Cell Wall Signaling in Plant Development and Defense

Eight hydroxyproline-O-galactosyltransferases play essential roles in female reproductive development

In angiosperms, ovules give rise to seeds upon fertilization. Thus, seed formation is dependent on both successful ovule development and tightly controlled communication between female and male gametophytes. During establishment of these interactions, cell walls play a pivotal role, especially arabinogalactan-proteins (AGPs). AGPs are highly glycosylated proteins decorated by arabinogalactan side chains, representing 90 % of the AGP molecule. AGP glycosylation is initiated by a reaction catalysed by hydroxyproline-O-galactosyltransferases (Hyp-GALTs), specifically eight of them (GALT2-9), which add the first galactose to Hyp residues. Five Hyp-GALTs (GALT2, 5, 7, 8 and 9) were previously described as essential for AGP functions in pollen and ovule development, pollen-pistil interactions, and seed morphology. In the present work, a higher order Hyp-GALT mutant (23456789) was studied, with a high degree of under-glycosylated AGPs, to gain deeper insight into the crucial roles of these eight enzymes in female reproductive tissues. Notably, the 23456789 mutant demonstrated a high quantity of unfertilized ovules, displaying abnormal callose accumulation both at the micropylar region and, sometimes, throughout the entire embryo sac. Additionally, this mutant displayed ovules with abnormal embryo sacs, had a disrupted spatiotemporal distribution of AGPs in female reproductive tissues, and showed abnormal seed and embryo development, concomitant with a reduction in AGP-GlcA levels. This study revealed that at least three more enzymes exhibit Hyp-O-GALT activity in Arabidopsis (GALT3, 4 and 6), and reinforces the crucial importance of AGP carbohydrates in carrying out the biological functions of AGPs during plant reproduction.

A peptide-receptor module links cell wall integrity sensing to pattern-triggered immunity

Plants employ cell-surface receptors to perceive non- or altered-self, including the integrity of their cell wall. Here we identify a specific ligand-receptor module responsive to cell wall damage that potentiates immunity in Arabidopsis. Disruption of cell wall integrity by inhibition of cellulose biosynthesis promotes pattern-triggered immunity transcriptionally in a manner dependent on the receptor kinase MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2 (MIK2). Notably, while MIK2 can perceive peptides of the large SERINE RICH ENDOGENOUS PEPTIDE family, a single member of this family, SCOOP18, is transcriptionally induced upon cell wall damage and is required for subsequent responses such as lignification and immunity potentiation. Collectively, our results identify the SCOOP18-MIK2 ligand-receptor module as an important central hub, connecting plant cell wall integrity sensing with immunity.

Functional Analysis of Cucumis melo CmXTH11 in Regulating Drought Stress Tolerance in Arabidopsis thaliana

The CmXTH11 gene, a member of the XTH (xyloglucan endotransglycosylase/hydrolase) family, plays a crucial role in plant responses to environmental stress. In this study, we heterologously expressed the melon gene CmXTH11 in Arabidopsis to generate overexpressing transgenic lines, thereby elucidating the regulatory role of CmXTH11 in water stress tolerance. Using these lines of CmXTH11 (OE1 and OE2) and wild-type (WT) Arabidopsis as experimental materials, we applied water stress treatments (including osmotic stress and soil drought) and rewatering treatments to investigate the response mechanisms of melon CmXTH11 in Arabidopsis under drought stress from a physiological and biochemical perspective. Overexpression of CmXTH11 significantly improved root growth under water stress conditions. The OE lines exhibited longer roots and a higher number of lateral roots compared to WT plants. The enhanced root system contributed to better water uptake and retention. Under osmotic and drought stress, the OE lines showed improved survival rates and less wilting compared to WT plants. Biochemical analyses revealed that CmXTH11 overexpression led to lower levels of malondialdehyde (MDA) and reduced electrolyte leakage, indicating decreased oxidative damage. The activities of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), were significantly higher in OE lines, suggesting enhanced oxidative stress tolerance. The CmXTH11 gene positively regulates water stress tolerance in Arabidopsis by enhancing root growth, improving water uptake, and reducing oxidative damage. Overexpression of CmXTH11 increases the activities of antioxidant enzymes, thereby mitigating oxidative stress and maintaining cellular integrity under water deficit conditions. These findings suggest that CmXTH11 is a potential candidate for genetic improvement of drought resistance in crops.

Mechanics of reproductive differentiation in the land plants: a paradigm shift?

This article addresses the physical mechanics of gametogenesis in vascular plants. The earliest events that initiate reproductive differentiation in the land plants are not well understood. How are the few cells that initiate reproductive differentiation specified and how is that information translated into action at the cellular level? In this article I propose a physical mechanism that resolves the problem of spatial targeting without invoking dependence on diffusible morphogens or other external signals. I suggest that the initiation of archesporial differentiation can instead be attributed to the confluence of organ geometry, surficial topography, and the physical mechanics of sporangial growth, resulting in the spontaneous emergence of an isotropic singularity that locates and precipitates archesporial differentiation. In discussing the logic of single-cell target selection and the limits of stochastic molecular signaling I propose that the sporangium would be better understood as a pressurized stress-mechanical lens that focuses turgor-generated growth forces on a central location, generating a physical singularity that locates and specifies the cell or cells that become the archesporium and initiates their transition from somatic proliferation to reproductive differentiation.

Pectin methylesterase activity is required for RALF1 peptide signalling output

The extracellular matrix plays an integrative role in cellular responses in plants, but its contribution to the signalling of extracellular ligands largely remains to be explored. Rapid alkalinisation factors (RALFs) are extracellular peptide hormones that play pivotal roles in various physiological processes. Here, we address a crucial connection between the de-methylesterification machinery of the cell wall component pectin and RALF1 activity. Pectin is a polysaccharide, contributing to the structural integrity of the cell wall. Our data illustrate that the pharmacological and genetic interference with pectin methyl esterases (PMEs) abolishes RALF1-induced root growth repression. Our data suggest that positively charged RALF1 peptides bind negatively charged, de-methylesterified pectin with high avidity. We illustrate that the RALF1 association with de-methylesterified pectin is required for its FERONIA-dependent perception, contributing to the control of the extracellular matrix and the regulation of plasma membrane dynamics. Notably, this mode of action is independent of the FER-dependent extracellular matrix sensing mechanism provided by FER interaction with the leucine-rich repeat extensin (LRX) proteins. We propose that the methylation status of pectin acts as a contextualizing signalling scaffold for RALF peptides, linking extracellular matrix dynamics to peptide hormone-mediated responses.

Plant Cell Wall Loosening by Expansins

Expansins comprise an ancient group of cell wall proteins ubiquitous in land plants and their algal ancestors. During cell growth, they facilitate passive yielding of the wall's cellulose networks to turgor-generated tensile stresses, without evidence of enzymatic activity. Expansins are also implicated in fruit softening and other developmental processes and in adaptive responses to environmental stresses and pathogens. The major expansin families in plants include α-expansins (EXPAs), which act on cellulose-cellulose junctions, and β-expansins, which can act on xylans. EXPAs mediate acid growth, which contributes to wall enlargement by auxin and other growth agents. The genomes of diverse microbes, including many plant pathogens, also encode expansins designated expansin-like X. Expansins are proposed to disrupt noncovalent bonding between laterally aligned polysaccharides (notably cellulose), facilitating wall loosening for a variety of biological roles.

