Cloning and Characterization of a Somatic Embryogenesis Receptor-Like Kinase Gene in Cotton (Gossypium hirsutum)

A novel gene, GhSERK1, was identified in cotton. It encoded a protein belonging to the somatic embryogenesis receptor- like kinase （SERK） family. The genomic sequence of GhSERK1 was 6 920 bp in length, containing a predicted transcriptional start site （TSS）. Its full-length cDNA was 2 502 bp, encoding a protein of 627 amino acids. Sequence analysis of GhSERK1 revealed high levels of similarity to other reported SERKs, as well as a conserved intron/exon structure that was unique to members of the SERK family. Expression analysis showed that GhSERK1 mRNA was present in all organs of cotton plants and at different developmental stages, but its transcripts were most abundant in reproductive organs. Compared with that of the male-fertile line, the level of GhSERK1 mRNA was lower in the anther of the male-sterile cotton line, in which the pollen development was defected. Taken together, these findings illustrated that the GhSERK1 play a critical role during the anther formation, and may also have a broad role in other aspects of plant development.

Receptor-like kinase OsASLRK regulates methylglyoxal response and content in rice

Receptor-like kinases(RLKs) are essential for plant abiotic stress responses. Methylglyoxal(MG) is a cellular metabolite that is often considered to be a stress signal molecule. However, limited information is available about the relationship between RLKs and MG. Here, we addressed the function of a receptor-like kinase, Os ASLRK, in the MG response and content in rice. A typical MG-responsive element(AAAAAAAA) exists in the promoter region of the OsASLRK gene. RTqPCR analysis indicated that the transcript level of OsASLRK was significantly increased by exogenous MG in a time-and dosage-dependent fashion. GUS staining also confirmed that the expression of Os ASLRK in rice root was enhanced by exogenous MG treatment. Genetic analysis suggested that the Osaslrk mutant displays increased sensitivity to MG and it showed higher endogenous MG content under exogenous MG treatments, while OsASLRK-overexpressing rice plants showed the opposite phenotypes. Diaminobenzidine(DAB) staining, scavenging enzyme activities and GSH content assays indicate that OsASLRK regulates MG sensitivity and content via the elevation of antioxidative enzyme activities and alleviation of membrane damage. Therefore, our results provide new evidence illustrating the roles that receptor-like kinase Os ASLRK plays in MG regulation in rice.

Role of activin receptor-like kinase 1 in vascular development and cerebrovascular diseases

Activin receptor-like kinase 1(ALK1)is a transmembrane serine/threonine receptor kinase of the transforming growth factor beta(TGFβ)receptor superfamily.ALK1 is specifically expressed in vascular endothelial cells,and its dynamic changes are closely related to the proliferation of endothelial cells,the recruitment of pericytes to blood vessels,and functional differentiation during embryonic vascular development.The pathophysiology of many cerebrovascular diseases is today understood as a disorder of endothelial cell function and an imbalance in the proportion of vascular cells.Indeed,mutations in ALK1 and its co-receptor endoglin are major genetic risk factors for vascular arteriovenous malformation.Many studies have shown that ALK1 is closely related to the development of cerebral aneurysms,arteriovenous malformations,and cerebral atherosclerosis.In this review,we describe the various roles of ALK1 in the regulation of angiogenesis and in the maintenance of cerebral vascular homeostasis,and we discuss its relationship to functional dysregulation in cerebrovascular diseases.This review should provide new perspectives for basic research on cerebrovascular diseases and offer more effective targets and strategies for clinical diagnosis,treatment,and prevention.

OsSRK1,an Atypical S-Receptor-Like Kinase Positively Regulates Leaf Width and Salt Tolerance in Rice

Receptor-like kinases(RLKs)are important for plant growth,development and defense responses.The S-receptor protein kinases(SRKs),which represent an RLK subfamily,control the selfincompatibility among Brassica species.However,little information is available regarding SRK functions in rice.We identified a gene OsSRK1 encoding an atypical SRK.The transcript of OsSRK1 was induced by abscisic acid(ABA),salt and polyethylene glycol.OsSRK1 localized to the plasma membrane and cytoplasm.Leaf width was increased in OsSRK1-overexpression(OsSRK1-OX)transgenic rice plants,likely because of an increase in cell number per leaf.Furthermore,the expression levels of OsCYCA3-1 and OsCYCD2-1,which encode positive regulators of cell division,were up-regulated in leaf primordium of OsSRK1-OX rice plants relative to those in wild type.Meanwhile,the expression level of OsKRP1,which encodes cell cycle inhibitor,was down-regulated in the OsSRK1-OX plants.Therefore,it is deduced that OsSRK1 regulates leaf width by promoting cell division in the leaf primordium.Additionally,OsSRK1-OX plants exhibited enhanced ABA sensitivity and salt tolerance compared with wild type.These results suggest that OsSRK1 plays important roles in leaf development and salt responses in rice.

