The Arabidopsis CURVY1 (CVY1) gene encoding a novel receptor-like protein kinase regulates cell morphogenesis, flowering time and seed production

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

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

Antagonistic RALF peptides control an intergeneric hybridization barrier on Brassicaceae stigmas

Pollen-pistil interactions establish interspecific/intergeneric pre-zygotic hybridization barriers in plants. The rejection of undesired pollen at the stigma is crucial to avoid outcrossing but can be overcome with the support of mentor pollen. The mechanisms underlying this hybridization barrier are largely unknown. Here, in Arabidopsis, we demonstrate that receptor-like kinases FERONIA/CURVY1/ANJEA/HERCULES RECEPTOR KINASE 1 and cell wall proteins LRX3/4/5 interact on papilla cell surfaces with autocrine stigmatic RALF1/22/23/33 peptide ligands (sRALFs) to establish a lock that blocks the penetration of undesired pollen tubes. Compatible pollen-derived RALF10/11/12/13/25/26/30 peptides (pRALFs) act as a key, outcompeting sRALFs and enabling pollen tube penetration. By treating Arabidopsis stigmas with synthetic pRALFs, we unlock the barrier, facilitating pollen tube penetration from distantly related Brassicaceae species and resulting in interspecific/intergeneric hybrid embryo formation. Therefore, we uncover a "lock-and-key" system governing the hybridization breadth of interspecific/intergeneric crosses in Brassicaceae. Manipulating this system holds promise for facilitating broad hybridization in crops.

Emerging Roles of Receptor-like Protein Kinases in Plant Response to Abiotic Stresses

The productivity of plants is hindered by unfavorable conditions. To perceive stress signals and to transduce these signals to intracellular responses, plants rely on membrane-bound receptor-like kinases (RLKs). These play a pivotal role in signaling events governing growth, reproduction, hormone perception, and defense responses against biotic stresses; however, their involvement in abiotic stress responses is poorly documented. Plant RLKs harbor an N-terminal extracellular domain, a transmembrane domain, and a C-terminal intracellular kinase domain. The ectodomains of these RLKs are quite diverse, aiding their responses to various stimuli. We summarize here the sub-classes of RLKs based on their domain structure and discuss the available information on their specific role in abiotic stress adaptation. Furthermore, the current state of knowledge on RLKs and their significance in abiotic stress responses is highlighted in this review, shedding light on their role in influencing plant-environment interactions and opening up possibilities for novel approaches to engineer stress-tolerant crop varieties.

Gibberellic acid sensitive dwarf encodes an ARPC2 subunit that mediates gibberellic acid biosynthesis, effects to grain yield in rice

The plant hormone gibberellic acid (GA) is important for plant growth and productivity. Actin-related proteins (ARPs) also play central roles in plant growth, including cell elongation and development. However, the relationships between ARPs and GA signaling and biosynthesis are not fully understood. Here, we isolated OsGASD, encoding an ARP subunit from rice (Oryza sativa), using the Ac/Ds knockout system. The osgasd knockout (Ko) mutation reduced GA3 content in shoots as well as plant growth and height. However, GA application restored the plant height of the osgasd Ko mutant to a height similar to that of the wild type (WT). Rice plants overexpressing OsGASD (Ox) showed increased plant height and grain yield compared to the WT. Transcriptome analysis of flag leaves of OsGASD Ox and osgasd Ko plants revealed that OsGASD regulates cell development and the expression of elongation-related genes. These observations suggest that OsGASD is involved in maintaining GA homeostasis to regulate plant development, thereby affecting rice growth and productivity.

The function of the plant cell wall in plant-microbe interactions

The plant cell wall is an interface of plant-microbe interactions. The ability of microbes to decompose cell wall polysaccharides contributes to microbial pathogenicity. Plants have evolved mechanisms to prevent cell wall degradation. However, the role of the cell wall in plant-microbe interactions is not well understood. Here, we discuss four functions of the plant cell wall-physical defence, storage of antimicrobial compounds, production of cell wall-derived elicitors, and provision of carbon sources-in the context of plant-microbe interactions. In addition, we discuss the four families of cell surface receptors associated with plant cell walls (malectin-like receptor kinase family, wall-associated kinase family, leucine-rich repeat receptor-like kinase family, and lysin motif receptor-like kinase family) that have been the subject of several important studies in recent years. This review summarises the findings on both plant cell wall and plant immunity, improving our understanding and may provide impetus to various researchers.

