New paradigms in cell adaptation: decades of discoveries on the CrRLK1L receptor kinase signalling network

FERONIA homologs in stress responses of horticultural plants: current knowledge and missing links

Owing to its versatile roles in almost all aspects of plants, FERONIA (FER), a receptor-like kinase of the Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) subfamily, has received extensive research interests during the past decades. Accumulating evidence has been emerged that FER homologs in horticultural crops also play crucial roles in reproductive biology and responses to environmental stimuli (abiotic and biotic stress factors). Here, we provide a review for the latest advances in the studies on FER homologs in modulating stress responses in horticultural crops, and further analyze the underlying mechanisms maintained by FER. Moreover, we also envisage the missing links in current work and provide a perspective for future studies on this star protein.

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.

Arabidopsis leucine-rich repeat malectin receptor-like kinases regulate pollen-stigma interactions

Flowering plants contain tightly controlled pollen-pistil interactions required for promoting intraspecific fertilization and preventing interspecific hybridizations. In Arabidopsis (Arabidopsis thaliana), several receptor kinases (RKs) are known to regulate the later stages of intraspecific pollen tube growth and ovular reception in the pistil, but less is known about RK regulation of the earlier stages. The Arabidopsis RECEPTOR-LIKE KINASE IN FLOWERS1 (RKF1)/RKF1-LIKE (RKFL) 1-3 cluster of 4 leucine-rich repeat malectin (LRR-MAL) RKs was previously found to function in the stigma to promote intraspecific pollen hydration. In this study, we tested additional combinations of up to 7 Arabidopsis LRR-MAL RK knockout mutants, including RKF1, RKFL1-3, LysM RLK1-INTERACTING KINASE1, REMORIN-INTERACTING RECEPTOR1, and NEMATODE-INDUCED LRR-RLK2. These LRR-MAL RKs were discovered to function in the female stigma to support intraspecific Arabidopsis pollen tube growth and to establish a prezygotic interspecific barrier against Capsella rubella pollen. Thus, this study uncovered additional biological functions for this poorly understood group of RKs in regulating the early stages of Arabidopsis sexual reproduction.

Rice kinase OsMRLK63 contributes to drought tolerance by regulating reactive oxygen species production

Drought is a major adverse environmental factor that plants face in nature but the molecular mechanism by which plants transduce stress signals and further endow themselves with tolerance remains unclear. Malectin/malectin-like domains containing receptor-like kinases (MRLKs) have been proposed to act as receptors in multiple biological signaling pathways, but limited studies show their roles in drought-stress signaling and tolerance. In this study, we demonstrate OsMRLK63 in rice (Oryza sativa L.) functions in drought tolerance by acting as the receptor of 2 rapid alkalization factors, OsRALF45 and OsRALF46. We show OsMRLK63 is a typical receptor-like kinase that positively regulates drought tolerance and reactive oxygen species (ROS) production. OsMRLK63 interacts with and phosphorylates several nicotinamide adenine dinucleotide phosphate (NADPH) oxidases with the primarily phosphorylated site at Ser26 in the N-terminal of RESPIRATORY BURST OXIDASE HOMOLOGUE A (OsRbohA). The application of the 2 small signal peptides (OsRALF45/46) on rice can greatly alleviate the dehydration of plants induced by mimic drought. This function depends on the existence of OsMRLK63 and the NADPH oxidase-dependent ROS production. The 2 RALFs interact with OsMRLK63 by binding to its extracellular domain, suggesting they may act as drought/dehydration signal sensors for the OsMRLK63-mediated process. Our study reveals a OsRALF45/46-OsMRLK63-OsRbohs module which contributes to drought-stress signaling and tolerance in rice.

The Receptor Kinases DRUS1 and DRUS2 Behave Distinctly in Osmotic Stress Tolerance by Modulating the Root System Architecture via Auxin Signaling

Receptor kinases DRUS1 (Dwarf and Runtish Spikelet1) and DRUS2 are orthologues of the renowned Arabidopsis thaliana gene FERONIA, which play redundant roles in rice growth and development. Whether the two duplicated genes perform distinct functions in response to environmental stress is largely unknown. Here, we found that osmotic stress (OS) and ABA increased DRUS1 expression while decreasing DRUS2. When subjected to osmotic stress, the increased DRUS1 in drus2 mutants suppresses the OsIAA repressors, resulting in a robust root system with an increased number of adventitious and lateral roots as well as elongated primary, adventitious, and lateral roots, conferring OS tolerance. In contrast, the decreased DRUS2 in drus1-1 mutants are not sufficient to suppress OsIAA repressors, leading to a feeble root system with fewer adventitious and lateral roots and hindering seminal root growth, rendering OS intolerance. All these findings offer valuable insights into the biological significance of the duplication of two homologous genes in rice, wherein, if one is impaired, the other one is able to continue auxin-signaling-mediated root growth and development to favor resilience to environmental stress, such as water shortage.

Large-scale analysis of the N-terminal regulatory elements of the kinase domain in plant Receptor-like kinase family

BACKGROUND

The N-terminal regulatory element (NRE) of Receptor-like kinases (RLKs), consisting of the juxtamembrane segment in receptor kinases (RKs) and the N-terminal extension segment in RLCKs, is a crucial component that regulates the activities of these proteins. However, the features and functions of the NRE have remained largely unexplored. Herein, we comprehensively analyze 510,233 NRE sequences in RLKs from 528 plant species, using information theory and data mining techniques to unravel their common characteristics and diversity. We also use recombinant RKs to investigate the function of the NRE in vitro.

RESULTS

Our findings indicate that the majority of NRE segments are around 40-80 amino acids in length and feature a serine-rich region and a 14-amino-acid consensus sequence, 'FSYEELEKAT[D/N]NF[S/D]', which contains a characteristic α-helix and ST motif that connects to the core kinase domain. This conserved signature sequence is capable of suppressing FERONIA's kinase activity. A motif discovery algorithm identifies 29 motifs with highly conserved phosphorylation sites in RK and RLCK classes, especially the motif 'VGPWKpTGLpSGQLQKAFVTGVP' in LRR-VI-2 class. Phosphorylation of an NRE motif in an LRR-VI-2 member, MDIS1, modulates the auto-phosphorylation of its co-receptor, MIK1, indicating the potential role of NRE as a 'kinase switch' in RLK activation. Furthermore, the characterization of phosphorylatable NRE motifs improves the accuracy of predicting phosphorylatable sites.