GhCASPL1 regulates secondary cell wall thickening in cotton fibers by stabilizing the cellulose synthase complex on the plasma membrane

Cotton (Gossypium hirsutum) fibers are elongated single cells that rapidly accumulate cellulose during secondary cell wall (SCW) thickening, which requires cellulose synthase complex (CSC) activity. Here, we describe the CSC-interacting factor CASPARIAN STRIP MEMBRANE DOMAIN-LIKE1 (GhCASPL1), which contributes to SCW thickening by influencing CSC stability on the plasma membrane. GhCASPL1 is preferentially expressed in fiber cells during SCW biosynthesis and encodes a MARVEL domain protein. The ghcaspl1 ghcaspl2 mutant exhibited reduced plant height and produced mature fibers with fewer natural twists, lower tensile strength, and a thinner SCW compared to the wild type. Similarly, the Arabidopsis (Arabidopsis thaliana) caspl1 caspl2 double mutant showed a lower cellulose content and thinner cell walls in the stem vasculature than the wild type but normal plant morphology. Introducing the cotton gene GhCASPL1 successfully restored the reduced cellulose content of the Arabidopsis caspl1 caspl2 mutant. Detergent treatments, ultracentrifugation assays, and enzymatic assays showed that the CSC in the ghcaspl1 ghcaspl2 double mutant showed reduced membrane binding and decreased enzyme activity compared to the wild type. GhCASPL1 binds strongly to phosphatidic acid (PA), which is present in much higher amounts in thickening fiber cells compared to ovules and leaves. Mutating the PA-binding site in GhCASPL1 resulted in the loss of its colocalization with GhCesA8, and it failed to localize to the plasma membrane. PA may alter membrane structure to facilitate protein-protein interactions, suggesting that GhCASPL1 and PA collaboratively stabilize the CSC. Our findings shed light on CASPL functions and the molecular machinery behind SCW biosynthesis in cotton fibers.

Biochemical insights into proline metabolism and its contribution to the endurant cell wall structure under metal stress

The cell wall serves as the primary barrier against the entry of heavy metal ions into cells. However, excessive accumulation of heavy metals within plants can lead to alterations in the spatial structure and physical properties of the cell wall, thereby affecting the capacity of plants to capture heavy metals. Proline (Pro) is involved in the synthesis of the cell wall, modulating the stability and integrity of its structure. Extensins, core proteins that maintain the cell wall structure, are proline/hydroxyproline-rich glycoproteins that contain the characteristic sequence Ser-[Pro]3-5. They act as intermediates in the regulation of biological processes such as cell wall synthesis, assembly, and signal transduction, typically forming a network structure of cell wall proteins through cross-linking with pectin. This network is essential for the self-assembly expansion of the plant cell wall and plays an indispensable role in cell wall stress signal transduction through its interaction with intracellular signalling molecules. However, the mechanisms by which Pro affects the synthesis of cell wall structural proteins, cell wall assembly, and the sensing of cell wall stress under heavy metal stress remain unclear. This review, from the perspectives of biochemistry and molecular biology, comprehensively elaborates on the impact of Pro and Pro-rich proteins on the structure and function of the cell wall. These findings emphasize the mechanism by which Pro enhances the ability of the cell wall to capture heavy metals, providing new research ideas for the use of genetic engineering to manipulate cell wall synthesis and repair, thereby reducing the phytotoxicity of heavy metals.

Cellular basis of legume-rhizobium symbiosis

The legume-rhizobium symbiosis represents the most important system for terrestrial biological nitrogen fixation on land. Efficient nitrogen fixation during this symbiosis depends on successful rhizobial infection and complete endosymbiosis, which are achieved by complex cellular events including cell-wall remodeling, cytoskeletal reorganizations, and extensive membrane expansion and trafficking. In this review, we explore the dynamic remodeling of the plant-specific cell wall-membrane system-cytoskeleton (WMC) continuum during symbiotic nitrogen fixation. We focus on key processes linked to efficient nitrogen fixation, including rhizobial uptake, infection thread formation and elongation, rhizobial droplet release, cytoplasmic bridge formation, and rhizobial endosymbiosis. Additionally, we discuss the advanced techniques for investigating the cellular basis of root-nodule symbiosis and provide insights into the unsolved mysteries of robust symbiotic nitrogen fixation.

DIA-Based Phosphoproteomics Identifies Early Phosphorylation Events in Response to EGTA and Mannitol in Arabidopsis

Osmotic stress significantly hampers plant growth and crop yields, emphasizing the need for a thorough comprehension of the underlying molecular responses. Previous research has demonstrated that osmotic stress rapidly induces calcium influx and signaling, along with the activation of a specific subset of protein kinases, notably the Raf-like protein (RAF)-sucrose nonfermenting-1-related protein kinase 2 (SnRK2) kinase cascades within minutes. However, the intricate interplay between calcium signaling and the activation of RAF-SnRK2 kinase cascades remains elusive. Here, in this study, we discovered that Raf-like protein (RAF) kinases undergo hyperphosphorylation in response to osmotic shocks. Intriguingly, treatment with the calcium chelator EGTA robustly activates RAF-SnRK2 cascades, mirroring the effects of osmotic treatment. Utilizing high-throughput data-independent acquisition-based phosphoproteomics, we unveiled the global impact of EGTA on protein phosphorylation. Beyond the activation of RAFs and SnRK2s, EGTA treatment also activates mitogen-activated protein kinase cascades, Calcium-dependent protein kinases, and receptor-like protein kinases, etc. Through overlapping assays, we identified potential roles of mitogen-activated protein kinase kinase kinase kinases and receptor-like protein kinases in the osmotic stress-induced activation of RAF-SnRK2 cascades. Our findings illuminate the regulation of phosphorylation and cellular events by Ca2+ signaling, offering insights into the (exocellular) Ca2+ deprivation during early hyperosmolality sensing and signaling.

Strategies of plants to overcome abiotic and biotic stresses

In their environment, plants are exposed to a multitude of abiotic and biotic stresses that differ in intensity, duration and severity. As sessile organisms, they cannot escape these stresses, but instead have developed strategies to overcome them or to compensate for the consequences of stress exposure. Defence can take place at different levels and the mechanisms involved are thought to differ in efficiency across these levels. To minimise metabolic constraints and to reduce the costs of stress defence, plants prioritise first-line defence strategies in the apoplastic space, involving ascorbate, defensins and small peptides, as well as secondary metabolites, before cellular processes are affected. In addition, a large number of different symplastic mechanisms also provide efficient stress defence, including chemical antioxidants, antioxidative enzymes, secondary metabolites, defensins and other peptides as well as proteins. At both the symplastic and the apoplastic level of stress defence and compensation, a number of specialised transporters are thought to be involved in exchange across membranes that still have not been identified, and information on the regeneration of different defence compounds remains ambiguous. In addition, strategies to overcome and compensate for stress exposure operate not only at the cellular, but also at the organ and whole-plant levels, including stomatal regulation, and hypersensitive and systemic responses to prevent or reduce the spread of stress impacts within the plant. Defence can also take place at the ecosystem level by root exudation of signalling molecules and the emission of volatile organic compounds, either directly or indirectly into the rhizosphere and/or the aboveground atmosphere. The mechanisms by which plants control the production of these compounds and that mediate perception of stressful conditions are still not fully understood. Here we summarise plant defence strategies from the cellular to ecosystem level, discuss their advantages and disadvantages for plant growth and development, elucidate the current state of research on the transport and regeneration capacity of defence metabolites, and outline insufficiently explored questions for further investigation.

The RALF2-FERONIA-MYB63 module orchestrates growth and defense in tomato roots

Plant secreted peptides RAPID ALKALINISATION FACTORs (RALFs), which act through the receptor FERONIA (FER), play important roles in plant growth. However, it remains unclear whether and how RALF-FER contributes to the trade-off of plant growth-defense. Here, we used a variety of techniques such as CRISPR/Cas9, protein-protein interaction and transcriptional regulation methods to investigate the role of RALF2 and its receptor FER in regulating lignin deposition, root growth, and defense against Fusarium oxysporum f. sp. lycopersici (Fol) in tomato (Solanum lycopersicum). The ralf2 and fer mutants show reduced primary root length, elevated lignin accumulation, and enhanced resistance against Fol than the wild-type. FER interacts with and phosphorylates MYB63 to promote its degradation. MYB63 serves as an activator of lignin deposition by regulating the transcription of dirigent protein gene DIR19. Mutation of DIR19 suppresses lignin accumulation, and reverses the short root phenotype and Fol resistance in ralf2 or fer mutant. Collectively, our results demonstrate that the RALF2-FER-MYB63 module fine-tunes root growth and resistance against Fol through regulating the deposition of lignin in tomato roots. The study sheds new light on how plants maintain the growth-defense balance via RALF-FER.