Malectin/Malectin-like domain-containing proteins: A repertoire of cell surface molecules with broad functional potential

Cell walls are at the front line of interactions between walled-organisms and their environment. They support cell expansion, ensure cell integrity and, for multicellular organisms such as plants, they provide cell adherence, support cell shape morphogenesis and mediate cell-cell communication. Wall-sensing, detecting perturbations in the wall and signaling the cell to respond accordingly, is crucial for growth and survival. In recent years, plant signaling research has suggested that a large family of receptor-like kinases (RLKs) could function as wall sensors partly because their extracellular domains show homology with malectin, a diglucose binding protein from the endoplasmic reticulum of animal cells. Studies of several malectin/malectin-like (M/ML) domain-containing RLKs (M/MLD-RLKs) from the model plant Arabidopsis thaliana have revealed an impressive array of biological roles, controlling growth, reproduction and stress responses, processes that in various ways rely on or affect the cell wall. Malectin homologous sequences are widespread across biological kingdoms, but plants have uniquely evolved a highly expanded family of proteins with ML domains embedded within various protein contexts. Here, we present an overview on proteins with malectin homologous sequences in different kingdoms, discuss the chromosomal organization of Arabidopsis M/MLD-RLKs and the phylogenetic relationship between these proteins from several model and crop species. We also discuss briefly the molecular networks that enable the diverse biological roles served by M/MLD-RLKs studied thus far.

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.

Genome-wide identification and analysis of Catharanthus roseus RLK1-like kinases in Nicotiana benthamiana

BACKGROUND

The Catharanthus roseus RLK1-like kinase (CrRLK1L) is a subfamily of the RLK gene family, and members are sensors of cell wall integrity and regulators of cell polarity growth. Recent studies have also shown that members of this subfamily are involved in plant immunity. Nicotiana benthamiana is a model plant widely used in the study of plant-pathogen interactions. However, the members of the NbCrRLK1L subfamily and their response to pathogens have not been reported.

RESULTS

In this study, a total of 31 CrRLK1L members were identified in the N. benthamiana genome, and these can be divided into 6 phylogenetic groups (I-VI). The members in each group have similar exon-intron structures and conserved motifs. NbCrRLK1Ls were predicted to be regulated by cis-acting elements such as STRE, TCA, ABRE, etc., and to be the target of transcription factors such as Dof and MYB. The expression profiles of the 16 selected NbCrRLK1Ls were determined by quantitative PCR. Most NbCrRLK1Ls were highly expressed in leaves but there were different and diverse expression patterns in other tissues. Inoculation with the bacterium Pseudomonas syringae or with Turnip mosaic virus significantly altered the transcript levels of the tested genes, suggesting that NbCrRLK1Ls may be involved in the response to pathogens.

CONCLUSIONS

This study systematically identified the CrRLK1L members in N. benthamiana, and analyzed their tissue-specific expression and gene expression profiles in response to different pathogens and two pathogens associated molecular patterns (PAMPs). This research lays the foundation for exploring the function of NbCrRLK1Ls in plant-microbe interactions.

The CrRLK1L subfamily: One of the keys to versatility in plants

Catharanthus roseous kinase 1L receptors (CrRLK1Ls) are a subfamily of membrane receptors unique to plant cells that perceive internal and external signals, integrate metabolic, physiological, and molecular processes, and regulate plant development. Recent genomic studies have suggested that this receptor subfamily arose during the emergence of terrestrial plants and has since diversified, preserving its essential functions. Participation of some of these CrRLK1Ls in different processes is presented and discussed herein, as well as the increasing number of interactors necessary for their function. At least five different responses have been detected after activating these receptors, such as physiological changes, formation or disassembly of protein complexes, metabolic responses, modification of gene expression, and modulation of phytohormone activity. To date, a common response mechanism for all processes involving CrRLK1Ls has not been described. In this review, the information available on the different functions of CrRLK1Ls was compiled. Additionally, the physiological and/or molecular mechanisms involved in the signaling processes triggered by these receptors are also discussed. In this review, we propose a possible common signaling mechanism for all processes regulated by CrRLK1Ls and pose questions to be answered in the future.

Identification and characterization of regulatory pathways involved in early flowering in the new leaves of alfalfa (Medicago sativa L.) by transcriptome analysis

BACKGROUND

Alfalfa (Medicago sativa L.) is a perennial legume extensively planted throughout the world as a high nutritive value livestock forage. Flowering time is an important agronomic trait that contributes to the production of alfalfa hay and seeds. However, the underlying molecular mechanisms of flowering time regulation in alfalfa are not well understood.

