The LRXs-RALFs-FER module controls plant growth and salt stress responses by modulating multiple plant hormones

Transcriptome Analysis Reveals the Biocontrol Mechanism of Endophytic Bacterium AM201, Rhodococcus sp., against Root Rot Disease of Atractylodes macrocephala

Atractylodes macrocephala Koidz (AMK) is a perennial herb from the plant family Asteraceae (formerly Compositae). This herb is mainly distributed in mountainous wetlands in Zhejiang, Sichuan, Yunnan, and Hunan provinces of China. Its medicinal production and quality, however, are severely impacted by root rot disease. In our previous study, endophytic bacterium designated AM201 exerted a high biocontrol effect on the root rot disease of AMK. However, the molecular mechanisms underlying this effect remain unclear. In this study, the identity of strain AM201 as Rhodococcus sp. was determined through analysis of its morphology, physiological and biochemical characteristics, as well as 16S rDNA sequencing. Subsequently, we performed transcriptome sequencing and bioinformatics analysis to compare and analyze the transcriptome profiles of root tissues from two groups: AM201 (AMK seedlings inoculated with Fusarium solani [FS] and AM201) and FS (AMK seedlings inoculated with FS alone). We also conducted morphological, physiological, biochemical, and molecular identification analyses for the AM201 strain. We obtained 1,560 differentially expressed genes, including 187 upregulated genes and 1,373 downregulated genes. We screened six key genes (GOLS2, CIPK25, ABI2, egID, PG1, and pgxB) involved in the resistance of AM201 against AMK root rot disease. These genes play a critical role in reactive oxygen species (ROS) clearance, Ca2+ signal transduction, abscisic acid signal inhibition, plant root growth, and plant cell wall defense. The strain AM201 was identified as Rhodococcus sp. based on its morphological characteristics, physiological and biochemical properties, and 16S rDNA sequencing results. The findings of this study could enable to prevent and control root rot disease in AMK and could offer theoretical guidance for the agricultural production of other medicinal herbs.

Small but powerful: RALF peptides in plant adaptive and developmental responses

Plants live in a highly dynamic environment and require to rapidly respond to a plethora of environmental stimuli, so that to maintain their optimal growth and development. A small plant peptide, rapid alkalization factor (RALF), can rapidly increase the pH value of the extracellular matrix in plant cells. RALFs always function with its corresponding receptors. Mechanistically, effective amount of RALF is induced and released at the critical period of plant growth and development or under different external environmental factors. Recent studies also highlighted the role of RALF peptides as important regulators in plant intercellular communications, as well as their operation in signal perception and as ligands for different receptor kinases on the surface of the plasma membrane, to integrate various environmental cues. In this context, understanding the fine-print of above processes may be essential to solve the problems of crop adaptation to various harsh environments under current climate trends scenarios, by genetic means. This paper summarizes the current knowledge about the structure and diversity of RALF peptides and their roles in plant development and response to stresses, highlighting unanswered questions and problems to be solved.

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.

Research Progress of Small Plant Peptides on the Regulation of Plant Growth, Development, and Abiotic Stress

Small peptides in plants are typically characterized as being shorter than 120 amino acids, with their biologically active variants comprising fewer than 20 amino acids. These peptides are instrumental in regulating plant growth, development, and physiological processes, even at minimal concentrations. They play a critical role in long-distance signal transduction within plants and act as primary responders to a range of stress conditions, including salinity, alkalinity, drought, high temperatures, and cold. This review highlights the crucial roles of various small peptides in plant growth and development, plant resistance to abiotic stress, and their involvement in long-distance transport. Furthermore, it elaborates their roles in the regulation of plant hormone biosynthesis. Special emphasis is given to the functions and mechanisms of small peptides in plants responding to abiotic stress conditions, aiming to provide valuable insights for researchers working on the comprehensive study and practical application of small peptides.

Overexpression of the potato VQ31 enhances salt tolerance in Arabidopsis

Plant-specific VQ proteins have crucial functions in the regulation of plant growth and development, as well as in plant abiotic stress responses. Their roles have been well established in the model plant Arabidopsis thaliana; however, the functions of the potato VQ proteins have not been adequately investigated. The VQ protein core region contains a short FxxhVQxhTG amino acid motif sequence. In this study, the VQ31 protein from potato was cloned and functionally characterized. The complete open reading frame (ORF) size of StVQ31 is 672 bp, encoding 223 amino acids. Subcellular localization analysis revealed that StVQ31 is located in the nucleus. Transgenic Arabidopsis plants overexpressing StVQ31 exhibited enhanced salt tolerance compared to wild-type (WT) plants, as evidenced by increased root length, germination rate, and chlorophyll content under salinity stress. The increased tolerance of transgenic plants was associated with increased osmotic potential (proline and soluble sugars), decreased MDA accumulation, decreased total protein content, and improved membrane integrity. These results implied that StVQ31 overexpression enhanced the osmotic potential of the plants to maintain normal cell growth. Compared to the WT, the transgenic plants exhibited a notable increase in antioxidant enzyme activities, reducing cell membrane damage. Furthermore, the real-time fluorescence quantitative PCR analysis demonstrated that StVQ31 regulated the expression of genes associated with the response to salt stress, including ERD, LEA4-5, At2g38905, and AtNCED3. These findings suggest that StVQ31 significantly impacts osmotic and antioxidant cellular homeostasis, thereby enhancing salt tolerance.

Small but mighty: Peptides regulating abiotic stress responses in plants

Throughout evolution, plants have developed strategies to confront and alleviate the detrimental impacts of abiotic stresses on their growth and development. The combat strategies involve intricate molecular networks and a spectrum of early and late stress-responsive pathways. Plant peptides, consisting of fewer than 100 amino acid residues, are at the forefront of these responses, serving as pivotal signalling molecules. These peptides, with roles similar to phytohormones, intricately regulate plant growth, development and facilitate essential cell-to-cell communications. Numerous studies underscore the significant role of these small peptides in coordinating diverse signalling events triggered by environmental challenges. Originating from the proteolytic processing of larger protein precursors or directly translated from small open reading frames, including microRNA (miRNA) encoded peptides from primary miRNA, these peptides exert their biological functions through binding with membrane-embedded receptor-like kinases. This interaction initiates downstream cellular signalling cascades, often involving major phytohormones or reactive oxygen species-mediated mechanisms. Despite these advances, the precise modes of action for numerous other small peptides remain to be fully elucidated. In this review, we delve into the dynamics of stress physiology, mainly focusing on the roles of major small signalling peptides, shedding light on their significance in the face of changing environmental conditions.