Multi-omics analysis of green lineage osmotic stress pathways unveils crucial roles of different cellular compartments

Maintenance of water homeostasis is a fundamental cellular process required by all living organisms. Here, we use the single-celled green alga Chlamydomonas reinhardtii to establish a foundational understanding of osmotic-stress signaling pathways through transcriptomics, phosphoproteomics, and functional genomics approaches. Comparison of pathways identified through these analyses with yeast and Arabidopsis allows us to infer their evolutionary conservation and divergence across these lineages. 76 genes, acting across diverse cellular compartments, were found to be important for osmotic-stress tolerance in Chlamydomonas through their functions in cytoskeletal organization, potassium transport, vesicle trafficking, mitogen-activated protein kinase and chloroplast signaling. We show that homologs for five of these genes have conserved functions in stress tolerance in Arabidopsis and reveal a novel PROFILIN-dependent stage of acclimation affecting the actin cytoskeleton that ensures tissue integrity upon osmotic stress. This study highlights the conservation of the stress response in algae and land plants, and establishes Chlamydomonas as a unicellular plant model system to dissect the osmotic stress signaling pathway.

FERONIA: A Receptor Kinase at the Core of a Global Signaling Network

Initially identified as a key regulator of female fertility in Arabidopsis, the FERONIA (FER) receptor kinase is now recognized as crucial for almost all aspects of plant growth and survival. FER partners with a glycosylphosphatidylinositol-anchored protein of the LLG family to act as coreceptors on the cell surface. The FER-LLG coreceptor interacts with different RAPID ALKALINIZATION FACTOR (RALF) peptide ligands to function in various growth and developmental processes and to respond to challenges from the environment. The RALF-FER-LLG signaling modules interact with molecules in the cell wall, cell membrane, cytoplasm, and nucleus and mediate an interwoven signaling network. Multiple FER-LLG modules, each anchored by FER or a FER-related receptor kinase, have been studied, illustrating the functional diversity and the mechanistic complexity of the FER family signaling modules. The challenges going forward are to distill from this complexity the unifying schemes where possible and attain precision and refinement in the knowledge of critical details upon which future investigations can be built. By focusing on the extensively characterized FER, this review provides foundational information to guide the next phase of research on FER in model as well as crop species and potential applications for improving plant growth and resilience.

Lipids and Lipid-Mediated Signaling in Plant-Pathogen Interactions

Plant lipids are essential cell constituents with many structural, storage, signaling, and defensive functions. During plant-pathogen interactions, lipids play parts in both the preexisting passive defense mechanisms and the pathogen-induced immune responses at the local and systemic levels. They interact with various components of the plant immune network and can modulate plant defense both positively and negatively. Under biotic stress, lipid signaling is mostly associated with oxygenated natural products derived from unsaturated fatty acids, known as oxylipins; among these, jasmonic acid has been of great interest as a specific mediator of plant defense against necrotrophic pathogens. Although numerous studies have documented the contribution of oxylipins and other lipid-derived species in plant immunity, their specific roles in plant-pathogen interactions and their involvement in the signaling network require further elucidation. This review presents the most relevant and recent studies on lipids and lipid-derived signaling molecules involved in plant-pathogen interactions, with the aim of providing a deeper insight into the mechanisms underpinning lipid-mediated regulation of the plant immune system.

Enabling Lignin Valorization Through Integrated Advances in Plant Biology and Biorefining

Despite lignin having long been viewed as an impediment to the processing of biomass for the production of paper, biofuels, and high-value chemicals, the valorization of lignin to fuels, chemicals, and materials is now clearly recognized as a critical element for the lignocellulosic bioeconomy. However, the intended application for lignin will likely require a preferred lignin composition and form. To that end, effective lignin valorization will require the integration of plant biology, providing optimal feedstocks, with chemical process engineering, providing efficient lignin transformations. Recent advances in our understanding of lignin biosynthesis have shown that lignin structure is extremely diverse and potentially tunable, while simultaneous developments in lignin refining have resulted in the development of several processes that are more agnostic to lignin composition. Here, we review the interface between in planta lignin design and lignin processing and discuss the advances necessary for lignin valorization to become a feature of advanced biorefining.

Top five unanswered questions in plant cell surface research

Plant cell wall researchers were asked their view on what the major unanswered questions are in their field. This article summarises the feedback that was received from them in five questions. In this issue you can find equivalent syntheses for researchers working on bacterial, unicellular parasite and fungal systems.

Homogalacturonan Pectins Tuned as an Effect of Susceptible rbohD, Col-0-Reactions, and Resistance rbohF-, rbohD/F-Reactions to TuMV

The plant cell wall is an actively reorganized network during plant growth and triggered immunity in response to biotic stress. While the molecular mechanisms managing perception, recognition, and signal transduction in response to pathogens are well studied in the context of damaging intruders, the current understanding of plant cell wall rebuilding and active defense strategies in response to plant virus infections remains poorly characterized. Pectins can act as major elements of the primary cell wall and are dynamic compounds in response to pathogens. Homogalacturonans (HGs), a main component of pectins, have been postulated as defensive molecules in plant-pathogen interactions and linked to resistance responses. This research focused on examining the regulation of selected pectin metabolism components in susceptible (rbohD-, Col-0-TuMV) and resistance (rbohF-, rbohD/F-TuMV) reactions. Regardless of the interaction type, ultrastructural results indicated dynamic cell wall rebuilding. In the susceptible reaction promoted by RbohF, there was upregulation of AtPME3 (pectin methylesterase) but not AtPME17, confirmed by induction of PME3 protein deposition. Moreover, the highest PME activity along with a decrease in cell wall methylesters compared to resistance interactions in rbohD-TuMV were noticed. Consequently, the susceptible reaction of rbohD and Col-0 to TuMV was characterized by a significant domination of low/non-methylesterificated HGs. In contrast, cell wall changes during the resistance response of rbohF and rbohD/F to TuMV were associated with dynamic induction of AtPMEI2, AtPMEI3, AtGAUT1, and AtGAUT7 genes, confirmed by significant induction of PMEI2, PMEI3, and GAUT1 protein deposition. In both resistance reactions, a dynamic decrease in PME activity was documented, which was most intense in rbohD/F-TuMV. This decrease was accompanied by an increase in cell wall methylesters, indicating that the domination of highly methylesterificated HGs was associated with cell wall rebuilding in rbohF and rbohD/F defense responses to TuMV. These findings suggest that selected PME with PMEI enzymes have a diverse impact on the demethylesterification of HGs and metabolism as a result of rboh-TuMV interactions, and are important factors in regulating cell wall changes depending on the type of interaction, especially in resistance responses. Therefore, PMEI2 and PMEI3 could potentially be important signaling resistance factors in the rboh-TuMV pathosystem.