RESULTS

In this study, an early-flowering alfalfa genotype 80 and a late-flowering alfalfa genotype 195 were characterized for the flowering phenotype. Our analysis revealed that the lower jasmonate (JA) content in new leaves and the downregulation of JA biosynthetic genes (i.e. lipoxygenase, the 12-oxophytodienoate reductase-like protein, and salicylic acid carboxyl methyltransferase) may play essential roles in the early-flowering phenotype of genotype 80. Further research indicated that genes encode pathogenesis-related proteins [e.g. leucine rich repeat (LRR) family proteins, receptor-like proteins, and toll-interleukin-like receptor (TIR)-nucleotide-binding site (NBS)-LRR class proteins] and members of the signaling receptor kinase family [LRR proteins, kinases domain of unknown function 26 (DUF26) and wheat leucine-rich repeat receptor-like kinase10 (LRK10)-like kinases] are related to early flowering in alfalfa. Additionally, those involved in secondary metabolism (2-oxoglutarate/Fe (II)-dependent dioxygenases and UDP-glycosyltransferase) and the proteasome degradation pathway [really interesting new gene (RING)/U-box superfamily proteins and F-box family proteins] are also related to early flowering in alfalfa.

CONCLUSIONS

Integrated phenotypical, physiological, and transcriptomic analyses demonstrate that hormone biosynthesis and signaling pathways, pathogenesis-related genes, signaling receptor kinase family genes, secondary metabolism genes, and proteasome degradation pathway genes are responsible for the early flowering phenotype in alfalfa. This will provide new insights into future studies of flowering time in alfalfa and inform genetic improvement strategies for optimizing this important trait.

Gene expression changes occurring at bolting time are associated with leaf senescence in Arabidopsis

In plants, the vegetative to reproductive phase transition (termed bolting in Arabidopsis) generally precedes age-dependent leaf senescence (LS). Many studies describe a temporal link between bolting time and LS, as plants that bolt early, senesce early, and plants that bolt late, senesce late. The molecular mechanisms underlying this relationship are unknown and are potentially agriculturally important, as they may allow for the development of crops that can overcome early LS caused by stress-related early-phase transition. We hypothesized that leaf gene expression changes occurring in synchrony with bolting were regulating LS. ARABIDOPSIS TRITHORAX (ATX) enzymes are general methyltransferases that regulate the adult vegetative to reproductive phase transition. We generated an atx1, atx3, and atx4 (atx1,3,4) triple T-DNA insertion mutant that displays both early bolting and early LS. This mutant was used in an RNA-seq time-series experiment to identify gene expression changes in rosette leaves that are likely associated with bolting. By comparing the early bolting mutant to vegetative WT plants of the same age, we were able to generate a list of differentially expressed genes (DEGs) that change expression with bolting as the plants age. We trimmed the list by intersection with publicly available WT datasets, which removed genes from our DEG list that were atx1,3,4 specific. The resulting 398 bolting-associated genes (BAGs) are differentially expressed in a mature rosette leaf at bolting. The BAG list contains many well-characterized LS regulators (ORE1, WRKY45, NAP, WRKY28), and GO analysis revealed enrichment for LS and LS-related processes. These bolting-associated LS regulators may contribute to the temporal coupling of bolting time to LS.

Genome-Wide Identification of the CrRLK1L Subfamily and Comparative Analysis of Its Role in the Legume-Rhizobia Symbiosis

The plant receptor-like-kinase subfamily CrRLK1L has been widely studied, and CrRLK1Ls have been described as crucial regulators in many processes in Arabidopsis thaliana (L.), Heynh. Little is known, however, about the functions of these proteins in other plant species, including potential roles in symbiotic nodulation. We performed a phylogenetic analysis of CrRLK1L subfamily receptors of 57 different plant species and identified 1050 CrRLK1L proteins, clustered into 11 clades. This analysis revealed that the CrRLK1L subfamily probably arose in plants during the transition from chlorophytes to embryophytes and has undergone several duplication events during its evolution. Among the CrRLK1Ls of legumes and A. thaliana, protein structure, gene structure, and expression patterns were highly conserved. Some legume CrRLK1L genes were active in nodules. A detailed analysis of eight nodule-expressed genes in Phaseolus vulgaris L. showed that these genes were differentially expressed in roots at different stages of the symbiotic process. These data suggest that CrRLK1Ls are both conserved and underwent diversification in a wide group of plants, and shed light on the roles of these genes in legume–rhizobia symbiosis.

