RHO OF PLANTS signalling and the activating ROP GUANINE NUCLEOTIDE EXCHANGE FACTORS: specificity in cellular signal transduction in plants.

Every cell constantly receives signals from its neighbours or the environment. In plants, most signals are perceived by RECEPTOR-LIKE KINASEs (RLKs) and then transmitted into the cell. The molecular switches RHO OF PLANTS (ROP) are critical proteins for polar signal transduction and regulate multiple cell polarity processes downstream of RLKs. Many ROP-regulating proteins and scaffold proteins of the ROP complex are known. However, the spatiotemporal ROP signalling complex composition is not yet understood. Moreover, how specificity is achieved in different ROP signalling pathways within one cell still needs to be determined. This review gives an overview of recent advances in ROP signalling and how specificity by downstream scaffold proteins can be achieved. The composition of the ROP signalling complexes is discussed, focusing on the possibility of the simultaneous presence of ROP activators and inactivators within the same complex to balance ROP activity. Furthermore, this review highlights the function of plant-specific ROP GUANINE NUCLEOTIDE EXCHANGE FACTORS polarizing ROP signalling and defining the specificity of the initiated ROP signalling pathway.

A Leucine-Rich Repeat Receptor-like Kinase TaBIR1 Contributes to Wheat Resistance against Puccinia striiformis f. sp. tritici.

Plant cell surface-localized receptor-like kinases (RLKs) recognize invading pathogens and transduce the immune signals inside host cells, subsequently triggering immune responses to fight off pathogen invasion. Nonetheless, our understanding of the role of RLKs in wheat resistance to the biotrophic fungus Puccinia striiformis f. sp. tritici (Pst) remains limited. During the differentially expressed genes in Pst infected wheat leaves, a Leucine-repeat receptor-like kinase (LRR-RLK) gene TaBIR1 was significantly upregulated in the incompatible wheat-Pst interaction. qRT-PCR verified that TaBIR1 is induced at the early infection stage of Pst. The transient expression of TaBIR1-GFP protein in N. bentamiana cells and wheat mesophyll protoplasts revealed its plasma membrane location. The knockdown of TaBIR1 expression by VIGS (virus induced gene silencing) declined wheat resistance to stripe rust, resulting in reduced reactive oxygen species (ROS) production, callose deposition, and transcripts of pathogenesis-related genes TaPR1 and TaPR2, along with increased Pst infection area. Ectopic overexpression of TaBIR1 in N. benthamiana triggered constitutive immune responses with significant cell death, callose accumulation, and ROS production. Moreover, TaBIR1 triggered immunity is dependent on NbBAK1, the silencing of which significantly attenuated the defense response triggered by TaBIR1. TaBIR1 interacted with the NbBAK1 homologues in wheat, co-receptor TaSERK2 and TaSERK5, the transient expression of which could restore the impaired defense due to NbBAK1 silencing. Taken together, TaBIR1 is a cell surface RLK that contributes to wheat stripe rust resistance, probably as a positive regulator of plant immunity in a BAK1-dependent manner.

Pan-brassinosteroid signaling revealed by functional analysis of NILR1 in land plants.

Brassinosteroid (BR) signaling has been identified from the ligand BRs sensed by the receptor Brassinosteroid Insensitive 1 (BRI1) to the final activation of Brassinozole Resistant 1/bri1 EMS-Suppressor 1 through a series of transduction events. Extensive studies have been conducted to characterize the role of BR signaling in various biological processes. Our previous study has shown that Excess Microsporocytes 1 (EMS1) and BRI1 control different aspects of plant growth and development via conserved intracellular signaling. Here, we reveal that another receptor, NILR1, can complement the bri1 mutant in the absence of BRs, indicating a pathway that resembles BR signaling activated by NILR1. Genetic analysis confirms the intracellular domains of NILR1, BRI1 and EMS1 have a common signal output. Furthermore, we demonstrate that NILR1 and BRI1 share the coreceptor BRI1 Associated Kinase 1 and substrate BSKs. Notably, the NILR1-mediated downstream pathway is conserved across land plants. In summary, we provide evidence for the signaling cascade of NILR1, suggesting pan-brassinosteroid signaling initiated by a group of distant receptor-ligand pairs in land plants.

PERKing up our understanding of the proline-rich extensin-like receptor kinases, a forgotten plant receptor kinase family.

Proline-rich extensin-like receptor kinases (PERKs) are an important class of receptor-like kinases (RLKs) containing an extracellular proline-rich domain. While they are thought to be putative sensors of the cell wall integrity, there are very few reports on their biological functions in the plant, as compared with other RLKs. Several studies support a role for PERKs in plant growth and development, but their effect on the cell wall composition to regulate cell expansion is still lacking. Gene expression data suggest that they may intervene in response to environmental changes, in agreement with their subcellular localization. And there is growing evidence for PERKs as novel sensors of environmental stresses such as insects and viruses. However, little is known about their precise role in plant immunity and in the extracellular network of RLKs, as no PERK-interacting RLK or any coreceptor has been identified as yet. Similarly, their signaling activities and downstream signaling components are just beginning to be deciphered, including Ca2+ fluxes, reactive oxygen species accumulation and phosphorylation events. Here we outline emerging roles for PERKs as novel sensors of environmental stresses, and we discuss how to better understand this overlooked class of receptor kinases via several avenues of research.

Acquisition of a complex root microbiome reshapes the transcriptomes of rice plants.