Designing salt stress-resilient crops: Current progress and future challenges

Excess soil salinity affects large regions of land and is a major hindrance to crop production worldwide. Therefore, understanding the molecular mechanisms of plant salt tolerance has scientific importance and practical significance. In recent decades, studies have characterized hundreds of genes associated with plant responses to salt stress in different plant species. These studies have substantially advanced our molecular and genetic understanding of salt tolerance in plants and have introduced an era of molecular design breeding of salt-tolerant crops. This review summarizes our current knowledge of plant salt tolerance, emphasizing advances in elucidating the molecular mechanisms of osmotic stress tolerance, salt-ion transport and compartmentalization, oxidative stress tolerance, alkaline stress tolerance, and the trade-off between growth and salt tolerance. We also examine recent advances in understanding natural variation in the salt tolerance of crops and discuss possible strategies and challenges for designing salt stress-resilient crops. We focus on the model plant Arabidopsis (Arabidopsis thaliana) and the four most-studied crops: rice (Oryza sativa), wheat (Triticum aestivum), maize (Zea mays), and soybean (Glycine max).

How plants sense and respond to osmotic stress

Drought is one of the most serious abiotic stresses to land plants. Plants sense and respond to drought stress to survive under water deficiency. Scientists have studied how plants sense drought stress, or osmotic stress caused by drought, ever since Charles Darwin, and gradually obtained clues about osmotic stress sensing and signaling in plants. Osmotic stress is a physical stimulus that triggers many physiological changes at the cellular level, including changes in turgor, cell wall stiffness and integrity, membrane tension, and cell fluid volume, and plants may sense some of these stimuli and trigger downstream responses. In this review, we emphasized water potential and movements in organisms, compared putative signal inputs in cell wall-containing and cell wall-free organisms, prospected how plants sense changes in turgor, membrane tension, and cell fluid volume under osmotic stress according to advances in plants, animals, yeasts, and bacteria, summarized multilevel biochemical and physiological signal outputs, such as plasma membrane nanodomain formation, membrane water permeability, root hydrotropism, root halotropism, Casparian strip and suberin lamellae, and finally proposed a hypothesis that osmotic stress responses are likely to be a cocktail of signaling mediated by multiple osmosensors. We also discussed the core scientific questions, provided perspective about the future directions in this field, and highlighted the importance of robust and smart root systems and efficient source-sink allocations for generating future high-yield stress-resistant crops and plants.

Rapid alkalinization factor 22 has a structural and signalling role in root hair cell wall assembly

Pressurized cells with strong walls make up the hydrostatic skeleton of plants. Assembly and expansion of such stressed walls depend on a family of secreted RAPID ALKALINIZATION FACTOR (RALF) peptides, which bind both a membrane receptor complex and wall-localized LEUCINE-RICH REPEAT EXTENSIN (LRXs) in a mutually exclusive way. Here we show that, in root hairs, the RALF22 peptide has a dual structural and signalling role in cell expansion. Together with LRX1, it directs the compaction of charged pectin polymers at the root hair tip into periodic circumferential rings. Free RALF22 induces the formation of a complex with LORELEI-LIKE-GPI-ANCHORED PROTEIN 1 and FERONIA, triggering adaptive cellular responses. These findings show how a peptide simultaneously functions as a structural component organizing cell wall architecture and as a feedback signalling molecule that regulates this process depending on its interaction partners. This mechanism may also underlie wall assembly and expansion in other plant cell types.

Insights into plant salt stress signaling and tolerance

Soil salinization is an essential environmental stressor, threatening agricultural yield and ecological security worldwide. Saline soils accumulate excessive soluble salts which are detrimental to most plants by limiting plant growth and productivity. It is of great necessity for plants to efficiently deal with the adverse effects caused by salt stress for survival and successful reproduction. Multiple determinants of salt tolerance have been identified in plants, and the cellular and physiological mechanisms of plant salt response and adaption have been intensely characterized. Plants respond to salt stress signals and rapidly initiate signaling pathways to re-establish cellular homeostasis with adjusted growth and cellular metabolism. This review summarizes the advances in salt stress perception, signaling, and response in plants. A better understanding of plant salt resistance will contribute to improving crop performance under saline conditions using multiple engineering approaches. The rhizosphere microbiome-mediated plant salt tolerance as well as chemical priming for enhanced plant salt resistance are also discussed in this review.

Phytochrome B phosphorylation expanded: site-specific kinases are identified

The phytochrome B (phyB) photoreceptor is a key participant in red and far-red light sensing, playing a dominant role in many developmental and growth responses throughout the whole life of plants. Accordingly, phyB governs diverse signaling pathways, and although our knowledge about these pathways is constantly expanding, our view about their fine-tuning is still rudimentary. Phosphorylation of phyB is one of the relevant regulatory mechanisms, and - despite the expansion of the available methodology - it is still not easy to examine. Phosphorylated phytochromes have been detected using various techniques for decades, but the first phosphorylated phyB residues were only identified in 2013. Since then, concentrated attention has been turned toward the functional role of post-translational modifications in phyB signaling. Very recently in 2023, the first kinases that phosphorylate phyB were identified. These discoveries opened up new research avenues, especially by connecting diverse environmental impacts to light signaling and helping to explain some long-term unsolved problems such as the co-action of Ca2+ and phyB signaling. This review summarizes our recent views about the roles of the identified phosphorylated phyB residues, what we know about the enzymes that modulate the phospho-state of phyB, and how these recent discoveries impact future investigations.

