylesterification: consequences for cell wall biomechanics and development

Recent publications have increased our knowledge of how pectin composition and the degree of homogalacturonan methylesterification impact the biochemical and biomechanical properties of plant cell walls, plant development, and plants' interactions with their abiotic and biotic environments. Experimental observations have shown that the relationships between the DM, the pattern of de-methylesterificaton, its effect on cell wall elasticity, other biomechanical parameters, and growth are not straightforward. Working towards a detailed understanding of these relationships at single cell resolution is one of the big tasks of pectin research. Pectins are highly complex polysaccharides abundant in plant primary cell walls. New analytical and microscopy techniques are revealing the composition and mechanical properties of the cell wall and increasing our knowledge on the topic. Progress in plant physiological research supports a link between cell wall pectin modifications and plant development and interactions with the environment. Homogalacturonan pectins, which are major components of the primary cell wall, have a potential for modifications such as methylesterification, as well as an ability to form cross-linked structures with divalent cations. This contributes to changing the mechanical properties of the cell wall. This review aims to give a comprehensive overview of the pectin component homogalacturonan, including its synthesis, modification, regulation and role in the plant cell wall.

The Arabidopsis RCC1 Family Protein TCF1 Regulates Freezing Tolerance and Cold Acclimation through Modulating Lignin Biosynthesis

Cell water permeability and cell wall properties are critical to survival of plant cells during freezing, however the underlying molecular mechanisms remain elusive. Here, we report that a specifically cold-induced nuclear protein, Tolerant to Chilling and Freezing 1 (TCF1), interacts with histones H3 and H4 and associates with chromatin containing a target gene, blue-copper-binding protein (BCB), encoding a glycosylphosphatidylinositol-anchored protein that regulates lignin biosynthesis. Loss of TCF1 function leads to reduced BCB transcription through affecting H3K4me2 and H3K27me3 levels within the BCB gene, resulting in reduced lignin content and enhanced freezing tolerance. Furthermore, plants with knocked-down BCB expression (amiRNA-BCB) under cold acclimation had reduced lignin accumulation and increased freezing tolerance. The pal1pal2 double mutant (lignin content reduced by 30% compared with WT) also showed the freezing tolerant phenotype, and TCF1 and BCB act upstream of PALs to regulate lignin content. In addition, TCF1 acts independently of the CBF (C-repeat binding factor) pathway. Our findings delineate a novel molecular pathway linking the TCF1-mediated cold-specific transcriptional program to lignin biosynthesis, thus achieving cell wall remodeling with increased freezing tolerance.

The plant cell wall integrity maintenance mechanism--a case study of a cell wall plasma membrane signaling network

Some of the most important functions of plant cell walls are protection against biotic/abiotic stress and structural support during growth and development. A prerequisite for plant cell walls to perform these functions is the ability to perceive different types of stimuli in both qualitative and quantitative manners and initiate appropriate responses. The responses in turn involve adaptive changes in cellular and cell wall metabolism leading to modifications in the structures originally required for perception. While our knowledge about the underlying plant mechanisms is limited, results from Saccharomyces cerevisiae suggest the cell wall integrity maintenance mechanism represents an excellent example to illustrate how the molecular mechanisms responsible for stimulus perception, signal transduction and integration can function. Here I will review the available knowledge about the yeast cell wall integrity maintenance system for illustration purposes, summarize the limited knowledge available about the corresponding plant mechanism and discuss the relevance of the plant cell wall integrity maintenance mechanism in biotic stress responses.

Ectopic lignification in primary cellulose-deficient cell walls of maize cell suspension cultures

Maize (Zea mays L.) suspension-cultured cells with up to 70% less cellulose were obtained by stepwise habituation to dichlobenil (DCB), a cellulose biosynthesis inhibitor. Cellulose deficiency was accompanied by marked changes in cell wall matrix polysaccharides and phenolics as revealed by Fourier transform infrared (FTIR) spectroscopy. Cell wall compositional analysis indicated that the cellulose-deficient cell walls showed an enhancement of highly branched and cross-linked arabinoxylans, as well as an increased content in ferulic acid, diferulates and p-coumaric acid, and the presence of a polymer that stained positive for phloroglucinol. In accordance with this, cellulose-deficient cell walls showed a fivefold increase in Klason-type lignin. Thioacidolysis/GC-MS analysis of cellulose-deficient cell walls indicated the presence of a lignin-like polymer with a Syringyl/Guaiacyl ratio of 1.45, which differed from the sensu stricto stress-related lignin that arose in response to short-term DCB-treatments. Gene expression analysis of these cells indicated an overexpression of genes specific for the biosynthesis of monolignol units of lignin. A study of stress signaling pathways revealed an overexpression of some of the jasmonate signaling pathway genes, which might trigger ectopic lignification in response to cell wall integrity disruptions. In summary, the structural plasticity of primary cell walls is proven, since a lignification process is possible in response to cellulose impoverishment.

The plant cell wall integrity maintenance mechanism-concepts for organization and mode of action

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.