CONCLUSIONS

Our study provides a comprehensive dataset to investigate NRE segments from individual RLKs and enhances our understanding of the underlying mechanisms of RLK signal transduction and kinase activation processes in plant adaptation.

Phase separation of GRP7 facilitated by FERONIA-mediated phosphorylation inhibits mRNA translation to modulate plant temperature resilience

Changes in ambient temperature profoundly affect plant growth and performance. Therefore, the molecular basis of plant acclimation to temperature fluctuation is of great interest. In this study, we discovered that GLYCINE-RICH RNA-BINDING PROTEIN 7 (GRP7) contributes to cold and heat tolerance in Arabidopsis thaliana. We found that exposure to a warm temperature rapidly induces GRP7 condensates in planta, which can be reversed by transfer to a lower temperature. Cell biology and biochemical assays revealed that GRP7 undergoes liquid-liquid phase separation (LLPS) in vivo and in vitro. LLPS of GRP7 in the cytoplasm contributes to the formation of stress granules that recruit RNA, along with the translation machinery component eukaryotic initiation factor 4E1 (eIF4E1) and the mRNA chaperones COLD SHOCK PROTEIN 1 (CSP1) and CSP3, to inhibit translation. Moreover, natural variations in GRP7 affecting the residue phosphorylated by the receptor kinase FERONIA alter its capacity to undergo LLPS and correlate with the adaptation of some Arabidopsis accessions to a wider temperature range. Taken together, our findings illustrate the role of translational control mediated by GRP7 LLPS to confer plants with temperature resilience.

The soil emergence-related transcription factor PIF3 controls root penetration by interacting with the receptor kinase FER

The cotyledons of etiolated seedlings from terrestrial flowering plants must emerge from the soil surface, while roots must penetrate the soil to ensure plant survival. We show here that the soil emergence-related transcription factor PHYTOCHROME-INTERACTING FACTOR 3 (PIF3) controls root penetration via transducing external signals perceived by the receptor kinase FERONIA (FER) in Arabidopsis thaliana. The loss of FER function in Arabidopsis and soybean (Glycine max) mutants resulted in a severe defect in root penetration into agar medium or hard soil. Single-cell RNA sequencing (scRNA-seq) profiling of Arabidopsis roots identified a distinct cell clustering pattern, especially for root cap cells, and identified PIF3 as a FER-regulated transcription factor. Biochemical, imaging, and genetic experiments confirmed that PIF3 is required for root penetration into soil. Moreover, FER interacted with and stabilized PIF3 to modulate the expression of mechanosensitive ion channel PIEZO and the sloughing of outer root cap cells.

Rice receptor kinase FLR7 regulates rhizosphere oxygen levels and enriches the dominant Anaeromyxobacter that improves submergence tolerance in rice

Oxygen is one of the determinants of root microbiome formation. However, whether plants regulate rhizosphere oxygen levels to affect microbiota composition and the underlying molecular mechanisms remain elusive. The receptor-like kinase (RLK) family member FERONIA modulates the growth-defense tradeoff in Arabidopsis. Here, we established that rice FERONIA-like RLK 7 (FLR7) controls rhizosphere oxygen levels by methylene blue staining, oxygen flux, and potential measurements. The formation of oxygen-transporting aerenchyma in roots is negatively regulated by FLR7. We further characterized the root microbiota of 11 FLR mutants including flr7 and wild-type Nipponbare (Nip) grown in the field by 16S ribosomal RNA gene profiling and demonstrated that the 11 FLRs are involved in regulating rice root microbiome formation. The most abundant anaerobic-dependent genus Anaeromyxobacter in the Nip root microbiota was less abundant in the root microbiota of all these mutants, and this contributed the most to the community differences between most mutants and Nip. Metagenomic sequencing revealed that flr7 increases aerobic respiration and decreases anaerobic respiration in the root microbiome. Finally, we showed that a representative Anaeromyxobacter strain improved submergence tolerance in rice via FLR7. Collectively, our findings indicate that FLR7 mediates changes in rhizosphere oxygen levels and enriches the beneficial dominant genus Anaeromyxobacter and may provide insights for developing plant flood prevention strategies via the use of environment-specific functional soil microorganisms.

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 FERONIA-YUELAO module participates in translational control by modulating the abundance of tRNA fragments in Arabidopsis

tRNA fragments (tRFs) are a recently identified class of small noncoding RNAs. To date, the regulation of tRF abundance and its functional mechanisms have been largely unclear in plants. We investigated how the Arabidopsis thaliana receptor kinase FERONIA (FER) regulates the abundance of tRFs to inhibit global mRNA translation. We demonstrate that FER regulates tRF abundance by directly phosphorylating the tRNA-binding protein YUELAO (YL) to modulate its function. Downregulation of FER and YL prevented the modification of tRNA via cytosine-5-methylation and 2'-O-methylation, thereby increasing tRF abundance. Furthermore, we show that YL acts as an important genetic downstream target of FER signaling, and knockdown of a specific tRF partially rescues the root hair growth defects of fer and yl mutants. Our findings shed light on the abundance and regulatory mechanisms of tRF and their role in inhibiting translation in plants.

Sensing and regulation of plant extracellular pH

In plants, pH determines nutrient acquisition and sensing, and triggers responses to osmotic stress, whereas pH homeostasis protects the cellular machinery. Extracellular pH (pHe) controls the chemistry and rheology of the cell wall to adjust its elasticity and regulate cell expansion in space and time. Plasma membrane (PM)-localized proton pumps, cell-wall components, and cell wall-remodeling enzymes jointly maintain pHe homeostasis. To adapt to their environment and modulate growth and development, plant cells must sense subtle changes in pHe caused by the environment or neighboring cells. Accumulating evidence indicates that PM-localized cell-surface peptide-receptor pairs sense pHe. We highlight recent advances in understanding how plants perceive and maintain pHe, and discuss future perspectives.