Plant cell wall-mediated disease resistance: Current understanding and future perspectives

Beyond their function as structural barriers, plant cell walls are essential elements for the adaptation of plants to environmental conditions. Cell walls are dynamic structures whose composition and integrity can be altered in response to environmental challenges and developmental cues. These wall changes are perceived by plant sensors/receptors to trigger adaptative responses during development and upon stress perception. Plant cell wall damage caused by pathogen infection, wounding, or other stresses leads to the release of wall molecules, such as carbohydrates (glycans), that function as damage-associated molecular patterns (DAMPs). DAMPs are perceived by the extracellular ectodomains (ECDs) of pattern recognition receptors (PRRs) to activate pattern-triggered immunity (PTI) and disease resistance. Similarly, glycans released from the walls and extracellular layers of microorganisms interacting with plants are recognized as microbe-associated molecular patterns (MAMPs) by specific ECD-PRRs triggering PTI responses. The number of oligosaccharides DAMPs/MAMPs identified that are perceived by plants has increased in recent years. However, the structural mechanisms underlying glycan recognition by plant PRRs remain limited. Currently, this knowledge is mainly focused on receptors of the LysM-PRR family, which are involved in the perception of various molecules, such as chitooligosaccharides from fungi and lipo-chitooligosaccharides (i.e., Nod/MYC factors from bacteria and mycorrhiza, respectively) that trigger differential physiological responses. Nevertheless, additional families of plant PRRs have recently been implicated in oligosaccharide/polysaccharide recognition. These include receptor kinases (RKs) with leucine-rich repeat and Malectin domains in their ECDs (LRR-MAL RKs), Catharanthus roseus RECEPTOR-LIKE KINASE 1-LIKE group (CrRLK1L) with Malectin-like domains in their ECDs, as well as wall-associated kinases, lectin-RKs, and LRR-extensins. The characterization of structural basis of glycans recognition by these new plant receptors will shed light on their similarities with those of mammalians involved in glycan perception. The gained knowledge holds the potential to facilitate the development of sustainable, glycan-based crop protection solutions.

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.

Structure and growth of plant cell walls

Plant cells build nanofibrillar walls that are central to plant growth, morphogenesis and mechanics. Starting from simple sugars, three groups of polysaccharides, namely, cellulose, hemicelluloses and pectins, with very different physical properties are assembled by the cell to make a strong yet extensible wall. This Review describes the physics of wall growth and its regulation by cellular processes such as cellulose production by cellulose synthase, modulation of wall pH by plasma membrane H+-ATPase, wall loosening by expansin and signalling by plant hormones such as auxin and brassinosteroid. In addition, this Review discusses the nuanced roles, properties and interactions of cellulose, matrix polysaccharides and cell wall proteins and describes how wall stress and wall loosening cooperatively result in cell wall growth.

The complex transcriptional regulation of heat stress response in maize

As one of the most important food and feed crops worldwide, maize suffers much more tremendous damages under heat stress compared to other plants, which seriously inhibits plant growth and reduces productivity. To mitigate the heat-induced damages and adapt to high temperature environment, plants have evolved a series of molecular mechanisms to sense, respond and adapt high temperatures and heat stress. In this review, we summarized recent advances in molecular regulations underlying high temperature sensing, heat stress response and memory in maize, especially focusing on several important pathways and signals in high temperature sensing, and the complex transcriptional regulation of ZmHSFs (Heat Shock Factors) in heat stress response. In addition, we highlighted interactions between ZmHSFs and several epigenetic regulation factors in coordinately regulating heat stress response and memory. Finally, we laid out strategies to systematically elucidate the regulatory network of maize heat stress response, and discussed approaches for breeding future heat-tolerance maize.

Defects in the cell wall and its deposition caused by loss-of-function of three RLKs alter root hydrotropism in Arabidopsis thaliana

Root tips can sense moisture gradients and grow into environments with higher water potential. This process is called root hydrotropism. Here, we report three closely related receptor-like kinases (RLKs) that play critical roles in root hydrotropism: ALTERED ROOT HYDROTROPIC RESPONSE 1 (ARH1), FEI1, and FEI2. Overexpression of these RLKs strongly reduce root hydrotropism, but corresponding loss-of-function mutants exhibit an increased hydrotropic response in their roots. All these RLKs show polar localization at the plasma membrane regions in root tips. The biosynthesis of the cell wall, cutin, and wax (CCW) is significantly impaired in root tips of arh1-2 fei1-C fei2-C. A series of known CCW mutants also exhibit increased root hydrotropism and reduced osmotic tolerance, similar to the characteristics of the triple mutant. Our results demonstrat that the integrity of the cell wall, cutin, and root cap wax mediate a trade-off between root hydrotropism and osmotic tolerance.

Integrative omics studies revealed synergistic link between sucrose metabolic isogenes and carbohydrates in poplar roots infected by Fusarium wilt

Advances in carbohydrate metabolism prompted its essential role in defense priming and sweet immunity during plant-pathogen interactions. Nevertheless, upstream responding enzymes in the sucrose metabolic pathway and associated carbohydrate derivatives underlying fungal pathogen challenges remain to be deciphered in Populus, a model tree species. In silico deduction of genomic features, including phylogenies, exon/intron distributions, cis-regulatory elements, and chromosomal localization, identified 59 enzyme genes (11 families) in the Populus genome. Spatiotemporal expression of the transcriptome and the quantitative real-time PCR revealed a minuscule number of isogenes that were predominantly expressed in roots. Upon the pathogenic Fusarium solani (Fs) exposure, dynamic changes in the transcriptomics atlas and experimental evaluation verified Susy (PtSusy2 and 3), CWI (PtCWI3), VI (PtVI2), HK (PtHK6), FK (PtFK6), and UGPase (PtUGP2) families, displaying promotions in their expressions at 48 and 72 h of post-inoculation (hpi). Using the gas chromatography-mass spectrometry (GC-MS)-based non-targeted metabolomics combined with a high-performance ion chromatography system (HPICS), approximately 307 metabolites (13 categories) were annotated that led to the quantification of 46 carbohydrates, showing marked changes between three compared groups. By contrast, some sugars (e.g., sorbitol, L-arabitol, trehalose, and galacturonic acid) exhibited a higher accumulation at 72 hpi than 0 hpi, while levels of α-lactose and glucose decreased, facilitating them as potential signaling molecules. The systematic overview of multi-omics approaches to dissect the effects of Fs infection provides theoretical cues for understanding defense immunity depending on fine-tuned Suc metabolic gene clusters and synergistically linked carbohydrate pools in trees.

Signalling between the sexes during pollen tube reception

Plant reproduction is a complex, highly-coordinated process in which a single, male germ cell grows through the maternal reproductive tissues to reach and fertilise the egg cell. Focussing on Arabidopsis thaliana, we review signalling between male and female partners which is important throughout the pollen tube journey, especially during pollen tube reception at the ovule. Numerous receptor kinases and their coreceptors are implicated in signal perception in both the pollen tube and synergid cells at the ovule entrance, and several specific peptide and carbohydrate ligands for these receptors have recently been identified. Clarifying the interplay between these signals and the downstream responses they instigate presents a challenge for future research and may help to illuminate broader principles of plant cell-cell communication.

A self-regulatory cell-wall-sensing module at cell edges controls plant growth

Morphogenesis of multicellular organs requires coordination of cellular growth. In plants, cell growth is determined by turgor pressure and the mechanical properties of the cell wall, which also glues cells together. Because plants have to integrate tissue-scale mechanical stresses arising through growth in a fixed tissue topology, they need to monitor cell wall mechanical status and adapt growth accordingly. Molecular factors have been identified, but whether cell geometry contributes to wall sensing is unknown. Here we propose that plant cell edges act as cell-wall-sensing domains during growth. We describe two Receptor-Like Proteins, RLP4 and RLP4-L1, which occupy a unique polarity domain at cell edges established through a targeted secretory transport pathway. We show that RLP4s associate with the cell wall at edges via their extracellular domain, respond to changes in cell wall mechanics and contribute to directional growth control in Arabidopsis.