ROS interplay between plant growth and stress biology: Challenges and future perspectives

In plants, reactive oxygen species (ROS) have emerged as a multifunctional signaling molecules that modulate diverse stress and growth responses. Earlier studies on ROS in plants primarily focused on its toxicity and ROS-scavenging processes, but recent findings are offering new insights on its role in signal perception and transduction. Further, the interaction of cell wall receptors, calcium channels, HATPase, protein kinases, and hormones with NADPH oxidases (respiratory burst oxidase homologues (RBOHs), provides concrete evidence that ROS regulates major signaling cascades in different cellular compartments related to stress and growth responses. However, at the molecular level there are many knowledge gaps regarding how these players influence ROS signaling and how ROS regulate them during growth and stress events. Furthermore, little is known about how plant sensors or receptors detect ROS under various environmental stresses and induce subsequent signaling cascades. In light of this, we provided an update on the role of ROS signaling in plant growth and stress biology. First, we focused on ROS signaling, its production and regulation by cell wall receptor like kinases. Next, we discussed the interplay between ROS, calcium and hormones, which forms a major signaling trio regulatory network of signal perception and transduction. We also provided an overview on ROS and nitric oxide (NO) crosstalk. Furthermore, we emphasized the function of ROS signaling in biotic, abiotic and mechanical stresses, as well as in plant growth and development. Finally, we conclude by highlighting challenges and future perspectives of ROS signaling in plants that warrants future investigation.

Small secreted peptides (SSPs) in tomato and their potential roles in drought stress response

Tomato (Solanum lycopersicum) is one of the most important vegetable crops in the world and abiotic stresses often cause serious problems in tomato production. It is thus important to identify new regulators in stress response and to devise new approaches to promote stress tolerance in tomato. Previous studies have shown that small secreted peptides (SSPs) are important signal molecules regulating plant growth and stress response by mediating intercellular communication. However, little is known about tomato SSPs, especially their roles in responding to abiotic stresses. Here we report the identification of 1,050 putative SSPs in the tomato genome, 557 of which were classified into 38 known SSP families based on their conserved domains. GO and transcriptome analyses revealed that a large proportion of SlSSPs might be involved in abiotic stress response. Further analysis indicated that stress response related cis-elements were present on the SlCEP promotors and a number of SlCEPs were significantly upregulated by drought treatments. Among the drought-inducible SlCEPs, SlCEP10 and SlCEP11b were selected for further analysis via exogenous application of synthetic peptides. The results showed that treatments with both SlCEP10 and SlCEP11b peptides enhanced tomato drought stress tolerance, indicating the potential roles of SlSSPs in abiotic stress response.

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.

(-)-Loliolide is a general signal of plant stress that activates jasmonate-related responses

The production of defensive metabolites in plants can be induced by signaling chemicals released by neighboring plants. Induction is mainly known from volatile aboveground signals, with belowground signals and their underlying mechanisms largely unknown. We demonstrate that (-)-loliolide triggers defensive metabolite responses to competitors, herbivores, and pathogens in seven plant species. We further explore the transcriptional responses of defensive pathways to verify the signaling role of (-)-loliolide in wheat and rice models with well-known defensive metabolites and gene systems. In response to biotic and abiotic stressors, (-)-loliolide is produced and secreted by roots. This, in turn, induces the production of defensive compounds including phenolic acids, flavonoids, terpenoids, alkaloids, benzoxazinoids, and cyanogenic glycosides, regardless of plant species. (-)-Loliolide also triggers the expression of defense-related genes, accompanied by an increase in the concentration of jasmonic acid and hydrogen peroxide (H₂ O₂ ). Transcriptome profiling and inhibitor incubation indicate that (-)-loliolide-induced defense responses are regulated through pathways mediated by jasmonic acid, H₂ O₂ , and Ca ²⁺ . These findings argue that (-)-loliolide functions as a common belowground signal mediating chemical defense in plants. Such perception-dependent plant chemical defenses will yield critical insights into belowground signaling interactions.

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.

OsPEX1, an extensin-like protein, negatively regulates root growth in a gibberellin-mediated manner in rice

Leucine-rich repeat extensins (LRXs) are required for plant growth and development through affecting cell growth and cell wall formation. LRX gene family can be classified into two categories: predominantly vegetative-expressed LRX and reproductive-expressed PEX. In contrast to the tissue specificity of Arabidopsis PEX genes in reproductive organs, rice OsPEX1 is also highly expressed in roots in addition to reproductive tissue. However, whether and how OsPEX1 affects root growth is unclear. Here, we found that overexpression of OsPEX1 retarded root growth by reducing cell elongation likely caused by an increase of lignin deposition, whereas knockdown of OsPEX1 had an opposite effect on root growth, indicating that OsPEX1 negatively regulated root growth in rice. Further investigation uncovered the existence of a feedback loop between OsPEX1 expression level and GA biosynthesis for proper root growth. This was supported by the facts that exogenous GA₃ application downregulated transcript levels of OsPEX1 and lignin-related genes and rescued the root developmental defects of the OsPEX1 overexpression mutant, whereas OsPEX1 overexpression reduced GA level and the expression of GA biosynthesis genes. Moreover, OsPEX1 and GA showed antagonistic action on the lignin biosynthesis in root. OsPEX1 overexpression upregulated transcript levels of lignin-related genes, whereas exogenous GA₃ application downregulated their expression. Taken together, this study reveals a possible molecular pathway of OsPEX1mediated regulation of root growth through coordinate modulation of lignin deposition via a negative feedback regulation between OsPEX1 expression and GA biosynthesis.