A receptor-like kinase SlFERL mediates immune responses of tomato to Botrytis cinerea by recognizing BcPG1 and fine-tuning MAPK signaling

FERONIA (FER) is a receptor-like kinase showing versatile functions during plant growth, development, and responses to environmental stimuli. However, its functions during the interaction between fruit and necrotrophic fungal pathogens are still unclear. Combining reverse genetic approaches, physiological assays, co-immunoprecipitation, protein phosphorylation identification, and site-directed mutagenesis, we reported a tomato FER homolog SlFERL (Solanum lycopersicum FERONIA Like) involved in the immune responses to Botrytis cinerea invasion. The results indicated that SlFERL extracellular domain recognized and interacted with the secreted virulence protein BcPG1 from B. cinerea, further revealed that SlFERL triggered downstream signaling by phosphorylating SlMAP3K18 at Thr45, Ser49, Ser76, and Ser135. Moreover, we verified that SlMAP2K2 and SlMAP2K4 synergistically contributed to immune response of tomato to B. cinerea, in which SlFERL-SlMAP3K18 module substantially modulated protein level and/or kinase activity of SlMAP2K2/SlMAP2K4. These findings reveal a new pattern-triggered immune pathway, indicating that SlFERL participates in the immune responses to B. cinerea invasion via recognizing BcPG1 and fine-tuning MAPK signaling.

The power of patterns: new insights into pattern-triggered immunity

The plant immune system features numerous immune receptors localized on the cell surface to monitor the apoplastic space for danger signals from a broad range of plant colonizers. Recent discoveries shed light on the enormous complexity of molecular signals sensed by these receptors, how they are generated and removed to maintain cellular homeostasis and immunocompetence, and how they are shaped by host-imposed evolutionary constraints. Fine-tuning receptor sensing mechanisms at the molecular, cellular and physiological level is critical for maintaining a robust but adaptive host barrier to commensal, pathogenic, and symbiotic colonizers alike. These receptors are at the core of any plant-colonizer interaction and hold great potential for engineering disease resistance and harnessing beneficial microbiota to keep crops healthy.

A new "lock-and-key" system revealed for plant reproductive barriers

Mate selection in flowering plants can occur very rapidly after male pollen contact on the female pistil, but the cellular regulators driving this process were poorly understood. In this issue of Cell, Lan et al. have discovered the components of a complex ligand-receptor system controlling pollen selection in Arabidopsis thaliana.

Genome-Wide Identification and Analysis of Catharanthus roseus Receptor-like Kinase 1-like Proteins in Eggplant

As an important member of the plant receptor-like kinases, Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) plays vital roles in plant growth and development, as well as biotic and abiotic stress response. Numerous CrRLK1Ls have been identified and analyzed in various plant species, while our knowledge about eggplant (Solanum melongena L.) CrRLK1Ls is still scarce. Utilizing state-of-the-art genomic data, we conducted the first genome-wide identification and analysis of CrRLK1L proteins in eggplant. In this study, 32 CrRLK1L proteins were identified and analyzed in eggplant. A subsequent gene structure and protein domain analysis showed that the identified eggplant CrRLK1Ls possessed typical features of CrRLK1Ls. A subcellular localization prediction demonstrated that these proteins mostly localized on the plasma membrane. A collinearity analysis showed that some eggplant CrRLK1L genes had predicted intraspecies or interspecies evolutionary duplication events. Promoter analysis suggests that eggplant CrRLK1Ls may be involved in plant hormone signaling, host-pathogen interactions, and environmental responses. Based on transcriptomic gene expression analysis, it is indicated that eggplant CrRLK1Ls may be involved in the resistance response of eggplant to Botrytis cinerea. Together, these results will give us a theoretical foundation and guidance for elaborating the biological functions of CrRLK1Ls in eggplant growth, development, and resistance response.

Contrasting self-recognition rejection systems for self-incompatibility in Brassica and Papaver

Self-incompatibility (SI) plays a pivotal role in whether self-pollen is accepted or rejected. Most SI systems employ two tightly linked loci encoding highly polymorphic pollen (male) and pistil (female) S-determinants that control whether self-pollination is successful or not. In recent years our knowledge of the signalling networks and cellular mechanisms involved has improved considerably, providing an important contribution to our understanding of the diverse mechanisms used by plant cells to recognise each other and elicit responses. Here, we compare and contrast two important SI systems employed in the Brassicaceae and Papaveraceae. Both use 'self-recognition' systems, but their genetic control and S-determinants are quite different. We describe the current knowledge about the receptors and ligands, and the downstream signals and responses utilized to prevent self-seed set. What emerges is a common theme involving the initiation of destructive pathways that block the key processes that are required for compatible pollen-pistil interactions.

Characterization of Malectin/Malectin-like Receptor-like Kinase Family Members in Foxtail Millet (Setaria italica L.)

Plant malectin/malectin-like receptor-like kinases (MRLKs) play crucial roles throughout the life course of plants. Here, we identified 23 SiMRLK genes from foxtail millet. All the SiMRLK genes were named according to the chromosomal distribution of the SiMRLKs in the foxtail millet genome and grouped into five subfamilies based on phylogenetic relationships and structural features. Synteny analysis indicated that gene duplication events may take part in the evolution of SiMRLK genes in foxtail millet. The expression profiles of 23 SiMRLK genes under abiotic stresses and hormonal applications were evaluated through qRT-PCR. The expression of SiMRLK1, SiMRLK3, SiMRLK7 and SiMRLK19 were significantly affected by drought, salt and cold stresses. Exogenous ABA, SA, GA and MeJA also obviously changed the transcription levels of SiMRLK1, SiMRLK3, SiMRLK7 and SiMRLK19. These results signified that the transcriptional patterns of SiMRLKs showed diversity and complexity in response to abiotic stresses and hormonal applications in foxtail millet.