RALF-FERONIA Signaling: Linking Plant Immune Response with Cell Growth

Plants perceive various external and internal signals to self-modulate biological processes through members of the receptor-like kinase (RLK) family, among which Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) proteins with their ligands, rapid alkalinization factor (RALF) peptides, have attracted considerable interest. FERONIA (FER), a CrRLK1L member, was initially reported to act as a major plant cell growth modulator in distinct tissues. Subsequently, the RALF-FER pathway was confirmed to function as an essential regulator of plant stress responses, including but not limited to immune responses. Furthermore, the RALF-FER pathway modulates immune responses and cell growth in a context-specific manner, and the vital roles of this pathway are beginning to be appreciated in crop species. The recent remarkable advances in understanding the functions and molecular mechanisms of the RALF-FER pathway have also raised many interesting questions that need to be answered in the future. This review mainly focuses on the roles of FER and other CrRLK1L members in modulating immune responses in the context of cell growth in response to their RALF peptide ligands and presents a brief outlook for future research.

RALF-FERONIA Signaling: Linking Plant Immune Response with Cell Growth

Plants perceive various external and internal signals to self-modulate biological processes through members of the receptor-like kinase (RLK) family, among which Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) proteins with their ligands, rapid alkalinization factor (RALF) peptides, have attracted considerable interest. FERONIA (FER), a CrRLK1L member, was initially reported to act as a major plant cell growth modulator in distinct tissues. Subsequently, the RALF-FER pathway was confirmed to function as an essential regulator of plant stress responses, including but not limited to immune responses. Furthermore, the RALF-FER pathway modulates immune responses and cell growth in a context-specific manner, and the vital roles of this pathway are beginning to be appreciated in crop species. The recent remarkable advances in understanding the functions and molecular mechanisms of the RALF-FER pathway have also raised many interesting questions that need to be answered in the future. This review mainly focuses on the roles of FER and other CrRLK1L members in modulating immune responses in the context of cell growth in response to their RALF peptide ligands and presents a brief outlook for future research.

FERONIA cytoplasmic domain: node of varied signal outputs

The receptor-like kinase (RLK) FERONIA (FER), located on the plasma membrane, belongs to the Catharanthus roseus RLK1-like kinase family (CrRLK1L) and participates in widespread biological processes in plants in a context-dependent fashion. Genetic studies in Arabidopsis illustrated the versatile roles that FER plays in fertilization, vegetative growth, defense and stress responses, cell-wall homeostasis, as well as protein synthesis. These studies also helped to identify genes and signal pathways involved in FER signal transduction. Despite increasingly larger numbers of studies discussing how FER senses its ligand, Rapid alkalinization factor (RALF) peptides, and further regulates downstream factors, few have shown the mechanisms of how FER mediates the specific regulation of downstream signals in context of the phosphorylation of its cytoplasmic domain. As understanding this would help in better understanding the diversity and complexity of FER function, this paper aims to review the roles of FER in regulating different signal outputs from the view of the role of its cytoplasmic domain.

Mutations of two FERONIA-like receptor genes enhance rice blast resistance without growth penalty

Genes that provide resistance to fungi and/or bacteria usually reduce plant growth and ultimately affect grain yield. Thus, crop breeding programs need to find genetic resources that balance disease resistance with growth. The receptor kinase FERONIA regulates cell growth and survival in Arabidopsis. Here, we investigate, in rice, the role of members of the FERONIA-like receptor (FLR) gene family in the balance between growth and the response to the fungal pathogen Magnaporthe oryzae (Pyricularia oryzae), which causes the most devastating disease in rice. We carried out genome-wide gene expression and functional screenings in rice via a gene knockout strategy, and we successfully knocked out 14 FLR genes in rice. Using these genetic resources, we found that mutations in the FLR2 and FLR11 genes provide resistance to rice blast without a profound growth penalty. Detailed analyses revealed that FLR2 mutation increased both defense-related gene expression and M. oryzae-triggered production of reactive oxygen species. Thus, our results highlight novel genetic tools for studying the underlying molecular mechanisms of enhancing disease resistance without growth penalty.