Overexpression of the FERONIA receptor kinase MdMRLK2 enhances apple cold tolerance

Cold is one of the main abiotic stresses in temperate fruit crops, affecting the yield and fruit quality of apple in China and European countries. The plant receptor-like kinase FERONIA is widely reported to be involved in abiotic stresses. However, its function in apple cold resistance remains unknown. Modification of cell wall components and accumulation of soluble sugars and amino acids are important strategies by which plants cope with cold. In this study, expression of the apple FERONIA receptor-like kinase gene MdMRLK2 was rapidly induced by cold. Apple plants overexpressing MdMRLK2 (35S:MdMRLK2) showed enhanced cold resistance relative to the wild type. Under cold conditions, 35S:MdMRLK2 apple plants had higher amounts of water insoluble pectin, lignin, cellulose, and hemicellulose, which may have resulted from reduced activities of polygalacturonase, pectinate lyase, pectinesterase, and cellulase. More soluble sugars and free amino acids and less photosystem damage were also observed in 35S:MdMRLK2 apple plants. Intriguingly, MdMRLK2 interacted with the transcription factor MdMYBPA1 and promoted its binding to MdANS and MdUFGT promoters, leading to more anthocyanin biosynthesis, particularly under cold conditions. These findings complemented the function of apple FERONIA MdMRLK2 responding to cold resistance.

FERONIA coordinates plant growth and salt tolerance via the phosphorylation of phyB

Phosphorylation modification is required for the modulation of phytochrome B (phyB) thermal reversion, but the kinase(s) that phosphorylate(s) phyB and the biological significance of the phosphorylation are still unknown. Here we report that FERONIA (FER) phosphorylates phyB to regulate plant growth and salt tolerance, and the phosphorylation not only regulates dark-triggered photobody dissociation but also modulates phyB protein abundance in the nucleus. Further analysis indicates that phosphorylation of phyB by FER is sufficient to accelerate the conversion of phyB from the active form (Pfr) to the inactive form (Pr). Under salt stress, FER kinase activity is inhibited, leading to delayed photobody dissociation and increased phyB protein abundance in the nucleus. Our data also show that phyB mutation or overexpression of PIF5 attenuates growth inhibition and promotes plant survival under salt stress. Together, our study not only reveals a kinase that controls phyB turnover via a signature of phosphorylation, but also provides mechanistic insights into the role of the FER-phyB module in coordinating plant growth and stress tolerance.

Natural variations of chlorophyll fluorescence and ion transporter genes influenced the differential response of japonica rice germplasm with different salt tolerances

Soil salinity seriously restricts rice growth, development, and production globally. Chlorophyll fluorescence and ion content reflect the level of injury and resistance of rice under salt stress. To understand the differences in the response mechanisms of japonica rice with varying degrees of salt tolerance, we analyzed the chlorophyll fluorescence characteristics and ion homeostasis of 12 japonica rice germplasm accessions by comprehensive evaluation of phenotype, haplotype, and expression of salt tolerance-related genes. The results revealed that salt-sensitive accessions were rapidly affected by the damage due to salinity. Salt tolerance score (STS) and relative chlorophyll relative content (RSPAD) were extremely significantly reduced (p<0.01), and chlorophyll fluorescence and ion homeostasis were influenced by various degrees under salt stress. The STS, RSPAD, and five chlorophyll fluorescence parameters of salt-tolerant accessions (STA) were significantly higher than that of salt-sensitive accessions (SSA). Principal component analysis (PCA) with 13 indices suggested three principal components (PCs), with a cumulative contribution rate of 90.254%, which were used to screen Huangluo (typical salt-tolerant germplasm) and Shanfuliya (typical salt-sensitive germplasm) based on the comprehensive evaluation D-value (D_CI ). The expression characteristics of chlorophyll fluorescence genes (OsABCI7 and OsHCF222) and ion transporter protein genes (OsHKT1;5, OsHKT2;1, OsHAK21, OsAKT2, OsNHX1, and OsSOS1) were analyzed. The expressions of these genes were higher in Huangluo than in Shanfuliya under salt stress. Haplotype analysis revealed four key variations associated with salt tolerance, including an SNP (+1605 bp) within OsABCI7 exon, an SSR (-1231 bp) within OsHAK21 promoter, an indel site at OsNHX1 promoter (-822 bp), and an SNP (-1866 bp) within OsAKT2 promoter. Variation in OsABCI7 protein structure and differential expression of these three ion-transporter genes may contribute to the differential response of japonica rice to salt stress.

The regulation of plant cell wall organisation under salt stress

Plant cell wall biosynthesis is a complex and tightly regulated process. The composition and the structure of the cell wall should have a certain level of plasticity to ensure dynamic changes upon encountering environmental stresses or to fulfil the demand of the rapidly growing cells. The status of the cell wall is constantly monitored to facilitate optimal growth through the activation of appropriate stress response mechanisms. Salt stress can severely damage plant cell walls and disrupt the normal growth and development of plants, greatly reducing productivity and yield. Plants respond to salt stress and cope with the resulting damage by altering the synthesis and deposition of the main cell wall components to prevent water loss and decrease the transport of surplus ions into the plant. Such cell wall modifications affect biosynthesis and deposition of the main cell wall components: cellulose, pectins, hemicelluloses, lignin, and suberin. In this review, we highlight the roles of cell wall components in salt stress tolerance and the regulatory mechanisms underlying their maintenance under salt stress conditions.

Temporal transcriptome analysis reveals several key pathways involve in cadmium stress response in Nicotiana tabacum L

Tobacco has a strong cadmium (Cd) enrichment capacity, meaning that it can absorb large quantities from the environment, but too much Cd will cause damage to the plant. It is not yet clear how the plant can dynamically respond to Cd stress. Here, we performed a temporal transcriptome analysis of tobacco roots under Cd treatment from 0 to 48 h. The number of differentially expressed genes (DEGs) was found to change significantly at 3 h of Cd treatment, which we used to define the early and middle stages of the Cd stress response. The gene ontology (GO) term analysis indicates that genes related to photosynthesis and fatty acid synthesis were enriched during the early phases of the stress response, and in the middle phase biological process related to metal ion transport, DNA damage repair, and metabolism were enriched. It was also found that plants use precursor mRNA (pre-mRNA) processes to first resist Cd stress, and with the increasing of Cd treatment time, the overlapped genes number of DEGs and DAS increased, suggesting the transcriptional levels and post-transcriptional level might influence each other. This study allowed us to better understand how plants dynamically respond to cadmium stress at the transcriptional and post-transcriptional levels and provided a reference for the screening of Cd-tolerant genes in the future.