Constitutive expression of apple endo-POLYGALACTURONASE1 in fruit induces early maturation, alters skin structure and accelerates softening

During fruit ripening, polygalacturonases (PGs) are key contributors to the softening process in many species. Apple is a crisp fruit that normally exhibits only minor changes to cell walls and limited fruit softening. Here, we explore the effects of PG overexpression during fruit development using transgenic apple lines overexpressing the ripening-related endo-POLYGALACTURONASE1 gene. MdPG1-overexpressing (PGox) fruit displayed early maturation/ripening with black seeds, conversion of starch to sugars and ethylene production occurring by 80 days after pollination (DAP). PGox fruit exhibited a striking, white-skinned phenotype that was evident from 60 DAP and most likely resulted from increased air spaces and separation of cells in the hypodermis due to degradation of the middle lamellae. Irregularities in the integrity of the epidermis and cuticle were also observed. By 120 DAP, PGox fruit cracked and showed lenticel-associated russeting. Increased cuticular permeability was associated with microcracks in the cuticle around lenticels and was correlated with reduced cortical firmness at all time points and extensive post-harvest water loss from the fruit, resulting in premature shrivelling. Transcriptomic analysis suggested that early maturation was associated with upregulation of genes involved in stress responses, and overexpression of MdPG1 also altered the expression of genes involved in cell wall metabolism (e.g. β-galactosidase, MD15G1221000) and ethylene biosynthesis (e.g. ACC synthase, MD14G1111500). The results show that upregulation of PG not only has dramatic effects on the structure of the fruit outer cell layers, indirectly affecting water status and turgor, but also has unexpected consequences for fruit development.

An essential role for mannan degradation in both cell growth and secondary cell wall formation

Coordination of secondary cell wall deposition and cell expansion during plant growth is required for cell development, particularly in vascular tissues. Yet the fundamental coordination process has received little attention. We observed that the Arabidopsis endo-1,4-mannanase gene, AtMAN6, is involved in the formation of cell walls in vascular tissues. In the inflorescence stem, the man6 mutant had smaller vessel cells with thicker secondary cell walls and shorter fiber cells. Elongation growth was reduced in the root, and secondary cell wall deposition in vessel cells occurred early. Overexpression of AtMAN6 resulted in the inverse phenotypes of the man6 mutant. AtMAN6 was discovered on the plasma membrane and was specifically expressed in vessel cells during its early development. The AtMAN6 protein degraded galactoglucomannan to produce oligosaccharides, which caused secondary cell wall deposition in vessel and fiber cells to be suppressed. Transcriptome analysis revealed that the expression of genes involved in the regulation of secondary cell wall synthesis was changed in both man6 mutant and AtMAN6 overexpression plants. AtMAN6's C-terminal cysteine repeat motif (CCRM) was found to facilitate homodimerization and is required for its activity. According to the findings, the oligosaccharides produced by AtMAN6 hydrolysis may act as a signal to mediate this coordination between cell growth and secondary cell wall deposition.

Unveiling mechanisms of silicon-mediated resistance to chromium stress in rice using a newly-developed hierarchical system

Silicon (Si) has been well-known to enhance plant resistance to heavy-metal stress. However, the mechanisms by which silicon mitigates heavy-metal stress in plants are not clear. In particular, information regarding the role of Si in mediating resistance to heavy-metal stress at a single cell level is still lacking. Here, we developed a hierarchical system comprising the plant, protoplast, and suspension cell subsystems to investigate the mechanisms by which silicon helps to alleviate the toxic effects of trivalent chromium [Cr(III)] in rice. Our results showed that in whole-plant subsystem silicon reduced shoot Cr(III) concentration, effectively alleviating Cr(III) stress in seedlings and causing changes in antioxidant enzyme activities similar to those observed at lower Cr(III) concentrations without silicon added. However, in protoplast subsystem lacking the cell wall, no silicon deposition occurred, leading to insignificant changes in cell survival or antioxidation processes under Cr(III) stress. Conversely, in suspension cell subsystem, silicon supplementation substantially improved cell survival and changes in antioxidant enzyme activities under Cr(III) stress. This is due to the fact that >95% of silicon was on the cell wall, reducing Cr(III) concentration in cells by 7.7%-10.4%. Collectively, the results suggested that the silicon deposited on the cell wall hindered Cr(III) bio-uptake, which consequently delayed Cr(III)-induced changes in antioxidant enzyme activities. This research emphasizes the significance of cell walls in Si-alleviated heavy-metal stress and deepens our understanding of silicon functioning in plants. Furthermore, the hierarchical system has great potential for application in studying the functioning of other elements in plant cell walls.

Copper oxide nanoparticles alter the uptake and distribution of cadmium through disturbing the ordered structure of the cell wall in Arabidopsis root

Copper oxide nanoparticles (CuO NPs) influence the uptake of heavy metal ions by plants, but molecular mechanism is still unknown. Here, we proved the mechanism of CuO NPs affecting Cd absorption in Arabidopsis root. 4-d-old seedlings were treated by 10 and 20 mg/L CuO NPs for 3 d, which decreased the contents of cellulose and hemicellulose in roots. Moreover, the contents of some important monosaccharides were altered by CuO NPs, including arabinose, glucose and mannose. Biosynthesis of cellulose and hemicellulose is regulated by cellulose synthase A complexe (CSC) dynamics. The synthesis of tubulin cytoskeleton was inhibited by CuO NPs, which resulted in the decrease of CSCs bidirectional velocities. Furthermore, the arrangement and network of cellulose fibrillar bundles were disrupted by CuO NPs. CuO NPs treatment significantly increased the influx of Cd2+. The accumulation and translocation of Cd were increased by 10 and 20 mg/L CuO NPs treatment. The subcellular distribution of Cd in root cells indicated CuO NPs decrease the enrichment of Cd in cell wall, but increase the enrichment of Cd in soluble fraction and organelle. In light of these findings, we proposed a mechanistic model in which CuO NPs destroy the ordered structure of the cell wall, alter the uptake and distribution of Cd in Arabidopsis.

The cell surface is the place to be for brassinosteroid perception and responses

Adjusting the microenvironment around the cell surface is critical to responding to external cues or endogenous signals and to maintaining cell activities. In plant cells, the plasma membrane is covered by the cell wall and scaffolded with cytoskeletal networks, which altogether compose the cell surface. It has long been known that these structures mutually interact, but the mechanisms that integrate the whole system are still obscure. Here we spotlight the brassinosteroid (BR) plant hormone receptor BRASSINOSTEROID INSENSITIVE1 (BRI1) since it represents an outstanding model for understanding cell surface signalling and regulation. We summarize how BRI1 activity and dynamics are controlled by plasma membrane components and their associated factors to fine-tune signalling. The downstream signals, in turn, manipulate cell surface structures by transcriptional and post-translational mechanisms. Moreover, the changes in these architectures impact BR signalling, resulting in a feedback loop formation. This Review discusses how BRI1 and BR signalling function as central hubs to integrate cell surface regulation.

Harnessing the potential of non-coding RNA: An insight into its mechanism and interaction in plant biotic stress

Plants have developed diverse physical and chemical defence mechanisms to ensure their continued growth and well-being in challenging environments. Plants also have evolved intricate molecular mechanisms to regulate their responses to biotic stress. Non-coding RNA (ncRNA) plays a crucial role in this process that affects the expression or suppression of target transcripts. While there have been numerous reviews on the role of molecules in plant biotic stress, few of them specifically focus on how plant ncRNAs enhance resistance through various mechanisms against different pathogens. In this context, we explored the role of ncRNA in exhibiting responses to biotic stress endogenously as well as cross-kingdom regulation of transcript expression. Furthermore, we address the interplay between ncRNAs, which can act as suppressors, precursors, or regulators of other ncRNAs. We also delve into the regulation of ncRNAs in response to attacks from different organisms, such as bacteria, viruses, fungi, nematodes, oomycetes, and insects. Interestingly, we observed that diverse microorganisms interact with distinct ncRNAs. This intricacy leads us to conclude that each ncRNA serves a specific function in response to individual biotic stimuli. This deeper understanding of the molecular mechanisms involving ncRNAs in response to biotic stresses enhances our knowledge and provides valuable insights for future research in the field of ncRNA, ultimately leading to improvements in plant traits.