Down-regulation of transforming growth factor β1/activin receptor-like kinase 1 pathway gene expression by herbal compound 861 is related to deactivation of LX-2 cells

AIM: To investigate the effect of herbal compound 861 (Cpd861) on the transforming growth factor-β1 (TGFβ1)/ activin receptor-like kinase 1 (ALK1, type Ⅰ receptor) signaling-pathway-related gene expression in the LX-2 cell line, and the inhibitory mechanism of Cpd861 on the activation of LX-2 cells. METHODS: LX-2 cells were treated with TGFβ1 (5 ng/mL) Cpd861 (0.1 mg/mL), TGFβ1 (5 ng/mL) plus Cpd861 (5 ng/mL) for 24 h to investigate the effect of Cpd861 on the TGFβ1/ALK1 pathway. Real-time PCR was performed to examine the expression of α-SMA (α-smooth muscle actin), ALK1, Id1 (inhibitor of differentiation 1). Western blotting was carried out to measure the levels of α-SMA and phosphorylated Smad1, and immunocytochemical analysis for the expression of α-SMA. RESULTS: In LX-2 cells, TGFβ1/ALK1-pathway-related gene expression could be stimulated by TGFβ1, which led to excessive activation of the cells. Cpd861 decreased the activation of LX-2 cells by reducing the expression of α-SMA mRNA and protein expression. This effect was related to inhibition of the above TGFβ1/ALK1-pathway- related expression of genes such as Id1 and ALK1, and phosphorylation of Smad1 in LX-2 cells, even with TGFβ1 co-treatment for 24 h. CONCLUSION: Cpd861 can restrain the activation of LX-2 cells by inhibiting the TGFβ1/ALK1/Smad1 pathway.

A pair of G-type lectin receptor-like kinases modulates nlp20-mediated immune responses by coupling to the RLP23 receptor complex

Plant cells recognize microbial patterns with the plasma-membrane-localized pattern-recognition receptors consisting mainly of receptor kinases(RKs) and receptor-like proteins(RLPs). RKs, such as bacterial flagellin receptor FLS2, and their downstream signaling components have been studied extensively. However, newly discovered regulatory components of RLP-mediated immune signaling, such as the nlp20 receptor RLP23, await identification. Unlike RKs, RLPs lack a cytoplasmic kinase domain, instead recruiting the receptor-like kinases(RLKs) BAK1 and SOBIR1. SOBIR1 specifically works as an adapter for RLP-mediated immunity. To identify new regulators of RLP-mediated signaling, we looked for SOBIR1-binding proteins(SBPs) in Arabidopsis thaliana using protein immunoprecipitation and mass spectrometry,identifying two G-type lectin RLKs, SBP1 and SBP2, that physically interacted with SOBIR1.SBP1 and SBP2 showed high sequence similarity,were tandemly repeated on chromosome 4, and also interacted with both RLP23 and BAK1. sbp1 sbp2 double mutants obtained via CRISPR-Cas9 gene editing showed severely impaired nlp20-induced reactive oxygen species burst, mitogenactivated protein kinase(MAPK) activation, and defense gene expression, but normal flg22-induced immune responses. We showed that SBP1 regulated nlp20-induced immunity in a kinase activityindependent manner. Furthermore, the nlp20-induced the RLP23–BAK1 interaction, although not the flg22-induced FLS2–BAK1 interaction, was significantly reduced in sbp1 sbp2. This study identified SBPs as new regulatory components in RLP23 receptor complex that may specifically modulate RLP23-mediated immunity by positively regulating the interaction between the RLP23 receptor and the BAK1 co-receptor.