Discovery of 2,6-Dihalopurines as Stomata Opening Inhibitors: Implication of an LRX-Mediated H+-ATPase Phosphorylation Pathway

Stomata are pores in the leaf epidermis of plants and their opening and closing regulate gas exchange and water transpiration. Stomatal movements play key roles in both plant growth and stress responses. In recent years, small molecules regulating stomatal movements have been used as a powerful tool in mechanistic studies, as well as key players for agricultural applications. Therefore, the development of new molecules regulating stomatal movement and the elucidation of their mechanisms have attracted much attention. We herein describe the discovery of 2,6-dihalopurines, AUs, as a new stomatal opening inhibitor, and their mechanistic study. Based on biological assays, AUs may involve in the pathway related with plasma membrane H⁺-ATPase phosphorylation. In addition, we identified leucine-rich repeat extensin proteins (LRXs), LRX3, LRX4 and LRX5 as well as RALF, as target protein candidates of AUs by affinity based pull down assay and molecular dynamics simulation. The mechanism of stomatal movement related with the LRXs-RALF is an unexplored pathway, and therefore further studies may lead to the discovery of new signaling pathways and regulatory factors in the stomatal movement.

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.

RALF peptides modulate immune response in the moss Physcomitrium patens

BACKGROUND

RAPID ALKALINIZATION FACTOR (RALFs) are cysteine-rich peptides that regulate multiple physiological processes in plants. This peptide family has considerably expanded during land plant evolution, but the role of ancient RALFs in modulating stress responses is unknown.Results: Here, we used the moss Physcomitrium patens as a model to gain insight into the role of RALF peptides in the coordination of plant growth and stress response in non-vascular plants. The quantitative proteomic analysis revealed concerted downregulation of M6 metalloprotease and some membrane proteins, including those involved in stress response, in PpRALF1, 2 and 3 knockout (KO) lines. The subsequent analysis revealed the role of PpRALF3 in growth regulation under abiotic and biotic stress conditions, implying the importance of RALFs in responding to various adverse conditions in bryophytes. We found that knockout of the PpRALF2 and PpRALF3 genes resulted in increased resistance to bacterial and fungal phytopathogens, Pectobacterium carotovorum and Fusarium solani, suggesting the role of these peptides in negative regulation of the immune response in P. patens. Comparing the transcriptomes of PpRALF3 KO and wild-type plants infected by F. solani showed that the regulation of genes in the phenylpropanoid pathway and those involved in cell wall modification and biogenesis was different in these two genotypes.

CONCLUSION

Thus, our study sheds light on the function of the previously uncharacterized PpRALF3 peptide and gives a clue to the ancestral functions of RALF peptides in plant stress response.

The cell biology of primary cell walls during salt stress

Salt stress simultaneously causes ionic toxicity, osmotic stress, and oxidative stress, which directly impact plant growth and development. Plants have developed numerous strategies to adapt to saline environments. Whereas some of these strategies have been investigated and exploited for crop improvement, much remains to be understood, including how salt stress is perceived by plants and how plants coordinate effective responses to the stress. It is, however, clear that the plant cell wall is the first contact point between external salt and the plant. In this context, significant advances in our understanding of halotropism, cell wall synthesis, and integrity surveillance, as well as salt-related cytoskeletal rearrangements, have been achieved. Indeed, molecular mechanisms underpinning some of these processes have recently been elucidated. In this review, we aim to provide insights into how plants respond and adapt to salt stress, with a special focus on primary cell wall biology in the model plant Arabidopsis thaliana.

Brassinolide improves the tolerance of Malus hupehensis to alkaline stress

Malus hupehensis is one of the most widely used apple rootstocks in china but is severely damaged by alkaline soil. Alkaline stress can cause more serious harmful effects on apple plants than salt stress because it also induces high pH stress except for ion toxicity, osmotic stress, and oxidative damage. Brassinolide (BL) plays important roles in plant responses to salt stress. However, its role and function mechanism in apple plants in response to alkaline stress has never been reported. This study showed that applying exogenous 0.2 mg/L BL significantly enhanced the resistance of M. hupehensis seedlings to alkaline stress. The main functional mechanisms were also explored. First, exogenous BL could decrease the rhizosphere pH and promote Ca²⁺ and Mg²⁺ absorption by regulating malic acid and citric acid contents and increasing H⁺ excretion. Second, exogenous BL could alleviate ion toxicity caused by alkaline stress through enhancing Na⁺ efflux and inhibiting K⁺ expel and vacuole compartmentalization. Last, exogenous BL could balance osmotic stress by accumulating proline and reduce oxidative damage through increasing the activities of antioxidant enzymes and antioxidants contents. This study provides an important theoretical basis for further analyzing the mechanism of exogenous BL in improving alkaline tolerance of apple plants.

Pollen-pistil interactions: It takes two to tangle but a molecular cast of many to deliver

Explosive advances have been made in the molecular understanding of pollen-pistil interactions that underlie reproductive success in flowering plants in the past three decades. Among the most notable is the discovery of pollen tube attractants [1∗,2∗]. The roles these molecules play in facilitating conspecific precedence thus promoting interspecific genetic isolation are also emerging [3-5]. Male-female interactions during the prezygotic phase and contributions from the male and female gametophytes have been comprehensively reviewed recently. Here, we focus on key advances in understanding the mechanistic underpinnings of how these interactions overcome barriers at various pollen-pistil interfaces along the pollen tube growth pathway to facilitate fertilization by desirable mates.