Microscale Thermophoresis (MST) to Study Rapid Alkalinization Factor (RALF)-Receptor Interactions

Microscale thermophoresis (MST) is a simple but powerful tool to study the in vitro interaction among biomolecules, and to quantify binding affinities. MST curves describe the change in the fluorescence level of a fluorescent target as a result of an IR-laser-induced temperature change. The degree and nature of the change in fluorescence signal depends on the size, charge, and solvation shell of the molecules, properties that change in function of the binding of a ligand to the fluorescent target.We used MST to describe the interaction between components of a regulatory module involved in plant cell wall integrity control. This module comprises the secreted peptide Rapid Alkalinization Factor 23 (RALF23) and its receptor complex consisting of the GPI-anchored receptor Lorelei-Like Glycoprotein 1 (LLG1) and a receptor kinase of the CrRLK1L family, FERONIA. Here we show how MST can also be used to study three-partner interactions.

Leaf microbiome dysbiosis triggered by T2SS-dependent enzyme secretion from opportunistic Xanthomonas pathogens

In healthy plants, the innate immune system contributes to maintenance of microbiota homoeostasis, while disease can be associated with microbiome perturbation or dysbiosis, and enrichment of opportunistic plant pathogens like Xanthomonas. It is currently unclear whether the microbiota change occurs independently of the opportunistic pathogens or is caused by the latter. Here we tested if protein export through the type-2 secretion system (T2SS) by Xanthomonas causes microbiome dysbiosis in Arabidopsis thaliana in immunocompromised plants. We found that Xanthomonas strains secrete a cocktail of plant cell wall-degrading enzymes that promote Xanthomonas growth during infection. Disease severity and leaf tissue degradation were increased in A. thaliana mutants lacking the NADPH oxidase RBOHD. Experiments with gnotobiotic plants, synthetic bacterial communities and wild-type or T2SS-mutant Xanthomonas revealed that virulence and leaf microbiome composition are controlled by the T2SS. Overall, a compromised immune system in plants can enrich opportunistic pathogens, which damage leaf tissues and ultimately cause microbiome dysbiosis by facilitating growth of specific commensal bacteria.

The damage-associated molecular pattern cellotriose alters the phosphorylation pattern of proteins involved in cellulose synthesis and trans-Golgi trafficking in Arabidopsis thaliana

We have recently demonstrated that the cellulose breakdown product cellotriose is a damage-associated molecular pattern (DAMP) which induces responses related to the integrity of the cell wall. Activation of downstream responses requires the Arabidopsis malectin domain-containing CELLOOLIGOMER RECEPTOR KINASE1 (CORK1)¹. The cellotriose/CORK1 pathway induces immune responses, including NADPH oxidase-mediated reactive oxygen species production, mitogen-activated protein kinase 3/6 phosphorylation-dependent defense gene activation, and the biosynthesis of defense hormones. However, apoplastic accumulation of cell wall breakdown products should also activate cell wall repair mechanisms. We demonstrate that the phosphorylation pattern of numerous proteins involved in the accumulation of an active cellulose synthase complex in the plasma membrane and those for protein trafficking to and within the trans-Golgi network (TGN) are altered within minutes after cellotriose application to Arabidopsis roots. The phosphorylation pattern of enzymes involved in hemicellulose or pectin biosynthesis and the transcript levels for polysaccharide-synthesizing enzymes responded barely to cellotriose treatments. Our data show that the phosphorylation pattern of proteins involved in cellulose biosynthesis and trans-Golgi trafficking is an early target of the cellotriose/CORK1 pathway.

The response of rhubarb to smut infection is revealed through a comparative transcriptome and metabolome study

MAIN CONCLUSION

Integrated transcriptome and metabolome analysis have unveiled the physiological and molecular responses of rhubarb to infection by smut fungi. Rhubarb is an important medicinal plant that is easily infected by smut fungi during its growth. Thus far, no research on the influence of smut fungi on the growth of rhubarb and its secondary metabolism has been conducted. In this study, petioles of Chinese rhubarb (Rheum officinale) [healthy or infected with smut fungus (Thecaphora schwarzmaniana)] were characterized. Microscopic structure, global gene expression profiling, global metabolic profiling, and key enzyme activity and metabolite levels in infected plants were analyzed. Infection by smut fungi resulted in numerous holes inside the petiole tissue and led to visible tumors on the external surface of the petiole. Through metabolic changes, T. schwarzmaniana induced the production of specific sugars, lipids, and amino acids, and inhibited the metabolism of phenolics and flavonoids in R. officinale. The concentrations of key medicinal compounds (anthraquinones) were decreased because of smut fungus infection. In terms of gene expression, the presence of T. schwarzmaniana led to upregulation of the genes associated with nutrient (sugar, amino acid, etc.) transport and metabolism. The gene expression profiling showed a stimulated cell division activity (the basis of tumor formation). Although plant antioxidative response was enhanced, the plant defense response against pathogen was suppressed by T. schwarzmaniana, as indicated by the expression profiling of genes involved in biotic and abiotic stress-related hormone signaling and the synthesis of plant disease resistance proteins. This study demonstrated physiological and molecular changes in R. officinale under T. schwarzmaniana infection, reflecting the survival tactics employed by smut fungus for parasitizing rhubarb.

The Localization of Cell Wall Components in the Quadrifids of Whole-Mount Immunolabeled Utricularia dichotoma Traps

Utricularia (bladderworts) are carnivorous plants. They produce small hollow vesicles, which function as suction traps that work underwater and capture fine organisms. Inside the traps, there are numerous glandular trichomes (quadrifids), which take part in the secretion of digestive enzymes, the resorption of released nutrients, and likely the pumping out of water. Due to the extreme specialization of quadrifids, they are an interesting model for studying the cell walls. This aim of the study was to fill in the gap in the literature concerning the immunocytochemistry of quadrifids in the major cell wall polysaccharides and glycoproteins. To do this, the localization of the cell wall components in the quadrifids was performed using whole-mount immunolabeled Utricularia traps. It was observed that only parts (arms) of the terminal cells had enough discontinuous cuticle to be permeable to antibodies. There were different patterns of the cell wall components in the arms of the terminal cells of the quadrifids. The cell walls of the arms were especially rich in low-methyl-esterified homogalacturonan. Moreover, various arabinogalactan proteins also occurred. Cell walls in glandular cells of quadrifids were rich in low-methyl-esterified homogalacturonan; in contrast, in the aquatic carnivorous plant Aldrovanda vesiculosa, cell walls in the glandular cells of digestive glands were poor in low-methyl-esterified homogalacturonan. Arabinogalactan proteins were found in the cell walls of trap gland cells in all studied carnivorous plants: Utricularia, and members of Droseraceae and Drosophyllaceae.

Origin, evolution, and diversification of the wall-associated kinase gene family in plants

KEY MESSAGE

The study of the origin, evolution, and diversification of the wall-associated kinase gene family in plants facilitates their functional investigations in the future. Wall-associated kinases (WAKs) make up one subfamily of receptor-like kinases (RLKs), and function directly in plant cell elongation and responses to biotic and abiotic stresses. The biological functions of WAKs have been extensively characterized in angiosperms; however, the origin and evolutionary history of the WAK family in green plants remain unclear. Here, we performed a comprehensive analysis of the WAK family to reveal its origin, evolution, and diversification in green plants. In total, 1061 WAK genes were identified in 37 species from unicellular algae to multicellular plants, and the results showed that WAK genes probably originated before bryophyte differentiation and were widely distributed in land plants, especially angiosperms. The phylogeny indicated that the land plant WAKs gave rise to five clades and underwent lineage-specific expansion after species differentiation. Cis-acting elements and expression patterns analyses of WAK genes in Arabidopsis and rice demonstrated the functional diversity of WAK genes in these two species. Many gene gains and losses have occurred in angiosperms, leading to an increase in the number of gene copies. The evolutionary trajectory of the WAK family during polyploidization was uncovered using Gossypium species. Our results provide insights into the evolution of WAK genes in green plants, facilitating their functional investigations in the future.