Rapid alkalinization factor: function, regulation, and potential applications in agriculture

Rapid alkalinization factor (RALF) is widespread throughout the plant kingdom and controls many aspects of plant life. Current studies on the regulatory mechanism underlying RALF function mainly focus on Arabidopsis, but little is known about the role of RALF in crop plants. Here, we systematically and comprehensively analyzed the relation between RALF family genes from five important crops and those in the model plant Arabidopsis thaliana. Simultaneously, we summarized the functions of RALFs in controlling growth and developmental behavior using conservative motifs as cues and predicted the regulatory role of RALFs in cereal crops. In conclusion, RALF has considerable application potential in improving crop yields and increasing economic benefits. Using gene editing technology or taking advantage of RALF as a hormone additive are effective way to amplify the role of RALF in crop plants.

Rice FERONIA-LIKE RECEPTOR 3 and 14 affect grain quality by regulating redox homeostasis during endosperm development

Chalky endosperm negatively affects the appearance, milling, and eating qualities of rice (Oryza sativa L.) grains. Here, we report the role of two receptor-like kinases, FERONIA-LIKE RECEPTOR 3 (FLR3) and FERONIA-LIKE RECEPTOR 14 (FLR14), in grain chalkiness and quality. Knockouts of FLR3 and/or FLR14 increased the number of white-core grains caused by aberrant accumulation of storage substances, resulting in poor grain quality. Conversely, the overexpression of FLR3 or FLR14 reduced grain chalkiness and improved grain quality. Transcriptome and metabolome analyses showed that genes and metabolites involved in the oxidative stress response were significantly up-regulated in flr3 and flr14 grains. The content of reactive oxygen species was significantly increased in flr3 and flr14 mutant endosperm but decreased in overexpression lines. This strong oxidative stress response induced the expression of programmed cell death (PCD)-related genes and caspase activity in endosperm, which further accelerated PCD, resulting in grain chalkiness. We also demonstrated that FLR3 and FLR14 reduced grain chalkiness by alleviating heat-induced oxidative stress in rice endosperm. Therefore, we report two positive regulators of grain quality that maintain redox homeostasis in the endosperm, with potential applications in breeding rice for optimal grain quality.

Role of the plant-specific calcium-binding C2-DOMAIN ABSCISIC ACID-RELATED (CAR) protein family in environmental signaling

Many signaling processes rely on information decoding at the plasma membrane, and membrane-associated proteins and their complexes are fundamental for regulating this process. Still many questions exist as to how protein complexes are assembled and function at membrane sites to change identity and dynamics of membrane systems. Peripheral membrane proteins containing a calcium and phospholipid-binding C2-domain can act in membrane-related signaling by providing a tethering function so that protein complexes form. C2 domain proteins termed C2-DOMAIN ABSCISIC ACID-RELATED (CAR) proteins are plant-specific, and the functional relevance of this C2 domain protein subgroup is just emerging. The ten Arabidopsis CAR proteins CAR1 to CAR10 have a single C2 domain with a plant-specific insertion, the so-called "CAR-extra-signature" or also termed "sig domain". Via this "sig domain" CAR proteins can bind signaling protein complexes of different kinds and act in biotic and abiotic stress, blue light and iron nutrition. Interestingly, CAR proteins can oligomerize in membrane microdomains, and their presence in the nucleus can be linked with nuclear protein regulation. This shows that CAR proteins may play unprecedented roles in coordinating environmental responses and assembling required protein complexes to transmit information cues between plasma membrane and nucleus. The aim of this review is to summarize structure-function characteristics of the CAR protein family and assemble findings from CAR protein interactions and physiological functions. From this comparative investigation we extract common principles about the molecular operations that CAR proteins may fulfill in the cell. We also deduce functional properties of the CAR protein family based on its evolution and gene expression profiles. We highlight open questions and suggest novel avenues to prove and understand the functional networks and roles played by this protein family in plants.

Integrative Proteomics and Metabolomics Analysis Reveals the Role of Small Signaling Peptide Rapid Alkalinization Factor 34 (RALF34) in Cucumber Roots

The main role of RALF small signaling peptides was reported to be the alkalization control of the apoplast for improvement of nutrient absorption; however, the exact function of individual RALF peptides such as RALF34 remains unknown. The Arabidopsis RALF34 (AtRALF34) peptide was proposed to be part of the gene regulatory network of lateral root initiation. Cucumber is an excellent model for studying a special form of lateral root initiation taking place in the meristem of the parental root. We attempted to elucidate the role of the regulatory pathway in which RALF34 is a participant using cucumber transgenic hairy roots overexpressing CsRALF34 for comprehensive, integrated metabolomics and proteomics studies, focusing on the analysis of stress response markers. CsRALF34 overexpression resulted in the inhibition of root growth and regulation of cell proliferation, specifically in blocking the G2/M transition in cucumber roots. Based on these results, we propose that CsRALF34 is not part of the gene regulatory networks involved in the early steps of lateral root initiation. Instead, we suggest that CsRALF34 modulates ROS homeostasis and triggers the controlled production of hydroxyl radicals in root cells, possibly associated with intracellular signal transduction. Altogether, our results support the role of RALF peptides as ROS regulators.

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.

CAR modulates plasma membrane nano-organization and immune signaling downstream of RALF1-FERONIA signaling pathway

In Arabidopsis, the receptor-like kinase (RLK) FERONIA (FER) senses peptide ligands in the plasma membrane (PM), modulates plant growth and development, and integrates biotic and abiotic stress signaling for downstream adaptive responses. However, the molecular interplay of these diverse processes is largely unknown. Here, we show that FER, the receptor of Rapid Alkalinization Factor 1 (RALF1), physically interacts with C2 domain ABA-related (CAR) proteins to control the nano-organization of the PM. During this process, the RALF1-FER pathway upregulates CAR protein translation, and then more CAR proteins are recruited to the PM. This acts as a rapid feedforward loop that stabilizes the PM liquid-ordered phase. FER interacts with and phosphorylates CARs, thereby reducing their lipid-binding ability and breaking the feedback regulation at later time points. The formation of the flg22-induced FLS2-BAK1 immune complex, which depends on the integrity of FER-containing nanodomains, is impaired in fer and pentuple car14569 mutant. Together, we propose that the FER-CAR module controls the formation of PM nano-organization during RALF signaling through a self-contained amplifying loop including both positive and negative feedback.