Knights in Action： Lectin Receptor-Like Kinases in Plant Development and Stress Responses

The Receptor-Like Kinase （RLK） is a vast protein family with over 600 genes in Arabidopsis and 1100 in rice. The Lectin RLK （LecRLK） family is believed to play crucial roles in saccharide signaling as well as stress perception. All the LecRLKs possess three domains： an N-terminal lectin domain, an intermediate transmembrane domain, and a C-terminal kinase domain. On the basis of lectin domain variability, LecRLKs have been subgrouped into three subclasses： L-, G-, and C-type LecRLKs. While the previous studies on LecRLKs were dedicated to classification, comparative structural analysis and expression analysis by promoter-based studies, most of the recent studies on LecRLKs have laid special emphasis on the potential of this gene family in regulating biotic/abiotic stress and developmental pathways in plants, thus mak- ing the prospects of studying the LecRLK-mediated regulatory mechanism exceptionally promising. In this review, we have described in detail the LecRLK gene family with respect to a historical, evolutionary, and structural point of view. Furthermore, we have laid emphasis on the LecRLKs roles in development, stress conditions, and hormonal response. We have also discussed the exciting research prospects offered by the current knowledge on the LecRLK gene family. The multitude of the LecRLK gene family members and their functional diversity mark these genes as both interesting and worthy candidates for further analysis, especially in the field of crop improvement.

Receptor-Like Kinases in Plant Innate Immunity

Plants employ a highly effective surveillance system to detect potential pathogens, which is critical for the success of land plants in an environment surrounded by numerous microbes. Recent efforts have led to the identification of a number of immune receptors and components of immune receptor complexes. It is now clear that receptor-like kinases （RLKs） and receptor-like proteins （RLPs） are key pattern-recognition receptors （PRRs） for microbe- and plant-derived molecular patterns that are associated with pathogen invasion. RLKs and RLPs involved in immune signaling belong to large gene families in plants and have undergone lineage specific expansion. Molecular evolution and population studies on phytopathogenic molecular signatures and their receptors have provided crucial insight into the co-evolution between plants and pathogens.

Genome-Wide Expression Pattern Analyses of the Arabidopsis Leucine-Rich Repeat Receptor-Like Kinases

Receptor-like protein kinases （RLKs） are a large group of transmembrane proteins playing critical roles in cell-cell and cell--environment communications. Based on extracellular domain structures, RLKs were classified into more than 21 subfamilies, among which leucine-rich repeat RLKs （LRR-RLKs） belong to the largest subfamily in plants such as Arabidopsis and rice. In Arabidopsis, there are approximately 223 LRR-RLKs, but only about 60 of which have been functionally described to date. To systematically investigate the roles of LRR-RLKs in regulating plant growth, development, and stress adaptations, we generated promoter：：GUS transgenic plants for all 223 LRR-RLK genes in Arabidopsis and analyzed their detailed expression patterns at various developmental stages. The results provide valuable resources for functionally elucidating this large and essential signaling protein subfamily.

Global Analysis of Expression Profiles of Rice Receptor-Like Kinase Genes

The receptor-like kinases （RLKs） play critical roles in plant development and response to stress stimuli. By perceiving or sensing the extracellular signals, RLK activates the downstream signaling pathway through phosphorylating the specific targets. Up to now, only a few RLKs have been functionally identified, which are even fewer in rice （Oryza sativa L.）. We here report the systemic analysis of the expression profiles of rice RLK coding genes in different tissues, with the emphasis on seed development and in response to both abiotic stress and plant hormones. The results showed that most rice RLK genes are expressed in two or more tissues, of which the RLCK-RLKs have a higher, while WAK- and SD-RLKs have a lower, expression level in the vegetative tissues than other subfamily members. Interestingly, the constitutively highly expressed RLKs in rice and Arabidopsis are conserved, which is consistent with the previous hypothesis that RLKs existed before the differentiation of monocotyledon and dicotyledon plants. Nearly one-third of the detected rice RLKs are expressed during seed development, and the RLCK-RLK members possess a higher percentage during the endosperm de- velopment, suggesting a novel function of RLCK-RLK members in endosperm development. Further analysis revealed that many RLK genes expressed during seed development are also regulated by abiotic stresses （cold, salt, or drought） or hor- mones, indicating that RLKs may take part in the stress-related signaling pathways such as dehydration of endosperm. These results provide informative insights into the RLK studies and will be helpful to reveal the global regulatory network controlling rice seed development.