Under flooding conditions, controlled-release fertiliser coated microplastics affect the growth and accumulation of cadmium in rice by increasing the fluidity of cadmium and interfering with metabolic pathways

The combined pollution of microplastics (MPs) and Cd can affect plant growth and development and Cd accumulation, with most studies focusing on dryland soil. However, the effects of polyurethane (PU) controlled-release fertiliser coated MPs (PU MPs), which widely exist in rice systems, coupled with Cd on plant growth and Cd accumulation under flooding conditions are still unknown. Therefore, in the present study, in situ techniques were used to systematically study the effects of PU MPs and Cd coupling on the physiological and biochemical performance, metabolomics characteristics, rhizosphere bacterial community, and Cd bioavailability of rice in different soil types (red soil/cinnamon soil). The results showed that the effects of PU MPs on rice growth and Cd accumulation were concentration-dependent, especially in red soil. High PU concentration (1 %) inhibited rice root growth significantly (44 %). The addition of PU MPs inhibited photosynthetically active radiation, net photosynthesis, and transpiration rate of rice, mainly with low concentration (0.1 %) in red soil and high concentration (1 %) in cinnamon soil. PU MPs can enhance the expression of Cd resistance genes (cadC and copA) in soil, enhance the mobility of Cd, and affect the metabolic pathways of metabolites in the rhizosphere soil (red soil: fatty acid metabolism; cinnamon soil: amino acid degradation, heterobiodegradation, and nucleotide metabolism) to promote Cd absorption in rice. Especially in red soil, Cd accumulation in the root and aboveground parts of rice after the addition of high concentration PU (1 %) was 1.7 times and 1.3 times, respectively, that of the control (p < 0.05). Simultaneously, microorganisms can affect rice growth and Cd bioavailability by affecting functional bacteria related to carbon, iron, sulfur, and manganese. The results of the present study provide novel insights into the potential effects of PU MPs coupled with Cd on plants, rhizosphere bacterial communities, and Cd bioavailability.

Transcriptome Analysis of Sweet Cherry (Prunus avium L.) Cultivar 'Lapins' upon Infection of Pseudomonas syringae pv. syringae

Bacterial canker caused by Pseudomonas syringae pv. syringae (Pss) is responsible for substantial loss to the production of sweet cherry in Chile. To date, the molecular mechanisms of the Pss-sweet cherry interaction and the disease-related genes in the plant are poorly understood. In order to gain insight into these aspects, a transcriptomic analysis of the sweet cherry cultivar 'Lapins' for differentially expressed genes (DEGs) in response to Pss inoculation was conducted. Three Pss strains, A1M3, A1M197, and 11116_b1, were inoculated in young twigs, and RNA was extracted from tissue samples at the inoculation site and distal sections. RNA sequencing and transcriptomic expression analysis revealed that the three strains induced different patterns of responses in local and distal tissues. In the local tissues, A1M3 triggered a much more extensive response than the other two strains, enriching DEGs especially involved in photosynthesis. In the distal tissues, the three strains triggered a comparable extent of responses, among which 11116_b1 induced a group of DEGs involved in defense responses. Furthermore, tissues from various inoculations exhibited an enrichment of DEGs related to carbohydrate metabolism, terpene metabolism, and cell wall biogenesis. This study opened doors to future research on the Pss-sweet cherry interaction, immunity responses, and disease control.

Differences in the Occurrence of Cell Wall Components between Distinct Cell Types in Glands of Drosophyllum lusitanicum

Carnivorous plants are mixotrophs that have developed the ability to lure, trap, and digest small organisms and utilize components of the digested bodies. Leaves of Drosophyllum lusitanicum have two kinds of glands (emergences): stalked mucilage glands and sessile digestive glands. The stalked mucilage glands perform the primary role in prey lure and trapping. Apart from their role in carnivory, they absorb water condensed from oceanic fog; thus, plants can survive in arid conditions. To better understand the function of carnivorous plant emergences, the molecular composition of their cell walls was investigated using immunocytochemical methods. In this research, Drosophyllum lusitanicum was used as a study system to determine whether cell wall immunocytochemistry differs between the mucilage and digestive glands of other carnivorous plant species. Light and electron microscopy were used to observe gland structure. Fluorescence microscopy revealed the localization of carbohydrate epitopes associated with the major cell wall polysaccharides and glycoproteins. The mucilage gland (emergence) consists of a glandular head, a connecting neck zone, and stalk. The gland head is formed by an outer and inner layer of glandular (secretory) cells and supported by a layer of endodermoid (barrier) cells. The endodermoid cells have contact with a core of spongy tracheids with spiral-shaped thickenings. Lateral tracheids are surrounded by epidermal and parenchymal neck cells. Different patterns of cell wall components were found in the various cell types of the glands. Cell walls of glandular cells generally are poor in both low and highly esterified homogalacturonans (HGs) but enriched with hemicelluloses. Cell walls of inner glandular cells are especially rich in arabinogalactan proteins (AGPs). The cell wall ingrowths in glandular cells are significantly enriched with hemicelluloses and AGPs. In the case of cell wall components, the glandular cells of Drosophyllum lusitanicum mucilage glands are similar to the glandular cells of the digestive glands of Aldrovanda vesiculosa and Dionaea muscipula.

Brassinosteroid recruits FERONIA to safeguard cell expansion in Arabidopsis

Plant cell expansion is driven by turgor pressure and regulated by hormones. How plant cells avoid cell wall rupture during hormone-induced cell expansion remains a mystery. Here we show that brassinosteroid (BR), while stimulating cell elongation, promotes the plasma membrane (PM) accumulation of the receptor kinase FERONIA (FER), which monitors cell wall damage and in turn attenuates BR-induced cell elongation to prevent cell rupture. The GSK3-like kinase BIN2 phosphorylates FER, resulting in reduced FER accumulation and translocation from endoplasmic reticulum to PM. By inactivating BIN2, BR signaling promotes dephosphorylation and increases PM accumulation of FER, thereby enhancing the surveillance of cell wall integrity. Our study reveals a vital signaling circuit that coordinates hormone signaling with mechanical sensing to prevent cell bursting during hormone-induced cell expansion.

Immune activation during Pseudomonas infection causes local cell wall remodeling and alters AGP accumulation

The plant cell boundary generally comprises constituents of the primary and secondary cell wall (CW) that are deposited sequentially during development. Although it is known that the CW acts as a barrier against phytopathogens and undergoes modifications to limit their invasion, the extent, sequence, and requirements of the pathogen-induced modifications of the CW components are still largely unknown, especially at the level of the polysaccharide fraction. To address this significant knowledge gap, we adopted the compatible Pseudomonas syringae-Arabidopsis thaliana system. We found that, despite systemic signaling actuation, Pseudomonas infection leads only to local CW modifications. Furthermore, by utilizing a combination of CW and immune signaling-deficient mutants infected with virulent or non-virulent bacteria, we demonstrated that the pathogen-induced changes in CW polysaccharides depend on the combination of pathogen virulence and the host's ability to mount an immune response. This results in a pathogen-driven accumulation of CW hexoses, such as galactose, and an immune signaling-dependent increase in CW pentoses, mainly arabinose, and xylose. Our analyses of CW changes during disease progression also revealed a distinct spatiotemporal pattern of arabinogalactan protein (AGP) deposition and significant modifications of rhamnogalacturonan sidechains. Furthermore, genetic analyses demonstrated a critical role of AGPs, specifically of the Arabinoxylan Pectin Arabinogalactan Protein1, in limiting pathogen growth. Collectively, our results provide evidence for the actuation of significant remodeling of CW polysaccharides in a compatible host-pathogen interaction, and, by identifying AGPs as critical elements of the CW in plant defense, they pinpoint opportunities to improve plants against diverse pathogens.