Cell Wall Integrity Signaling in Fruit Ripening

Plant cell walls are essential structures for plant growth and development as well as plant adaptation to environmental stresses. Thus, plants have evolved signaling mechanisms to monitor the changes in the cell wall structure, triggering compensatory changes to sustain cell wall integrity (CWI). CWI signaling can be initiated in response to environmental and developmental signals. However, while environmental stress-associated CWI signaling has been extensively studied and reviewed, less attention has been paid to CWI signaling in relation to plant growth and development under normal conditions. Fleshy fruit development and ripening is a unique process in which dramatic alternations occur in cell wall architecture. Emerging evidence suggests that CWI signaling plays a pivotal role in fruit ripening. In this review, we summarize and discuss the CWI signaling in relation to fruit ripening, which will include cell wall fragment signaling, calcium signaling, and NO signaling, as well as Receptor-Like Protein Kinase (RLKs) signaling with an emphasis on the signaling of FERONIA and THESEUS, two members of RLKs that may act as potential CWI sensors in the modulation of hormonal signal origination and transduction in fruit development and ripening.

Structural and biochemical basis of Arabidopsis FERONIA receptor kinase-mediated early signaling initiation

Accumulating evidence indicates that early and essential events for receptor-like kinase (RLK) function involve both autophosphorylation and substrate phosphorylation. However, the structural and biochemical basis for these events is largely unclear. Here, we used RLK FERONIA (FER) as a model and crystallized its core kinase domain (FER-KD) and two FER-KD mutants (K565R, S525A) in complexes with ATP/ADP and Mg²⁺ in the unphosphorylated state. Unphosphorylated FER-KD was found to adopt an unexpected active conformation in its crystal structure. Moreover, unphosphorylated FER-KD mutants with reduced (S525A) or no catalytic activity (K565R) also adopt similar active conformations. Biochemical studies revealed that FER-KD is a dual-specificity kinase, and its autophosphorylation is accomplished via an intermolecular mechanism. Further investigations confirmed that initiating substrate phosphorylation requires autophosphorylation of the activation segment on T696, S701, and Y704. This study reveals the structural and biochemical basis for the activation and regulatory mechanism of FER, providing a paradigm for the early steps in RLK signaling initiation.

Genome-Wide Re-Identification and Analysis of CrRLK1Ls in Tomato

The Catharanthus roseus receptor-like kinase 1-like (CrRLK1L), which is a vital member of the plant receptor-like kinase family, plays versatile roles in plant growth, development, and stress response. Although the primary screening of tomato CrRLK1Ls has been reported previously, our knowledge of these proteins is still scarce. Using the latest genomic data annotations, a genome-wide re-identification and analysis of the CrRLK1Ls in tomatoes were conducted. In this study, 24 CrRLK1L members were identified in tomatoes and researched further. Subsequent gene structures, protein domains, Western blot analyses, and subcellular localization analyses all confirmed the accuracy of the newly identified SlCrRLK1L members. Phylogenetic analyses showed that the identified SlCrRLK1L proteins had homologs in Arabidopsis. Evolutionary analysis indicated that two pairs of the SlCrRLK1L genes had predicted segmental duplication events. Expression profiling analyses demonstrated that the SlCrRLK1L genes were expressed in various tissues, and most of them were up- or down-regulated by bacteria and PAMP treatments. Together, these results will lay the foundation for elaborating the biological roles of SlCrRLK1Ls in tomato growth, development, and stress response.

Zygnematophycean algae: Possible models for cellular and evolutionary biology

Plant terrestrialization was a critical event for our planet. For the study of plant evolution, charophytes have received a great deal of attention because of their phylogenetic position. Among charophytes, the class Zygnematophyceae is the closest lineage to land plants. During sexual reproduction, they show isogamous conjugation by immotile gametes, which is characteristic of zygnematophycean algae. Here, we introduce the genera Mougeotia, Penium, and Closterium, which are representative model organisms of Zygnematophyceae in terms of chloroplast photorelocation movement, the cell wall, and sexual reproduction, respectively.

Gene coexpression analysis reveals key pathways and hub genes related to late-acting self-incompatibility in Camellia oleifera

INTRODUCTION

Self-incompatibility (SI) is an important strategy for plants to maintain abundant variation to enhance their adaptability to the environment. Camellia oleifera is one of the most important woody oil plants and is widely cultivated in China. Late acting self-incompatibility (LSI) in C. oleifera results in a relatively poor fruit yield in the natural state, and understanding of the LSI mechanism remains limited.

METHODS

To better understand the molecular expression and gene coexpression network in the LSI reaction in C. oleifera, we conducted self- and cross-pollination experiments at two different flower bud developmental stages (3-4 d before flowering and 1 d before flowering), and cytological observation, fruit setting rate (FSR) investigation and RNA-Seq analysis were performed to investigate the mechanism of the male -female interaction and identify hub genes responsible for the LSI in C. oleifera.

RESULTS

Based on the 21 ovary transcriptomes, a total of 7669 DEGs were identified after filtering out low-expression genes. Weighted gene coexpression network analysis (WGCNA) divided the DEGs into 15 modules. Genes in the blue module (1163 genes) were positively correlated with FSR, and genes in the pink module (339 genes) were negatively correlated with FSR. KEGG analysis indicated that flavonoid biosynthesis, plant MAPK signaling pathways, ubiquitin-mediated proteolysis, and plant-pathogen interaction were the crucial pathways for the LSI reaction. Fifty four transcription factors (TFs) were obtained in the two key modules, and WRKY and MYB were potentially involved in the LSI reaction in C. oleifera. Network establishment indicated that genes encoding G-type lectin S-receptor-like serine (lecRLK), isoflavone 3'-hydroxylase-like (CYP81Q32), cytochrome P450 87A3-like (CYP87A3), and probable calcium-binding protein (CML41) were the hub genes that positively responded to the LSI reaction. The other DEGs inside the two modules, including protein RALF-like 10 (RALF), F-box and pectin acetylesterase (MTERF5), might also play vital roles in the LSI reaction in C. oleifera.

DISCUSSION

Overall, our study provides a meaningful resource for gene network studies of the LSI reaction process and subsequent analyses of pollen-pistil interactions and TF roles in the LSI reaction, and it also provides new insights for exploring the mechanisms of the LSI response.