To preserve or to destroy, that is the question: the role of the cell wall integrity pathway in pollen tube growth

In plants, cell-shape is defined by the cell wall, a complex network of polymers located outside the plasma membrane. During cell growth, cell wall properties have to be adjusted, assuring cell expansion without compromising cell integrity. Plasma membrane-located receptors sense cell wall properties, transducing extracellular signals into intracellular cascades through the cell wall integrity (CWI) pathway that, in turn, leads to adjustments in the regulation and composition of the cell wall. Using pollen tube growth as a single celled model system, we describe the importance of RAPID ALKALINIZATION FACTOR (RALF) peptides as sensors of cell wall integrity. RALF peptides can mediate the communication between cell wall components and plasma membrane-localized receptor-like kinases (RLKs) of the CrRLK1L family. The subsequent activation of intracellular pathways regulates H⁺, Ca²⁺, and ROS levels in the cell and apoplast, thereby modulating cell wall integrity. Interestingly, the RALF-CrRLK1L module and some of the components working up- and downstream of the RLK is conserved in many other developmental and physiological signaling processes.

Analysis of DNA Methylation Patterns Associated with In Vitro Propagated Globe Artichoke Plants Using an EpiRADseq-Based Approach

Globe artichoke represents one of the main horticultural species of the Mediterranean basin, and 'Spinoso sardo' is the most widespread and economically relevant varietal type in Sardinia, Italy. In the last decades, in vitro culture of meristematic apices has increased the frequency of aberrant plants in open-field production. These off-type phenotypes showed highly pinnate-parted leaves and late inflorescence budding, and emerged from some branches of the true-to-type 'Spinoso sardo' plants. This phenomenon cannot be foreseen and is reversible through generations, suggesting the occurrence of epigenetic alterations. Here, we report an exploratory study on DNA methylation patterns in off-type/true-to-type globe artichoke plants, using a modified EpiRADseq technology, which allowed the identification of 2,897 differentially methylated loci (DML): 1,998 in CG, 458 in CHH, and 441 in CHG methylation contexts of which 720, 88, and 152, respectively, were in coding regions. Most of them appeared involved in primary metabolic processes, mostly linked to photosynthesis, regulation of flower development, and regulation of reproductive processes, coherently with the observed phenotype. Differences in the methylation status of some candidate genes were integrated with transcriptional analysis to test whether these two regulation levels might interplay in the emergence and spread of the 'Spinoso sardo' non-conventional phenotype.

Multiplex mutagenesis of four clustered CrRLK1L with CRISPR/Cas9 exposes their growth regulatory roles in response to metal ions

Resolving functions of closely linked genes is challenging or nearly impossible with classical genetic tools. Four members of the Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) family are clustered on Arabidopsis chromosome five. To resolve the potentially redundant functions of this subclass of CrRLK1Ls named MEDOS1 to 4 (MDS1 to 4), we generated a single CRISPR/Cas9 transformation vector using a Golden Gate based cloning system to target all four genes simultaneously. We introduce single mutations within and deletions between MDS genes as well as knock-outs of the whole 11 kb gene cluster. The large MDS cluster deletion was inherited in up to 25% of plants lacking the CRISPR/Cas9 construct in the T2 generation. In contrast to described phenotypes of already characterized CrRLK1L mutants, quadruple mds knock-outs were fully fertile, developed normal root hairs and trichomes and responded to pharmacological inhibition of cellulose biosynthesis similar to wildtype. Recently, we demonstrated the role of four CrRLK1L in growth adaptation to metal ion stress. Here we show the involvement of MDS genes in response to Ni2+ during hypocotyl elongation and to Cd2+ and Zn2+ during root growth. Our finding supports the model of an organ specific network of positively and negatively acting CrRLK1Ls.

Crystal structures of two tandem malectin-like receptor kinases involved in plant reproduction

Complex cell-to-cell communication between the male pollen tube and the female reproductive organs is required for plant fertilization. A family of Catharanthus roseus receptor kinase 1-like (CrRLK1L) membrane receptors has been genetically implicated in this process. Here, crystal structures of the CrRLK1Ls ANXUR1 and ANXUR2 are reported at 1.48 and 1.1 Å resolution, respectively. The structures reveal a novel arrangement of two malectin-like domains connected by a short β-hairpin linker and stabilized by calcium ions. The canonical carbohydrate-interaction surfaces of related animal and bacterial carbohydrate-binding modules are not conserved in plant CrRLK1Ls. In line with this, the binding of chemically diverse oligosaccharides to ANXUR1 and HERCULES1 could not be detected. Instead, CrRLK1Ls have evolved a protein-protein interface between their malectin domains which forms a deep cleft lined by highly conserved aromatic and polar residues. Analysis of the glycosylation patterns of different CrRLK1Ls and their oligomeric states suggests that this cleft could resemble a binding site for a ligand required for receptor activation of CrRLK1Ls.