Comparative Root Transcriptome Profiling and Gene Regulatory Network Analysis between Eastern and Western Carrot (Daucus carota L.) Cultivars Reveals Candidate Genes for Vascular Tissue Patterning

Carrot (Daucus carota L.) is a highly consumed vegetable rich in carotenoids, known for their potent antioxidant, anti-inflammatory, and immune-protecting properties. While genetic and molecular studies have largely focused on wild and Western carrot cultivars (cvs), little is known about the evolutionary interactions between closely related Eastern and Western cvs. In this study, we conducted comparative transcriptome profiling of root tissues from Eastern (UHSBC-23-1) and Western (UHSBC-100) carrot cv. to better understand differentially expressed genes (DEGs) associated with storage root development and vascular cambium (VC) tissue patterning. Through reference-guided TopHat mapping, we achieved an average mapping rate of 73.87% and identified a total of 3544 DEGs (p < 0.05). Functional annotation and gene ontology classification revealed 97 functional categories, including 33 biological processes, 19 cellular components, 45 metabolic processes, and 26 KEGG pathways. Notably, Eastern cv. exhibited enrichment in cell wall, plant-pathogen interaction, and signal transduction terms, while Western cv. showed dominance in photosynthesis, metabolic process, and carbon metabolism terms. Moreover, constructed gene regulatory network (GRN) for both cvs. obtained orthologs with 1222 VC-responsive genes of Arabidopsis thaliana. In Western cv, GRN revealed VC-responsive gene clusters primarily associated with photosynthetic processes and carbon metabolism. In contrast, Eastern cv. exhibited a higher number of stress-responsive genes, and transcription factors (e.g., MYB15, WRKY46, AP2/ERF TF connected via signaling pathways with NAC036) were identified as master regulators of xylem vessel differentiation and secondary cell wall thickening. By elucidating the comparative transcriptome profiles of Eastern and Western cvs. for the first time, our study provides valuable insights into the differentially expressed genes involved in root development and VC tissue patterning. The identification of key regulatory genes and their roles in these processes represents a significant advancement in our understanding of the evolutionary relations and molecular mechanisms underlying secondary growth of carrot and regulation by vascular cambium.

Evolution and Expression of the Expansin Genes in Emmer Wheat

Expansin proteins, a crucial class of intracellular proteins, are known to play a vital role in facilitating processes like cell wall relaxation and cell growth. Recent discoveries have revealed that expansin proteins also have significant functions in plant growth, development, and response to resistance. However, the expansin gene family, particularly in emmer wheat, has not been thoroughly studied, particularly in terms of evolution. In this study, we identified 63 TdEXPs and 49 TtEXPs from the latest genome versions of wild emmer wheat (WEW) and durum wheat (DW), respectively. The physicochemical properties of the encoded expansin proteins exhibited minimal differences, and the gene structures remained relatively conserved. Phylogenetic analysis categorized the proteins into three subfamilies, namely EXPA, EXPB, and EXLA, in addition to the EXLB subfamily. Furthermore, codon preference analysis revealed an increased usage frequency of the nucleotide "T" in expansin proteins throughout the evolution of WEW and DW. Collinearity analysis demonstrated higher orthology between the expansin proteins of WEW and DW, with a Ka/Ks ratio ranging from 0.4173 to 0.9494, indicating purifying selection during the evolution from WEW to DW. Haplotype analysis of the expansin gene family identified five genes in which certain haplotypes gradually became dominant over the course of evolution, enabling adaptation for survival and improvement. Expression pattern analysis indicated tissue-specific expression of expansin genes in emmer wheat, and some of these genes were quantified through qRT-PCR to assess their response to salt stress. These comprehensive findings present the first systematic analysis of the expansin protein gene family during the evolution from WEW to DW, providing a foundation for further understanding the functions and biological roles of expansin protein genes in emmer wheat.

Why is FERONIA pleiotropic?

The plant cell wall has many roles: structure, hydraulics, signalling and immunity. Monitoring its status is therefore essential for plant life. Among many candidate cell wall sensors, FERONIA, a member of the Catharanthus roseus receptor-like kinase-1-like kinase (CrRLK1L) family, has received considerable attention, notably because of its numerous interactors and its implication in many biological pathways. Conversely, such an analytical dissection may blur its core function. Here we revisit the array of feronia phenotypes as an attempt to identify a unifying feature behind the plethora of biological and biochemical functions. We propose that the contribution of FERONIA in monitoring turgor-dependent cell wall tension may explain its pleiotropy.

Apoplastic and Symplasmic Markers of Somatic Embryogenesis

Somatic embryogenesis (SE) is a process that scientists have been trying to understand for many years because, on the one hand, it is a manifestation of the totipotency of plant cells, so it enables the study of the mechanisms regulating this process, and, on the other hand, it is an important method of plant propagation. Using SE in basic research and in practice is invaluable. This article describes the latest, but also historical, information on changes in the chemical composition of the cell wall during the transition of cells from the somatic to embryogenic state, and the importance of symplasmic communication during SE. Among wall chemical components, different pectic, AGP, extensin epitopes, and lipid transfer proteins have been discussed as potential apoplastic markers of explant cells during the acquisition of embryogenic competence. The role of symplasmic communication/isolation during SE has also been discussed, paying particular attention to the formation of symplasmic domains within and between cells that carry out different developmental processes. Information about the number and functionality of plasmodesmata (PD) and callose deposition as the main player in symplasmic isolation has also been presented.

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 receptor kinase FERONIA regulates phosphatidylserine localization at the cell surface to modulate ROP signaling

Cells maintain a constant dialog between the extracellular matrix and their plasma membrane to fine tune signal transduction processes. We found that the receptor kinase FERONIA (FER), which is a proposed cell wall sensor, modulates phosphatidylserine plasma membrane accumulation and nano-organization, a key regulator of Rho GTPase signaling in Arabidopsis. We demonstrate that FER is required for both Rho-of-Plant 6 (ROP6) nano-partitioning at the membrane and downstream production of reactive oxygen species upon hyperosmotic stimulus. Genetic and pharmacological rescue experiments indicate that phosphatidylserine is required for a subset of, but not all, FER functions. Furthermore, application of FER ligand shows that its signaling controls both phosphatidylserine membrane localization and nanodomains formation, which, in turn, tunes ROP6 signaling. Together, we propose that a cell wall-sensing pathway controls via the regulation of membrane phospholipid content, the nano-organization of the plasma membrane, which is an essential cell acclimation to environmental perturbations.

A necessary considering factor for breeding: growth-defense tradeoff in plants

Crop diseases cause enormous yield losses and threaten global food security. Deployment of resistant cultivars can effectively control the disease and to minimize crop losses. However, high level of genetic immunity to disease was often accompanied by an undesired reduction in crop growth and yield. Recently, literatures have been rapidly emerged in understanding the mechanism of disease resistance and development genes in crop plants. To determine how and why the costs and the likely benefit of resistance genes caused in crop varieties, we re-summarized the present knowledge about the crosstalk between plant development and disease resistance caused by those genes that function as plasma membrane residents, MAPK cassette, nuclear envelope (NE) channels components and pleiotropic regulators. Considering the growth-defense tradeoffs on the basis of current advances, finally, we try to understand and suggest that a reasonable balancing strategies based on the interplay between immunity with growth should be considered to enhance immunity capacity without yield penalty in future crop breeding.