A screening of inhibitors targeting the receptor kinase FERONIA reveals small molecules that enhance plant root immunity

Receptor-like kinases (RLKs) constitute the largest receptor family involved in the regulation of plant immunity and growth, but small-molecule inhibitors that target RLKs to improve agronomic traits remain unexplored. The RLK member FERONIA (FER) negatively regulates plant resistance to certain soil-borne diseases that are difficult to control and cause huge losses in crop yields and economy. Here, we identified 33 highly effective FER kinase inhibitors from 1494 small molecules by monitoring FER autophosphorylation in vitro. Four representative inhibitors (reversine, cenisertib, staurosporine and lavendustin A) inhibited the kinase activity of FER and its homologues in several crops by targeting the conserved ATP pocket in the kinase structure. FER contributes to the physiological impact of representative inhibitors in plants. The treatment of roots with reversine, staurosporine and lavendustin A enhanced innate immunity in plant roots and thus alleviated soil-borne diseases in tobacco, tomato and rice without growth penalties. Consistently, RNA sequencing assays showed that lavendustin A and reversine exert profound impacts on immunity-related gene expression. Our results will set a new milestone in the development of the plant RLK kinase regulation theory and provide a novel strategy for the prevention and control of plant soil-borne diseases without growth penalties.

The rubber tree RALF peptide hormone and its receptor protein kinase FER implicates in rubber production

RAPID ALKALINIZATION FACTORs (RALFs), which are secreted peptides serving as extracellular signals transduced to the inside of the cell, interact with the receptor-like kinase FERONIA (FER) and participates in various biological pathways. Here, we identified 23 RALF and 2 FER genes in Hevea brasiliensis (para rubber tree), and characterized their expression patterns in different tissues, across the process of leaf development, and in response to the rubber yield-stimulating treatments of tapping and ethylene. Four Hevea latex (the cytoplasm of rubber-producing laticifers)-abundant RALF isoforms, HbRALF19, HbRALF3, HbRALF22, and HbRALF16 were listed with descending expression levels. Of the four HbRALFs, expressions of HbRALF3 were markedly regulated in an opposite way by the treatments of tapping (depression) and ethylene (stimulation). All of the four latex-abundant RALFs specifically interacted with the extracellular domain of HbFER1. Transgenic Arabidopsis plants overexpressing these HbRALFs displayed phenotypes similar to those reported for AtRALFs, such as shorter roots, smaller plant architecture, and delayed flowering. The application of HbRALF3 and HbRALF19 recombinant proteins significantly reduced the pH of Hevea latex, an important factor regulating latex metabolism. An in vitro rubber biosynthesis assay in a mixture of latex cytosol (C-serum) revealed a positive role of HbFER1 in rubber biosynthesis. Taken together, these data provide evidence for the participation of the HbRALF-FER module in rubber production.

Plant Cell Wall Integrity Perturbations and Priming for Defense

A plant cell wall is a highly complex structure consisting of networks of polysaccharides, proteins, and polyphenols that dynamically change during growth and development in various tissues. The cell wall not only acts as a physical barrier but also dynamically responds to disturbances caused by biotic and abiotic stresses. Plants have well-established surveillance mechanisms to detect any cell wall perturbations. Specific immune signaling pathways are triggered to contrast biotic or abiotic forces, including cascades dedicated to reinforcing the cell wall structure. This review summarizes the recent developments in molecular mechanisms underlying maintenance of cell wall integrity in plant-pathogen and parasitic interactions. Subjects such as the effect of altered expression of endogenous plant cell-wall-related genes or apoplastic expression of microbial cell-wall-modifying enzymes on cell wall integrity are covered. Targeted genetic modifications as a tool to study the potential of cell wall elicitors, priming of signaling pathways, and the outcome of disease resistance phenotypes are also discussed. The prime importance of understanding the intricate details and complete picture of plant immunity emerges, ultimately to engineer new strategies to improve crop productivity and sustainability.

How a single receptor-like kinase exerts diverse roles: lessons from FERONIA

FERONIA (FER) is a member of the Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) protein subfamily, which participates in reproduction, abiotic stress, biotic stress, cell growth, hormone response, and other molecular mechanisms of plants. However, the mechanism by which a single RLK is capable of mediating multiple signals and activating multiple cellular responses remains unclear. Here, we summarize research progress revealing the spatial-temporal expression of FER, along with its co-receptors and ligands determined the function of FER signaling pathway in multiple organs. The specificity of the FER signaling pathway is proposed to operate under a four-layered mechanism: (1) Spatial-temporal expression of FER, co-receptors, and ligands specify diverse functions, (2) Specific ligands or ligand combinations trigger variable FER signaling pathways, (3) Diverse co-receptors confer diverse FER perception and response modes, and (4) Unique downstream components that modify FER signaling and responses. Moreover, the regulation mechanism of the signaling pathway- appears to depend on the interaction among the ligands, RLK receptors, co-receptors, and downstream components, which may be a general mechanism of RLKs to maintain signal specificity. This review will provide a insight into understanding the specificity determination of RLKs signaling in both model and horticultural crops.

Genome-wide characterization of soybean RALF genes and their expression responses to Fusarium oxysporum

RALFs (RAPID ALKALINIZATION FACTORs) are small peptides required for plant growth, development and immunity. RALF has recently been discovered to regulate plant resistance to fungal infection. However, little is known in crops, particularly in soybean. Here, 27 RALFs were identified in the genome of Glycine max. All Glycine max RALFs (GmRALFs) and 34 Arabidopsis RALFs were classified into 12 clades via the phylogenetic analyses. Gene structures, conserved motifs, chromosome distribution and cis-elements were analyzed in this study. Furthermore, 18 GmRALFs were found in response to Fusarium oxysporum (F. oxysporum) infection in soybean and to have distinct expression patterns. Among them, secretory function of two GmRALFs were identified, and three GmRALFs were detected to interact with FERONIA in Glycine max (GmFERONIA, GmFER). Our current study systematically identified and characterized GmRALFs in the soybean genome, laying a groundwork for further functional analyses and soybean breeding.

THESEUS1 activation by stress: isomerization and peptide perception?