Combined Proteomic and Metabolomic Analysis of the Molecular Mechanism Underlying the Response to Salt Stress during Seed Germination in Barley

Salt stress is a major abiotic stress factor affecting crop production, and understanding of the response mechanisms of seed germination to salt stress can help to improve crop tolerance and yield. The differences in regulatory pathways during germination in different salt-tolerant barley seeds are not clear. Therefore, this study investigated the responses of different salt-tolerant barley seeds during germination to salt stress at the proteomic and metabolic levels. To do so, the proteomics and metabolomics of two barley seeds with different salt tolerances were comprehensively examined. Through comparative proteomic analysis, 778 differentially expressed proteins were identified, of which 335 were upregulated and 443 were downregulated. These proteins, were mainly involved in signal transduction, propanoate metabolism, phenylpropanoid biosynthesis, plant hormones and cell wall stress. In addition, a total of 187 salt-regulated metabolites were identified in this research, which were mainly related to ABC transporters, amino acid metabolism, carbohydrate metabolism and lipid metabolism; 72 were increased and 112 were decreased. Compared with salt-sensitive materials, salt-tolerant materials responded more positively to salt stress at the protein and metabolic levels. Taken together, these results suggest that salt-tolerant germplasm may enhance resilience by repairing intracellular structures, promoting lipid metabolism and increasing osmotic metabolites. These data not only provide new ideas for how seeds respond to salt stress but also provide new directions for studying the molecular mechanisms and the metabolic homeostasis of seeds in the early stages of germination under abiotic stresses.

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.

Comparative transcriptome and metabolome analysis reveal key regulatory defense networks and genes involved in enhanced salt tolerance of Actinidia (kiwifruit)

The Actinidia (kiwifruit) is an emerging fruit plant that is severely affected by salt stress in northern China. Plants have evolved several signaling network mechanisms to cope with the detrimental effects of salt stress. To date, no reported work is available on metabolic and molecular mechanisms involved in kiwifruit salt tolerance. Therefore, the present study aims to decipher intricate adaptive responses of two contrasting salt tolerance kiwifruit species Actinidia valvata [ZMH (an important genotype), hereafter referred to as R] and Actinidia deliciosa ['Hayward' (an important green-fleshed cultivar), hereafter referred to as H] under 0.4% (w/w) salt stress for time courses of 0, 12, 24, and 72 hours (hereafter refered to as h) by combined transcriptome and metabolome analysis. Data revealed that kiwifruit displayed specific enrichment of differentially expressed genes (DEGs) under salt stress. Interestingly, roots of R plants showed a differential expression pattern for up-regulated genes. The KEGG pathway analysis revealed the enrichment of DEGs related to plant hormone signal transduction, glycine metabolism, serine and threonine metabolism, glutathione metabolism, and pyruvate metabolism in the roots of R under salt stress. The WGCNA resulted in the identification of five candidate genes related to glycine betaine (GB), pyruvate, total soluble sugars (TSS), and glutathione biosynthesis in kiwifruit. An integrated study of transcriptome and metabolome identified several genes encoding metabolites involved in pyruvate metabolism. Furthermore, several genes encoding transcription factors were mainly induced in R under salt stress. Functional validation results for overexpression of a candidate gene betaine aldehyde dehydrogenase (AvBADH, R_transcript_80484) from R showed significantly improved salt tolerance in Arabidopsis thaliana (hereafter referred to as At) and Actinidia chinensis ['Hongyang' (an important red-fleshed cultivar), hereafter referred to as Ac] transgenic plants than in WT plants. All in all, salt stress tolerance in kiwifruit roots is an intricate regulatory mechanism that consists of several genes encoding specific metabolites.

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.

Cell wall integrity regulation across plant species

Plant cell walls are highly dynamic and chemically complex structures surrounding all plant cells. They provide structural support, protection from both abiotic and biotic stress as well as ensure containment of turgor. Recently evidence has accumulated that a dedicated mechanism exists in plants, which is monitoring the functional integrity of cell walls and initiates adaptive responses to maintain integrity in case it is impaired during growth, development or exposure to biotic and abiotic stress. The available evidence indicates that detection of impairment involves mechano-perception, while reactive oxygen species and phytohormone-based signaling processes play key roles in translating signals generated and regulating adaptive responses. More recently it has also become obvious that the mechanisms mediating cell wall integrity maintenance and pattern triggered immunity are interacting with each other to modulate the adaptive responses to biotic stress and cell wall integrity impairment. Here we will review initially our current knowledge regarding the mode of action of the maintenance mechanism, discuss mechanisms mediating responses to biotic stresses and highlight how both mechanisms may modulate adaptive responses. This first part will be focused on Arabidopsis thaliana since most of the relevant knowledge derives from this model organism. We will then proceed to provide perspective to what extent the relevant molecular mechanisms are conserved in other plant species and close by discussing current knowledge of the transcriptional machinery responsible for controlling the adaptive responses using selected examples.

New Advances in the Regulation of Leaf Senescence by Classical and Peptide Hormones

Leaf senescence is the last stage of leaf development, manifested by leaf yellowing due to the loss of chlorophyll, along with the degradation of macromolecules and facilitates nutrient translocation from the sink to the source tissues, which is essential for the plants' fitness. Leaf senescence is controlled by a sophisticated genetic network that has been revealed through the study of the molecular mechanisms of hundreds of senescence-associated genes (SAGs), which are involved in multiple layers of regulation. Leaf senescence is primarily regulated by plant age, but also influenced by a variety of factors, including phytohormones and environmental stimuli. Phytohormones, as important signaling molecules in plant, contribute to the onset and progression of leaf senescence. Recently, peptide hormones have been reported to be involved in the regulation of leaf senescence, enriching the significance of signaling molecules in controlling leaf senescence. This review summarizes recent advances in the regulation of leaf senescence by classical and peptide hormones, aiming to better understand the coordinated network of different pathways during leaf senescence.