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

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

Plant Malectin-Like Receptor Kinases: From Cell Wall Integrity to Immunity and Beyond

Plant cells are surrounded by cell walls protecting them from a myriad of environmental challenges. For successful habitat adaptation, extracellular cues are perceived at the cell wall and relayed to downstream signaling constituents to mediate dynamic cell wall remodeling and adapted intracellular responses. Plant malectin-like receptor kinases, also known as Catharanthus roseus receptor-like kinase 1-like proteins (CrRLK1Ls), take part in these perception and relay processes. CrRLK1Ls are involved in many different plant functions. Their ligands, interactors, and downstream signaling partners are being unraveled, and studies about CrRLK1Ls' roles in plant species other than the plant model Arabidopsis thaliana are beginning to flourish. This review focuses on recent CrRLK1L-related advances in cell growth, reproduction, hormone signaling, abiotic stress responses, and, particularly, immunity. We also give an overview of the comparative genomics and evolution of CrRLK1Ls, and present a brief outlook for future research.

Role of CrRLK1L Cell Wall Sensors HERCULES1 and 2, THESEUS1, and FERONIA in Growth Adaptation Triggered by Heavy Metals and Trace Elements

Cell walls are not only a protective barrier surrounding protoplasts but serve as signaling platform between the extracellular environment and the intracellular physiology. Ions of heavy metals and trace elements, summarized to metal ions, bind to cell wall components, trigger their modification and provoke growth responses. To examine if metal ions trigger cell wall sensing receptor like kinases (RLKs) of the Catharanthus roseus RLK1-like (CrRLK1L) family we employed a molecular genetic approach. Quantitative transcription analyses show that HERCULES1 (HERK1), THESEUS1 (THE1), and FERONIA (FER) were differently regulated by cadmium (Cd), nickel (Ni), and lead (Pb). Growth responses were quantified for roots and etiolated hypocotyls of related mutants and overexpressors on Cd, copper (Cu), Ni, Pb, and zinc (Zn) and revealed a complex pattern of gene specific, overlapping and antagonistic responses. Root growth was often inversely affected to hypocotyl elongation. For example, both HERK genes seem to negatively regulate hypocotyl elongation upon Cd, Ni, Zn, and Pb while they support root growth on Cd, Cu, and Ni. The different THE1 alleles exhibited a similar effect between roots and hypocotyls on Ni, where the loss-of-function mutant was more tolerant while the gain of function mutants were hypersensitive indicating that THE1 is mediating Ni specific inhibition of hypocotyl elongation in the dark. In contrast hypocotyl elongation of the knock-out mutant, fer-4, was hypersensitive to Ni but exhibited a higher tolerance to Cd, Cu, Pb, and Zn. These data indicate an antagonistic action between THE1 and FER in relation to hypocotyl elongation upon excess of Ni. FERs function as receptor for rapid alkalinization factors (RALFs) was tested with the indicator bromocresol purple. While fer-4 roots strongly acidified control and metal ion containing media, the etiolated hypocotyls alkalized the media which is consistent with the already shorter hypocotyl of fer-4. No other CrRLK1L mutant exhibited this phenotype except of the THE1:GFP overexpressor on Ni suggesting that THE1 might be involved in Ni induced and hypocotyl specific RALF signaling and growth regulating pathway. Overall, our findings establish a molecular link between metal ion stress, growth and the cell wall integrity sensors of the CrRLK1L family.

T-DNA alleles of the receptor kinase THESEUS1 with opposing effects on cell wall integrity signaling

Perturbation of cellulose synthesis in plants triggers stress responses, including growth retardation, mediated by the cell wall integrity-sensing receptor-like kinase (RLK) THESEUS1 (THE1). The analysis of two alleles carrying T-DNA insertions at comparable positions has led to conflicting conclusions concerning the impact of THE1 signaling on growth. Here we confirm that, unlike the1-3 and other the1 alleles in which cellular responses to genetic or pharmacological inhibition of cellulose synthesis are attenuated, the1-4 showed enhanced responses, including growth inhibition, ectopic lignification, and stress gene expression. Both the1-3 and the1-4 express a transcript encoding a predicted membrane-associated truncated protein lacking the kinase domain. However, the1-3, in contrast to the1-4, strongly expresses antisense transcripts, which are expected to prevent the expression of the truncated protein as suggested by the genetic interactions between the two alleles. Seedlings overexpressing such a truncated protein react to isoxaben treatment similarly to the1-4 and the full-length THE overexpressor. We conclude that the1-4 is a hypermorphic allele; that THE1 signaling upon cell wall damage has a negative impact on cell expansion; and that caution is required when interpreting the phenotypic effects of T-DNA insertions in RLK genes.