An update on receptor-like kinase involvement in the maintenance of plant cell wall integrity

BACKGROUND

Plant cell walls form the interface between the cells and their environment. They perform different functions, such as protecting cells from biotic and abiotic stress and providing structural support during development. Maintenance of the functional integrity of cell walls during these different processes is a prerequisite that enables the walls to perform their particular functions. The available evidence suggests that an integrity maintenance mechanism exists in plants that is capable of both detecting wall integrity impairment caused by cell wall damage and initiating compensatory responses to maintain functional integrity. The responses involve 1-aminocyclopropane-1-carboxylic acid (ACC), jasmonic acid, reactive oxygen species and calcium-based signal transduction cascades as well as the production of lignin and other cell wall components. Experimental evidence implicates clearly different signalling molecules, but knowledge regarding contributions of receptor-like kinases to this process is less clear. Different receptor-like kinase families have been considered as possible sensors for perception of cell wall damage; however, strong experimental evidence that provides insights into functioning exists for very few kinases.

SCOPE AND CONCLUSIONS

This review examines the involvement of cell wall integrity maintenance in different biological processes, defines what constitutes plant cell wall damage that impairs functional integrity, clarifies which stimulus perception and signal transduction mechanisms are required for integrity maintenance and assesses the available evidence regarding the functions of receptor-like kinases during cell wall integrity maintenance. The review concludes by discussing how the plant cell wall integrity maintenance mechanism could form an essential component of biotic stress responses and of plant development, functions that have not been fully recognized to date.

Evidence for land plant cell wall biosynthetic mechanisms in charophyte green algae

BACKGROUND AND AIMS

The charophyte green algae (CGA) are thought to be the closest living relatives to the land plants, and ancestral CGA were unique in giving rise to the land plant lineage. The cell wall has been suggested to be a defining structure that enabled the green algal ancestor to colonize land. These cell walls provide support and protection, are a source of signalling molecules, and provide developmental cues for cell differentiation and elongation. The cell wall of land plants is a highly complex fibre composite, characterized by cellulose cross-linked by non-cellulosic polysaccharides, such as xyloglucan, embedded in a matrix of pectic polysaccharides. How the land plant cell wall evolved is currently unknown: early-divergent chlorophyte and prasinophyte algae genomes contain a low number of glycosyl transferases (GTs), while land plants contain hundreds. The number of GTs in CGA is currently unknown, as no genomes are available, so this study sought to give insight into the evolution of the biosynthetic machinery of CGA through an analysis of available transcriptomes.

METHODS

Available CGA transcriptomes were mined for cell wall biosynthesis GTs and compared with GTs characterized in land plants. In addition, gene cloning was employed in two cases to answer important evolutionary questions.

KEY RESULTS

Genetic evidence was obtained indicating that many of the most important core cell wall polysaccharides have their evolutionary origins in the CGA, including cellulose, mannan, xyloglucan, xylan and pectin, as well as arabino-galactan protein. Moreover, two putative cellulose synthase-like D family genes (CSLDs) from the CGA species Coleochaete orbicularis and a fragment of a putative CSLA/K-like sequence from a CGA Spirogyra species were cloned, providing the first evidence that all the cellulose synthase/-like genes present in early-divergent land plants were already present in CGA.

CONCLUSIONS

The results provide new insights into the evolution of cell walls and support the notion that the CGA were pre-adapted to life on land by virtue of the their cell wall biosynthetic capacity. These findings are highly significant for understanding plant cell wall evolution as they imply that some features of land plant cell walls evolved prior to the transition to land, rather than having evolved as a result of selection pressures inherent in this transition.

Early cell-wall modifications of maize cell cultures during habituation to dichlobenil

Studies involving the habituation of plant cell cultures to cellulose biosynthesis inhibitors have achieved significant progress as regards understanding the structural plasticity of cell walls. However, since habituation studies have typically used high concentrations of inhibitors and long-term habituation periods, information on initial changes associated with habituation has usually been lost. This study focuses on monitoring and characterizing the short-term habituation process of maize (Zea mays) cell suspensions to dichlobenil (DCB). Cellulose quantification and FTIR spectroscopy of cell walls from 20 cell lines obtained during an incipient DCB-habituation process showed a reduction in cellulose levels which tended to revert depending on the inhibitor concentration and the length of time that cells were in contact with it. Variations in the cellulose content were concomitant with changes in the expression of several ZmCesA genes, mainly involving overexpression of ZmCesA7 and ZmCesA8. In order to explore these changes in more depth, a cell line habituated to 1.5μM DCB was identified as representative of incipient DCB habituation and selected for further analysis. The cells of this habituated cell line grew more slowly and formed larger clusters. Their cell walls were modified, showing a 33% reduction in cellulose content, that was mainly counteracted by an increase in arabinoxylans, which presented increased extractability. This result was confirmed by immunodot assays graphically plotted by heatmaps, since habituated cell walls had a more extensive presence of epitopes for arabinoxylans and xylans, but also for homogalacturonan with a low degree of esterification and for galactan side chains of rhamnogalacturonan I. Furthermore, a partial shift of xyloglucan epitopes toward more easily extractable fractions was found. However, other epitopes, such as these specific for arabinan side chains of rhamnogalacturonan I or homogalacturonan with a high degree of esterification, seemed to be not affected. In conclusion, the early modifications occurring in maize cell walls as a consequence of DCB-habituation involved quantitative and qualitative changes of arabinoxylans, but also other polysaccharides. Thereby some of the changes that took place in the cell walls in order to compensate for the lack of cellulose differed according to the DCB-habituation level, and illustrate the ability of plant cells to adopt appropriate coping strategies depending on the herbicide concentration and length of exposure time.