Arabidopsis root responses to salinity depend on pectin modification and cell wall sensing

Owing to its detrimental effect on plant growth, salinity is an increasing worldwide problem for agriculture. To understand the molecular mechanisms activated in response to salt in Arabidopsis thaliana, we investigated the Catharanthus roseus receptor-like kinase 1-like family, which contains sensors that were previously shown to be involved in sensing the structural integrity of the cell walls. We found that herk1 the1-4 double mutants, lacking the function of HERKULES1 (HERK1) and combined with a gain-of-function allele of THESEUS1 (THE1), strongly respond to salt application, resulting in an intense activation of stress responses, similarly to plants lacking FERONIA (FER) function. We report that salt triggers pectin methyl esterase (PME) activation and show its requirement for the activation of several salt-dependent responses. Because chemical inhibition of PMEs alleviates these salt-induced responses, we hypothesize a model in which salt directly leads to cell wall modifications through the activation of PMEs. Responses to salt partly require the functionality of FER alone or HERK1/THE1 to attenuate salt effects, highlighting the complexity of the salt-sensing mechanisms that rely on cell wall integrity.

Summary: Salt-triggered activation of pectin methyl esterase changes pectin in Arabidopsis, inducing at least two pathways: a CrRLK1L-dependent pathway downregulating salt stress responses and a CrRLK1L-independent pathway that activates downstream signaling.

Transcriptomic Profiling Provides Molecular Insights Into Hydrogen Peroxide-Enhanced Arabidopsis Growth and Its Salt Tolerance

Salt stress is an important environmental factor limiting plant growth and crop production. Plant adaptation to salt stress can be improved by chemical pretreatment. This study aims to identify whether hydrogen peroxide (H2O2) pretreatment of seedlings affects the stress tolerance of Arabidopsis thaliana seedlings. The results show that pretreatment with H2O2 at appropriate concentrations enhances the salt tolerance ability of Arabidopsis seedlings, as revealed by lower Na+ levels, greater K+ levels, and improved K+/Na+ ratios in leaves. Furthermore, H2O2 pretreatment improves the membrane properties by reducing the relative membrane permeability (RMP) and malonaldehyde (MDA) content in addition to improving the activities of antioxidant enzymes, including superoxide dismutase, and glutathione peroxidase. Our transcription data show that exogenous H2O2 pretreatment leads to the induced expression of cell cycle, redox regulation, and cell wall organization-related genes in Arabidopsis, which may accelerate cell proliferation, enhance tolerance to osmotic stress, maintain the redox balance, and remodel the cell walls of plants in subsequent high-salt environments.

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.

Apple receptor-like kinase FERONIA regulates salt tolerance and ABA sensitivity in Malus domestica

FERONIA (FER) is a membrane-localized receptor-like kinase that plays pivotal roles in male and female gametophyte recognition, hormone signaling crosstalk, and biotic and abiotic responses. Most reports focus on the functions of FER in model plant Arabidopsis thaliana. However, the functions of FER homologs have not been deeply investigated in apple (Malus domestica), an important economic fruit crop distributed worldwide, especially in China. In this study, we identified an apple homolog of Arabidopsis FER, named MdFER (MDP0000390677). The two proteins encoded by AtFER and MdFER share similar domains: an extracellular malectin-like domain, a transmembrane domain, and an intracellular kinase domain. MdFER was further proven to localize to the plasma membrane in the epidermal cells of Nicotiana benthamiana. MdFER was widely expressed in different apple tissues, but the highest expression was found in roots. In addition, expression of MdFER was significantly induced by treatment with abscisic acid (ABA) and salt (NaCl). Overexpressing MdFER dramatically improved the resistance to salt stress and reduced the sensitivity to ABA in apple callus, while suppressing MdFER expression showed contrary effects. Furthermore, ectopic expression of MdFER in Arabidopsis significantly increased the salt tolerance and reduced the sensitivity to ABA. In addition, under salt stress and ABA treatment, Arabidopsis with highly expressed MdFER accumulated less reactive oxygen species (ROS), and the enzymatic activity of two ROS scavengers, superoxide dismutase and catalase, was higher compared with that of wild type (WT). Our work proves that MdFER positively regulates salt tolerance and negatively regulates ABA sensitivity in apple, which enriched the functions of FER in different plant species.

Cellulose synthase-like protein OsCSLD4 plays an important role in the response of rice to salt stress by mediating abscisic acid biosynthesis to regulate osmotic stress tolerance

Cell wall polysaccharide biosynthesis enzymes play important roles in plant growth, development and stress responses. The functions of cell wall polysaccharide synthesis enzymes in plant growth and development have been well studied. In contrast, their roles in plant responses to environmental stress are poorly understood. Previous studies have demonstrated that the rice cell wall cellulose synthase-like D4 protein (OsCSLD4) is involved in cell wall polysaccharide synthesis and is important for rice growth and development. This study demonstrated that the OsCSLD4 function-disrupted mutant nd1 was sensitive to salt stress, but insensitive to abscisic acid (ABA). The expression of some ABA synthesis and response genes was repressed in nd1 under both normal and salt stress conditions. Exogenous ABA can restore nd1-impaired salt stress tolerance. Moreover, overexpression of OsCSLD4 can enhance rice ABA synthesis gene expression, increase ABA content and improve rice salt tolerance, thus implying that OsCSLD4-regulated rice salt stress tolerance is mediated by ABA synthesis. Additionally, nd1 decreased rice tolerance to osmotic stress, but not ion toxic tolerance. The results from the transcriptome analysis showed that more osmotic stress-responsive genes were impaired in nd1 than salt stress-responsive genes, thus indicating that OsCSLD4 is involved in rice salt stress response through an ABA-induced osmotic response pathway. Intriguingly, the disruption of OsCSLD4 function decreased grain width and weight, while overexpression of OsCSLD4 increased grain width and weight. Taken together, this study demonstrates a novel plant salt stress adaptation mechanism by which crops can coordinate salt stress tolerance and yield.

Small signaling peptides mediate plant adaptions to abiotic environmental stress

Peptide-receptor complexes activate distinct downstream regulatory networks to mediate plant adaptions to abiotic environmental stress. Plants are constantly exposed to various adverse environmental factors; thus they must adjust their growth accordingly. Plants recruit small secretory peptides to adapt to these detrimental environments. These small peptides, which are perceived by their corresponding receptors and/or co-receptors, act as local- or long-distance mobile signaling molecules to establish cell-to-cell regulatory networks, resulting in optimal cellular and physiological outputs. In this review, we highlight recent advances on the regulatory role of small peptides in plant abiotic responses and nutrients signaling.