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

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

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

Genetic Architecture of a Rice Nested Association Mapping Population

Describing the genetic diversity in the gene pool of crops will provide breeders with novel resources for varietal improvement. Nested Association Mapping (NAM) populations are uniquely suited for characterizing parental diversity through the shuffling and fixation of parental haplotypes. Here, we describe a set of 1879 rice NAM lines created through the selfing and single-seed descent of F1 hybrids derived from elite IR64 indica crossed with 10 diverse tropical japonica lines. Genotyping data indicated tropical japonica alleles were captured at every queried locus despite the presence of segregation distortion factors. Several distortion loci were mapped, both shared and unique, among the 10 populations. Using two-point and multi-point genetic map calculations, our datasets achieved the ∼1500 cM expected map size in rice. Finally, we highlighted the utility of the NAM lines for QTL mapping, including joint analysis across the 10 populations, by confirming known QTL locations for the trait days to heading.

Verification and fine mapping of qGW1.05, a major QTL for grain weight in maize (Zea mays L.)

Grain weight, one of the important factors to determine corn yield, is a typical quantitative inheritance trait. However, the molecular genetic basis of grain weight still remains limited. In our previous researches, a major QTL associated with grain weight, qGW1.05, has been identified between SSR markers umc1601 and umc1754 at bin locus 1.05-1.06 in maize. Here, its genetic and environmental stabiliteis were verified using a BC₃F₂ population to identify the effect of qGW1.05 on grain weight. Further, qGW1.05-NILs were obtained by MAS successfully. Via a large BC₆F₂ segregation population, together with polymorphic microsatellite markers developed between the parents to screen the genotype of the recombinant plants, qGW1.05 was positioned to a 1.11 Mb genome interval. Furthermore, the progenies of 15 recombinants were tested to confirm the effect of qGW1.05 on grain weight. Combining collinearity among cereal crops and genome annotation, the several candidate genes taking part in grain development were identified in the qGW1.05 region. In this study, qGW1.05 was limited to a 1.11 Mb region on chromosome 1, which established the foundation for understanding the molecular basis underlying kernel development and improving grain weight through MAS using the tightly flanking molecular markers in maize.

The Rice Receptor-Like Kinases DWARF AND RUNTISH SPIKELET1 and 2 Repress Cell Death and Affect Sugar Utilization during Reproductive Development

Cell-to-cell communication precisely controls the creation of new organs during reproductive growth. However, the sensor molecules that mediate developmental signals in monocot plants are poorly understood. Here, we report that DWARF AND RUNTISH SPIKELET1 (DRUS1) and DRUS2, two closely related receptor-like kinases (RLKs), redundantly control reproductive growth and development in rice (Oryza sativa). A drus1-1 drus2 double knockout mutant, but not either single mutant, showed extreme dwarfism and barren inflorescences that harbored sterile spikelets. The gibberellin pathway was not impaired in this mutant. A phenotypic comparison of mutants expressing different amounts of DRUS1 and 2 revealed that reproductive growth requires a threshold level of DRUS1/2 proteins. DRUS1 and 2 maintain cell viability by repressing protease-mediated cell degradation and likely by affecting sugar utilization or conversion. In the later stages of anther development, survival of the endothecium requires DRUS1/2, which may stimulate expression of the UDP-glucose pyrophosphorylase gene UGP2 and starch biosynthesis in pollen. Unlike their Arabidopsis thaliana ortholog FERONIA, DRUS1 and 2 mediate a fundamental signaling process that is essential for cell survival and represents a novel biological function for the CrRLK1L RLK subfamily.