SERK Family Receptor-like Kinases Function as Co-receptors with PXY for Plant Vascular Development

In Arabidopsis, the CLAVATA3/EMBRYO SURROUNDING REGION-RELATED （CLE） peptides play important roles in regulating proliferation and differentiation of plant-specific stem cells. Although receptors of CLEs are reported to be leucine-rich repeat receptor kinases, the mechanisms underlying CLE-induced receptor activation remain largely unknown. Here we show that SOMATIC EMBRYOGENESIS RECEPTOR KINASEs （SERKs） serve as co-receptors in CLE41/TDIF-PXY signaling to regulate plant vascular development. TDIF induces interaction of its receptor PXY with SERKs in vitro and in vivo. Furthermore, the serk1-1 serk2-1 bakl-5 mutant plants are less sensitive to TDIF, phenocopying the pxy mutant with a compromised promotion of procambial cell proliferation. Crystal structure of the PXY-TDIF-SERK2 complex reveals that the last amino acid of TDIF conserved among CLEs and other evolutionary-related peptides is important for the interaction between SERK2 and PXY. Taken together, our current study identifies SERKs as signaling components of the TDIF-PXY pathway and suggests a conserved activation mechanism of CLE receptors.

Splicing of Receptor-Like Kinase-Encoding SNC4 and CERK1 is Regulated by Two Conserved Splicing Factors that Are Required for Plant Immunity

Plant immune receptors belonging to the receptor-like kinase （RLK） family play important roles in the recog- nition of microbial pathogens and activation of downstream defense responses. The Arabidopsis mutant snc4-1D con- tains a gain-of-function mutation in the RLK SNC4 （SUPPRESSOR OF NPRI-1, CONSTITUTIVE4）, which leads to constitutive activation of defense responses. Analysis of suppressor mutants of snc4-1D identified two conserved splicing factors, SUA （SUPPRESSOR OF ABI3-5） and RSN2 （REQUIRED FOR SNC4-1D 2）, that are required for the constitutive defense responses in snc4-1D. In sua and rsn2 mutants, SNC4 splicing is altered and the amount of 5NC4 transcripts is reduced. Further analysis showed that SUA and RSN2 are also required for the proper splicing of CERK1 （CHITIN ELICITOR RECEPTOR KINASE1）, which encodes another RLK that functions as a receptor for chitin. In sua and rsn2 mutants, induction of reactive oxygen species by chitin is reduced and the non-pathogenic bacteria Pseudomonas syringae pv. tomato DC3OOOhrcC grows to higher titers than in wild-type plants. Our study suggests that pre-mRNA splicing plays important roles in the regulation of plant immunity mediated by the RLKs SNC4 and CERK1.

Receptor-like kinases and receptor-like proteins： keys to pathogen recognition and defense signaling in plant innate immunity

Plants have evolved multiple layers of defense against various pathogens in the environment. Receptor-like kinases/proteins （RLKs/RLPs） are on the front lines of the battle between plants and pathogens since they are present at the plasma membrane and perceive signature molecules from either the invading pathogen or damaged plant tissue. With a few notable exceptions, most RLKs/RLPs are positive regulators of plant innate immunity. In this review, we summarize recently discovered RLKs/RLPs that are involved in plant defense responses against various classes of pathogens, We also describe what is currently known about the mechanisms of RLK-mediated initiation of signaling via protein-protein interactions and phosphorylation.

Notes from the Underground:Receptor-Like Kinases in Arabidopsis Root Development

During plant development, the frequency and context of cell division must be controlled, and cells must differentiate properly to perform their mature functions. In addition, stem cell niches need to be maintained as a reservoir for new cells. All of these processes require intercellular signaling, whether it is a cell relaying its position to other cells, or more mature cells signaling to the stem cell niche to regulate the rate of growth. Receptor-like kinases have emerged as a major component in these diverse roles, especially within the Arabidopsis root. In this review, the functions of receptor-like kinase signaling in regulating Arabidopsis root development will be examined in theareas of root apical meristem maintenance, regulation of epidermal cell fate, lateral root development and vascular differentiation.

Receptor-Like Kinases:Key Regulators of Plant Development and Defense

Plants are multi-cellular organisms that live in diverse and fluctuating environments. Cell-cell and cell-environment com- munication are therefore critical to plant growth and develop- ment. In animals, transmembrane receptor protein tyrosine kinases play significant roles in cell-cell signaling. There was a great deal of surprise in the plant community, however, when the first receptor-like protein kinase （RLK） was isolated from maize by John C.