Identification of Key Genes in 'Luang Pratahn', Thai Salt-Tolerant Rice, Based on Time-Course Data and Weighted Co-expression Networks

Salinity is an important environmental factor causing a negative effect on rice production. To prevent salinity effects on rice yields, genetic diversity concerning salt tolerance must be evaluated. In this study, we investigated the salinity responses of rice (Oryza sativa) to determine the critical genes. The transcriptomes of 'Luang Pratahn' rice, a local Thai rice variety with high salt tolerance, were used as a model for analyzing and identifying the key genes responsible for salt-stress tolerance. Based on 3' Tag-Seq data from the time course of salt-stress treatment, weighted gene co-expression network analysis was used to identify key genes in gene modules. We obtained 1,386 significantly differentially expressed genes in eight modules. Among them, six modules indicated a significant correlation within 6, 12, or 48h after salt stress. Functional and pathway enrichment analysis was performed on the co-expressed genes of interesting modules to reveal which genes were mainly enriched within important functions for salt-stress responses. To identify the key genes in salt-stress responses, we considered the two-state co-expression networks, normal growth conditions, and salt stress to investigate which genes were less important in a normal situation but gained more impact under stress. We identified key genes for the response to biotic and abiotic stimuli and tolerance to salt stress. Thus, these novel genes may play important roles in salinity tolerance and serve as potential biomarkers to improve salt tolerance cultivars.

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

Receptor-like kinases (RLKs), which constitute the largest receptor family in plants, are essential for perceiving and relaying information about various environmental stimuli. Tremendous progress has been made in the past few decades towards elucidating the mechanisms of action of several RLKs, with emerging paradigms pointing to their roles in cell adaptations. Among these paradigms, Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) proteins and their rapid alkalinization factor (RALF) peptide ligands have attracted much interest. In particular, FERONIA (FER) is a CrRLK1L protein that participates in a wide array of physiological processes associated with RALF signalling, including cell growth and monitoring cell wall integrity, RNA and energy metabolism, and phytohormone and stress responses. Here, we analyse FER in the context of CrRLK1L members and their ligands in multiple species. The FER working model raises many questions about the role of CrRLK1L signalling networks during cell adaptation. For example, how do CrRLK1Ls recognize various RALF peptides from different organisms to initiate specific phosphorylation signal cascades? How do RALF-FER complexes achieve their specific, sometimes opposite, functions in different cell types? Here, we summarize recent major findings and highlight future perspectives in the field of CrRLK1L signalling networks.

FERONIA restricts Pseudomonas in the rhizosphere microbiome via regulation of reactive oxygen species

Maintaining microbiome structure is critical for the health of both plants and animals. By re-screening a collection of Arabidopsis mutants affecting root immunity and hormone crosstalk, we identified a FERONIA (FER) receptor kinase mutant (fer-8) with a rhizosphere microbiome enriched in Pseudomonas fluorescens without phylum-level dysbiosis. Using microbiome transplant experiments, we found that the fer-8 microbiome was beneficial. The effect of FER on rhizosphere pseudomonads was largely independent of its immune scaffold function, role in development and jasmonic acid autoimmunity. We found that the fer-8 mutant has reduced basal levels of reactive oxygen species (ROS) in roots and that mutants deficient in NADPH oxidase showed elevated rhizosphere pseudomonads. The addition of RALF23 peptides, a FER ligand, was sufficient to enrich P. fluorescens. This work shows that FER-mediated ROS production regulates levels of beneficial pseudomonads in the rhizosphere microbiome.

Maintenance of Cell Wall Integrity under High Salinity

Cell wall biosynthesis is a complex biological process in plants. In the rapidly growing cells or in the plants that encounter a variety of environmental stresses, the compositions and the structure of cell wall can be dynamically changed. To constantly monitor cell wall status, plants have evolved cell wall integrity (CWI) maintenance system, which allows rapid cell growth and improved adaptation of plants to adverse environmental conditions without the perturbation of cell wall organization. Salt stress is one of the abiotic stresses that can severely disrupt CWI, and studies have shown that the ability of plants to sense and maintain CWI is important for salt tolerance. In this review, we highlight the roles of CWI in salt tolerance and the mechanisms underlying the maintenance of CWI under salt stress. The unsolved questions regarding the association between the CWI and salt tolerance are discussed.

Cracking the "Sugar Code": A Snapshot of N- and O-Glycosylation Pathways and Functions in Plants Cells

Glycosylation is a fundamental co-translational and/or post-translational modification process where an attachment of sugars onto either proteins or lipids can alter their biological function, subcellular location and modulate the development and physiology of an organism. Glycosylation is not a template driven process and as such produces a vastly larger array of glycan structures through combinatorial use of enzymes and of repeated common scaffolds and as a consequence it provides a huge expansion of both the proteome and lipidome. While the essential role of N- and O-glycan modifications on mammalian glycoproteins is already well documented, we are just starting to decode their biological functions in plants. Although significant advances have been made in plant glycobiology in the last decades, there are still key challenges impeding progress in the field and, as such, holistic modern high throughput approaches may help to address these conceptual gaps. In this snapshot, we present an update of the most common O- and N-glycan structures present on plant glycoproteins as well as (1) the plant glycosyltransferases (GTs) and glycosyl hydrolases (GHs) responsible for their biosynthesis; (2) a summary of microorganism-derived GHs characterized to cleave specific glycosidic linkages; (3) a summary of the available tools ranging from monoclonal antibodies (mAbs), lectins to chemical probes for the detection of specific sugar moieties within these complex macromolecules; (4) selected examples of N- and O-glycoproteins as well as in their related GTs to illustrate the complexity on their mode of action in plant cell growth and stress responses processes, and finally (5) we present the carbohydrate microarray approach that could revolutionize the way in which unknown plant GTs and GHs are identified and their specificities characterized.