Conserved Roles of CrRLK1L Receptor-Like Kinases in Cell Expansion and Reproduction from Algae to Angiosperms

Receptor-like kinases (RLKs) are regulators of plant development through allowing cells to sense their extracellular environment. They facilitate detection of local endogenous signals, in addition to external biotic and abiotic stimuli. The Catharanthus roseus RLK1-like (CrRLK1L) protein kinase subfamily, which contains FERONIA, plays a central role in regulating fertilization and in cell expansion mechanisms such as cell elongation and tip growth, as well as having indirect links to plant-pathogen interactions. Several components of CrRLK1L signaling pathways have been identified, including an extracellular ligand, coreceptors, and downstream signaling elements. The presence and abundance of the CrRLK1L proteins in the plant kingdom suggest an origin within the Streptophyta lineage, with a notable increase in prevalence in the seeded land plants. Given the function of the sole CrRLK1L protein in a charophycean alga, the possibility of a conserved role in detection and/or regulation of cell wall integrity throughout the Strephtophytes is discussed. Orthologs of signaling pathway components are also present in extant representatives of non-vascular land plants and early vascular land plants including the liverwort Marchantia polymorpha, the moss Physcomitrella patens and the lycophyte Selaginella moellendorffii. Deciphering the roles in development of the CrRLK1L protein kinases in early diverging land plants will provide insights into their ancestral function, furthering our understanding of this diversified subfamily of receptors in higher plants.

FERONIA and Her Pals: Functions and Mechanisms

Current research into the FERONIA family of receptor kinases highlights both questions and opportunities for understanding signaling strategies in plant growth and survival.

Genome-wide analysis of CrRLK1L gene family in Gossypium and identification of candidate CrRLK1L genes related to fiber development

Members of the CrRLK1L family, a subgroup of the receptor-like kinase (RLK) gene family, are thought to act as sensors for the integrity of the cell wall and regulators of polar elongation. To better understand the various functions in fiber development, we conducted genome-wide identification and characterization analyses of CrRLK1L family in cotton. Here 44, 40, and 79 CrRLK1L genes were identified from three cotton species: diploid G. raimondii (D5), diploid G. arboreum (A2), and tetraploid G. hirsutum TM-1 (AD1), respectively. The 44 CrRLK1Ls in G. raimondii were anchored to the 12 chromosomes unevenly and were classified into six groups (I-VI), with group II and group IV being further divided into two subgroups (groups IIa and IIb, and IVa and IVb, respectively). These CrRLK1Ls displayed a highly regular pattern of developmental and spatial regulation in cotton. Using the transcriptome data of five chromosomal segment introgression lines (CSILs) and the physical integration of CrRLK1Ls with the quantitative trait loci (QTLs) related to fiber quality traits, we revealed that six CrRLK1L genes were highly associated with fiber development. This study brings new insights into the integrated genome-wide identification of CrRLK1Ls in cotton and provides references for the genetic improvement of cotton fiber.

Plant development regulation: Overview and perspectives

Plant development, as occur in other eukaryotes, is conducted through a complex network of hormones, transcription factors, enzymes and micro RNAs, among other cellular components. They control developmental processes such as embryo, apical root and shoot meristem, leaf, flower, or seed formation, among others. The research in these topics has been very active in last decades. Recently, an explosion of new data concerning regulation mechanisms as well as the response of these processes to environmental changes has emerged. Initially, most of investigations were carried out in the model eudicot Arabidopsis but currently data from other plant species are available in the literature, although they are still limited. The aim of this review is focused on summarize the main molecular actors involved in plant development regulation in diverse plant species. A special attention will be given to the major families of genes and proteins participating in these regulatory mechanisms. The information on the regulatory pathways where they participate will be briefly cited. Additionally, the importance of certain structural features of such proteins that confer ductility and flexibility to these mechanisms will also be reported and discussed.

The SCAR/WAVE complex polarizes PAN receptors and promotes division asymmetry in maize

Pre-mitotic establishment of polarity is a key event in the preparation of mother cells for asymmetric cell divisions that produce daughters of distinct fates, and ensures correct cellular patterning of tissues and eventual organ function. Previous work has shown that two receptor-like kinases, PANGLOSS2 (PAN2) and PAN1, and the small GTPase RHO GTPASE OF PLANTS (ROP) promote mother cell polarity and subsequent division asymmetry in developing maize stomata. PAN proteins become polarized prior to asymmetric cell division, however, the mechanism of this polarization is unknown. Here we show that the SCAR/WAVE regulatory complex, which activates the actin-nucleating ARP2/3 complex, is the first known marker of polarity in this asymmetric division model and is required for PAN polarization. These findings implicate actin, and specifically branched actin networks, in PAN polarization and asymmetric cell division.