Receptor-like kinase OsCR4 controls leaf morphogenesis and embryogenesis by fixing the distribution of auxin in rice

Cell differentiation is a key event in organ development;it involves auxin gradient formation, cell signaling, and transcriptional regulation. Yet, how these processes are orchestrated during leaf morphogenesis is poorly understood. Here, we demonstrate an essential role for the receptor-like kinase Os CR4 in leaf development. oscr4 loss-of-function mutants displayed short shoots and roots, with tiny,crinkly, or even dead leaves. The delayed outgrowth of the first three leaves and seminal root in oscr4 was due to defects in plumule and radicle formation during embryogenesis. The deformed epidermal,mesophyll, and vascular tissues observed in oscr4 leaves arose at the postembryo stage;the corresponding expression pattern of pro Os CR4:GUS in embryos and young leaves suggests that Os CR4 functions in these tissues. Signals from the auxin reporter DR5 rev:VENUS were found to be altered in oscr4 embryos and disorganized in oscr4 leaves, in which indole-3-acetic acid accumulation was further revealed by immunofluorescence. Os WOX3 A, which is auxin responsive and related to leaf development,was activated extensively and ectopically in oscr4 leaves, partially accounting for the observed lack of cell differentiation. Our data suggest that Os CR4 plays a fundamental role in leaf morphogenesis and embryogenesis by fixing the distribution of auxin.

A G-type lectin receptor-like kinase regulates the perception of oomycete apoplastic expansin-like proteins

Phytophthora capsici is one of the most harmful pathogens in agriculture, which threatens the safe production of multiple crops and causes serious economic losses worldwide. Here, we identified a P. capsici expansin-like protein, Pc EXLX1, by liquid chromatography-tandem mass spectrometry from Nicotiana benthamiana apoplastic fluid infected with P. capsici. Clustered regularly interspaced short palindromic repeats/crispr associated protein9(CRISPR/Cas9)-mediated Pc EXLX1 knockout mutants exhibited significantly enhanced virulence,while the overexpression of Pc EXLX1 impaired the virulence. Prokaryotically expressed Pc EXLX1 activated multiple plant immune responses, which were BRI1-associated kinase 1(BAK1)-and suppressor of BIR1-1(SOBIR1)-dependent. Furthermore, overexpression of Pc EXLX1 homologs in N. benthamiana could also increase plant resistance to P. capsici. A G-type lectin receptor-like kinase from N. benthamiana, expansin-regulating kinase 1(ERK1), was shown to regulate the perception of Pc EXLX1 and positively mediate the plant resistance to P. capsici. These results reveal that the expansin-like protein, Pc EXLX1, is a novel apoplastic effector with plant immunity-inducing activity of oomycetes, perception of which is regulated by the receptor-like kinase, ERK1.

Receptor-like Kinases in Root Development:Current Progress and Future Directions

Cell-to-cell and cell-to-environment communications are critical to the growth and development of plants.Cell surface-localized receptor-like kinases(RLKs)are mainly involved in sensing various extracellular signals to initiate their corresponding cellular responses.As important vegetative organs for higher plants to adapt to a terrestrial living situation,roots play a critical role for the survival of plants.It has been demonstrated that RLKs control many biological processes during root growth and development.In this review,we summarize several key regulatory processes during Arabidopsis root developm ent in which RLKs play critical roles.We also put forward a number of relevant questions that are required to be explored in future studies.

Rapid responses:Receptor-like kinases directly regulate the functions of membrane transport proteins in plants

Receptor-like kinases(RLKs)are a large group of plant-specific transmembrane proteins mainly acting as receptors or co-receptors of various extracellular signals.They usually turn extracellular signals into intracellular responses via altering gene expression profiles.However,recent studies confirmed that many RLKs can physically interact with diverse membrane-localized transport proteins and regulate their activities for speedy responses in limited tissues or cells.In this minireview,we highlight recent discoveries regarding how RLKs can work with membrane transport proteins collaboratively and thereby trigger cellular responses in a precise and rapid manner.It is anticipated that such regulation broadly presents in plants and more examples will be gradually revealed when in-depth analyses are conducted for the functions of RLKs.