Plant cell-wall perturbation upon environmental stress triggers adaptive cellular responses mediated by plasma membrane-resident sensors. We discuss a recent study by Bacete et al. showing that THESEUS1 (THE1) regulates plant cell adaptive responses to cell-wall disruption and update the working model for THE1 activation.

Regulation of pattern-triggered immunity and growth by phytocytokines

Endogenous signalling peptides play diverse roles during plant growth, development and stress responses. Research in recent years has unravelled peptides with previously known growth-regulatory function as immune-modulatory agents that fine-tune pattern-triggered immunity (PTI). Moreover, peptides that are long known as endogenous danger signals were recently implicated in growth and development. In analogy to metazoan systems these peptides are referred to as phytocytokines. In this review we will highlight recent progress made on our understanding of phytocytokines simultaneously regulating growth and PTI which shows the complex interplay of peptide signalling pathways regulating multiple aspects of a plant's life.

Genome-Wide Analysis of CqCrRLK1L and CqRALF Gene Families in Chenopodium quinoa and Their Roles in Salt Stress Response

Chenopodium quinoa is a halophyte with exceptional nutritional qualities, and therefore it is potentially an ideal crop to grow in saline soils, not only addressing the problem of land salinization, but also providing nutrient food for the health of humans. Currently, the molecular mechanisms underlying salt tolerance in quinoa are still largely unknown. In Arabidopsis thaliana, Catharanthus roseus receptor-like kinase (CrRLK1Ls) FERONIA (FER) and its ligands rapid alkalinization factors (RALFs) have been reported that participate in the regulation of salt tolerance. Here, we performed a genome-wide analysis and identified 26 CqCrRLK1L and 18 CqRALF family genes in quinoa genome. Transcriptomic profiling of the leaf, root, stamen, and pistil tissues of quinoa reveals that different CqCrRLK1L and CqRALF genes exhibit tissue-specific expression patterns, which is consistent with that observed in other plant species. RNA-seq data show that three CqCrRLK1L genes are highly up-regulated after salt treatment, suggesting that some CqCrRLK1L family genes are transcriptionally responsive to salt stress in quinoa. Biochemical study indicates that CqRALF15, a paralog of Arabidopsis RALF22, is physically associated with CrRLK1L proteins CqFER and AtFER. CqRALF15 and AtRALF22 are functionally conserved in inducing the internalization of AtFER and in triggering root growth inhibition in both quinoa and Arabidopsis. Moreover, overexpression of CqRALF15 in Arabidopsis results in enhanced leaf bleaching under salt stress, indicating that CqRALF15 is involved in salt stress response. Together, our study characterizes CqCrRLK1L and CqRALF family genes in quinoa at genomic, transcriptional, and protein levels, and provides evidence to support their roles in salt stress response.

The RALF1-FERONIA complex interacts with and activates TOR signaling in response to low nutrients

Target of rapamycin (TOR) kinase is an evolutionarily conserved major regulator of nutrient metabolism and organismal growth in eukaryotes. In plants, nutrients are remobilized and reallocated between shoots and roots under low-nutrient conditions, and nitrogen and nitrogen-related nutrients (e.g., amino acids) are key upstream signals leading to TOR activation in shoots under low-nutrient conditions. However, how these forms of nitrogen can be sensed to activate TOR in plants is still poorly understood. Here we report that the Arabidopsis receptor kinase FERONIA (FER) interacts with the TOR pathway to regulate nutrient (nitrogen and amino acid) signaling under low-nutrient conditions and exerts similar metabolic effects in response to nitrogen deficiency. We found that FER and its partner, RPM1-induced protein kinase (RIPK), interact with the TOR/RAPTOR complex to positively modulate TOR signaling activity. During this process, the receptor complex FER/RIPK phosphorylates the TOR complex component RAPTOR1B. The RALF1 peptide, a ligand of the FER/RIPK receptor complex, increases TOR activation in the young leaf by enhancing FER-TOR interactions, leading to promotion of true leaf growth in Arabidopsis under low-nutrient conditions. Furthermore, we showed that specific amino acids (e.g., Gln, Asp, and Gly) promote true leaf growth under nitrogen-deficient conditions via the FER-TOR axis. Collectively, our study reveals a mechanism by which the RALF1-FER pathway activates TOR in the plant adaptive response to low nutrients and suggests that plants prioritize nutritional stress response over RALF1-mediated inhibition of cell growth under low-nutrient conditions.

Genome-Wide Identification of Rapid Alkalinization Factor Family in Brassica napus and Functional Analysis of BnRALF10 in Immunity to Sclerotinia sclerotiorum

Rapid alkalinization factors (RALFs) were recently reported to be important players in plant immunity. Nevertheless, the signaling underlying RALF-triggered immunity in crop species against necrotrophic pathogens remains largely unknown. In this study, RALF family in the important oil crop oilseed rape (Brassica napus) was identified and functions of BnRALF10 in immunity against the devastating necrotrophic pathogen Sclerotinia sclerotiorum as well as the signaling underlying this immunity were revealed. The oilseed rape genome carried 61 RALFs, half of them were atypical, containing a less conserved YISY motif and lacking a RRXL motif or a pair of cysteines. Family-wide gene expression analyses demonstrated that patterns of expression in response to S. sclerotiorum infection and DAMP and PAMP treatments were generally RALF- and stimulus-specific. Most significantly responsive BnRALF genes were expressionally up-regulated by S. sclerotiorum, while in contrast, more BnRALF genes were down-regulated by BnPep5 and SsNLP1. These results indicate that members of BnRALF family are likely differentially involved in plant immunity. Functional analyses revealed that BnRALF10 provoked diverse immune responses in oilseed rape and stimulated resistance to S. sclerotiorum. These data support BnRALF10 to function as a DAMP to play a positive role in plant immunity. BnRALF10 interacted with BnFER. Silencing of BnFER decreased BnRALF10-induced reactive oxygen species (ROS) production and compromised rape resistance to S. sclerotiorum. These results back BnFER to be a receptor of BnRALF10. Furthermore, quantitative proteomic analysis identified dozens of BnRALF10-elicited defense (RED) proteins, which respond to BnRALF10 in protein abundance and play a role in defense. Our results revealed that BnRALF10 modulated the abundance of RED proteins to fine tune plant immunity. Collectively, our results provided some insights into the functions of oilseed rape RALFs and the signaling underlying BnRALF-triggered immunity.

FERONIA Receptor Kinase Integrates with Hormone Signaling to Regulate Plant Growth, Development, and Responses to Environmental Stimuli

Plant hormones are critical chemicals that participate in almost all aspects of plant life by triggering cellular response cascades. FERONIA is one of the most well studied members in the subfamily of Catharanthus roseus receptor-like kinase1-like (CrRLK1Ls) hormones. It has been proved to be involved in many different processes with the discovery of its ligands, interacting partners, and downstream signaling components. A growing body of evidence shows that FERONIA serves as a hub to integrate inter- and intracellular signals in response to internal and external cues. Here, we summarize the recent advances of FERONIA in regulating plant growth, development, and immunity through interactions with multiple plant hormone signaling pathways.

Genome wide study of cysteine rich receptor like proteins in Gossypium sp

Cysteine-rich receptor-like-kinases (CRKs), a transmembrane subfamily of receptor-like kinase, play crucial roles in plant adaptation. As such cotton is the major source of fiber for the textile industry, but environmental stresses are limiting its growth and production. Here, we have performed a deep computational analysis of CRKs in five Gossypium species, including G. arboreum (60 genes), G. raimondii (74 genes), G. herbaceum (65 genes), G. hirsutum (118 genes), and G. barbadense (120 genes). All identified CRKs were classified into 11 major classes and 43 subclasses with the finding of several novel CRK-associated domains including ALMT, FUSC_2, Cript, FYVE, and Pkinase. Of these, DUF26_DUF26_Pkinase_Tyr was common and had elevated expression under different biotic and abiotic stresses. Moreover, the 35 land plants comparison identified several new CRKs domain-architectures. Likewise, several SNPs and InDels were observed in CLCuD resistant G. hirsutum. The miRNA target side prediction and their expression profiling in different tissues predicted miR172 as a major CRK regulating miR. The expression profiling of CRKs identified multiple clusters with co-expression under certain stress conditions. The expression analysis under CLCuD highlighted the role of GhCRK057, GhCRK059, GhCRK058, and GhCRK081 in resistant accession. Overall, these results provided primary data for future potential functional analysis as well as a reference study for other agronomically important crops.

THESEUS1 modulates cell wall stiffness and abscisic acid production in Arabidopsis thaliana

Plants need to constantly adapt to a changing environment. Adaptation includes responses to biotic and abiotic stress. Key elements determining the response to abiotic stress are the cell walls surrounding all plant cells and the phytohormone abscisic acid, which influence turgor pressure in plants. Turgor pressure in plant cells is much higher than in animal cells and a key driver of plant growth and development. Here, we investigate the mechanism regulating cell wall stiffness and coordinating changes in stiffness and turgor with abscisic acid production. We characterize key elements of the mechanism and dissect its mode of action. This knowledge will enable us to improve plant resistance to drought stress, which is necessary due to our changing environment.

Plant cells can be distinguished from animal cells by their cell walls and high-turgor pressure. Although changes in turgor and the stiffness of cell walls seem coordinated, we know little about the mechanism responsible for coordination. Evidence has accumulated that plants, like yeast, have a dedicated cell wall integrity maintenance mechanism. It monitors the functional integrity of the wall and maintains integrity through adaptive responses induced by cell wall damage arising during growth, development, and interactions with the environment. These adaptive responses include osmosensitive induction of phytohormone production, defense responses, as well as changes in cell wall composition and structure. Here, we investigate how the cell wall integrity maintenance mechanism coordinates changes in cell wall stiffness and turgor in Arabidopsis thaliana. We show that the production of abscisic acid (ABA), the phytohormone-modulating turgor pressure, and responses to drought depend on the presence of a functional cell wall. We find that the cell wall integrity sensor THESEUS1 modulates mechanical properties of walls, turgor loss point, ABA biosynthesis, and ABA-controlled processes. We identify RECEPTOR-LIKE PROTEIN 12 as a component of cell wall integrity maintenance–controlling, cell wall damage–induced jasmonic acid (JA) production. We propose that THE1 is responsible for coordinating changes in turgor pressure and cell wall stiffness.

Mechanisms of plant cell wall surveillance in response to pathogens, cell wall-derived ligands and the effect of expansins to infection resistance or susceptibility

Cell wall integrity is tightly regulated and maintained given that non-physiological modification of cell walls could render plants vulnerable to biotic and/or abiotic stresses. Expansins are plant cell wall-modifying proteins active during many developmental and physiological processes, but they can also be produced by bacteria and fungi during interaction with plant hosts. Cell wall alteration brought about by ectopic expression, overexpression, or exogenous addition of expansins from either eukaryote or prokaryote origin can in some instances provide resistance to pathogens, while in other cases plants become more susceptible to infection. In these circumstances altered cell wall mechanical properties might be directly responsible for pathogen resistance or susceptibility outcomes. Simultaneously, through membrane receptors for enzymatically released cell wall fragments or by sensing modified cell wall barrier properties, plants trigger intracellular signaling cascades inducing defense responses and reinforcement of the cell wall, contributing to various infection phenotypes, in which expansins might also be involved. Here, we review the plant immune response activated by cell wall surveillance mechanisms, cell wall fragments identified as responsible for immune responses, and expansin's roles in resistance and susceptibility of plants to pathogen attack.

Phytocytokines function as immunological modulators of plant immunity

Plant plasma membrane-resident immune receptors regulate plant immunity by recognizing microbe-associated molecular patterns (MAMPs), damage-associated molecular patterns (DAMPs), and phytocytokines. Phytocytokines are plant endogenous peptides, which are usually produced in the cytosol and released into the apoplast when plant encounters pathogen infections. Phytocytokines regulate plant immunity through activating an overlapping signaling pathway with MAMPs/DAMPs with some unique features. Here, we highlight the current understanding of phytocytokine production, perception and functions in plant immunity, and discuss how plants and pathogens manipulate phytocytokine signaling for their own benefits during the plant-pathogen warfare.