Soil microorganisms can colonize plant roots and assemble in communities engaged in symbiotic relationships with their host. Though the compositional dynamics of root-associated microbiomes have been extensively studied, the host transcriptional response to these communities is poorly understood. Here, we developed an experimental system by which rice plants grown under axenic conditions can acquire a defined endosphere microbiome. Using this setup, we performed a cross-sectional characterization of plant transcriptomes in the presence or absence of a complex microbial community. To account for compositional variation, plants were inoculated with soil-derived microbiomes harvested from three distinct agricultural sites. Soil microbiomes triggered a major shift in the transcriptional profiles of rice plants that included the downregulation of one-third to one-fourth of the families of leucine-rich repeat receptor-like kinases and nucleotide-binding leucine-rich repeat receptors expressed in roots. Though the expression of several genes was consistent across all soil sources, a large fraction of this response was differentially impacted by soil type. These results demonstrate the role of root microbiomes in sculpting the transcriptomes of host plants and highlight the potential involvement of the two main receptor families of the plant immune system in the recruitment and maintenance of an endosphere microbiome.

A molecular roadmap to the plant immune system.

Plant diseases caused by pathogens and pests are a constant threat to global food security. Direct crop losses and the measures used to control disease (e.g. application of pesticides) have significant agricultural, economic, and societal impacts. Therefore, it is essential that we understand the molecular mechanisms of the plant immune system, a system that allows plants to resist attack from a wide variety of organisms ranging from viruses to insects. Here, we provide a roadmap to plant immunity, with a focus on cell-surface and intracellular immune receptors. We describe how these receptors perceive signatures of pathogens and pests and initiate immune pathways. We merge existing concepts with new insights gained from recent breakthroughs on the structure and function of plant immune receptors, which have generated a shift in our understanding of cell-surface and intracellular immunity and the interplay between the two. Finally, we use our current understanding of plant immunity as context to discuss the potential of engineering the plant immune system with the aim of bolstering plant defenses against disease.

Receptors in the Induction of the Plant Innate Immunity.

Plants adjust amplitude and duration of immune responses via different strategies to maintain growth, development, and resistance to pathogens. Pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) play vital roles. Pattern recognition receptors, comprising a large number of receptor-like protein kinases and receptor-like proteins, recognize related ligands and trigger immunity. PTI is the first layer of the innate immune system, and it recognizes PAMPs at the plasma membrane to prevent infection. However, pathogens exploit effector proteins to bypass or directly inhibit the PTI immune pathway. Consistently, plants have evolved intracellular nucleotide-binding domain and leucine-rich repeat-containing proteins to detect pathogenic effectors and trigger a hypersensitive response to activate ETI. PTI and ETI work together to protect plants from infection by viruses and other pathogens. Diverse receptors and the corresponding ligands, especially several pairs of well-studied receptors and ligands in PTI immunity, are reviewed to illustrate the dynamic process of PTI response here.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

IDA (INFLORESCENCE DEFICIENT IN ABSCISSION)-like peptides and HAE (HAESA)-like receptors regulate corolla abscission in Nicotiana benthamiana flowers.

BACKGROUND: Abscission is an active, organized, and highly coordinated cell separation process enabling the detachment of aerial organs through the modification of cell-to-cell adhesion and breakdown of cell walls at specific sites on the plant body known as abscission zones. In Arabidopsis thaliana, abscission of floral organs and cauline leaves is regulated by the interaction of the hormonal peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), a pair of redundant receptor-like protein kinases, HAESA (HAE) and HAESA-LIKE2 (HSL2), and SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) co-receptors. However, the functionality of this abscission signaling module has not yet been demonstrated in other plant species. RESULTS: The expression of the pair of NbenIDA1 homeologs and the receptor NbenHAE.1 was supressed at the base of the corolla tube by the inoculation of two virus-induced gene silencing (VIGS) constructs in Nicotiana benthamiana. These gene suppression events arrested corolla abscission but did not produce any obvious effect on plant growth. VIGS plants retained a higher number of corollas attached to the flowers than control plants, an observation related to a greater corolla breakstrength. The arrest of corolla abscission was associated with the preservation of the parenchyma tissue at the base of the corolla tube that, in contrast, was virtually collapsed in normal corollas. In contrast, the inoculation of a viral vector construct that increased the expression of NbenIDA1A at the base of the corolla tube negatively affected the growth of the inoculated plants accelerating the timing of both corolla senescence and abscission. However, the heterologous ectopic overexpression of citrus CitIDA3 and Arabidopsis AtIDA in N. benthamiana did not alter the standard plant phenotype suggesting that the proteolytic processing machinery was unable to yield active peptides. CONCLUSION: Here, we demonstrate that the pair of NbenIDA1 homeologs encoding small peptides of the IDA-like family and the receptor NbenHAE.1 control cellular breakdown at the base of the corolla tube awhere an adventitious AZ should be formed and, therefore, corolla abscission in N. benthamiana flowers. Altogether, our results provide the first evidence supporting the notion that the IDA-HAE/HSL2 signaling module is conserved in angiosperms.

A new method to visualize CEP hormone-CEP receptor interactions in vascular tissue in vivo.

C-TERMINALLY ENCODED PEPTIDEs (CEPs) control diverse responses in plants including root development, root system architecture, nitrogen demand signalling, and nutrient allocation that influences yield, and there is evidence that different ligands impart different phenotypic responses. Thus, there is a need for a simple method that identifies bona fide CEP hormone-receptor pairings in vivo and examines whether different CEP family peptides bind the same receptor. We used formaldehyde or photoactivation to cross-link fluorescently tagged group 1 or group 2 CEPs to receptors in semi-purified Medicago truncatula or Arabidopsis thaliana leaf vascular tissues to verify that COMPACT ROOT ARCHITECTURE 2 (CRA2) is the Medicago CEP receptor, and to investigate whether sequence diversity within the CEP family influences receptor binding. Formaldehyde cross-linked the fluorescein isothiocyanate (FITC)-tagged Medicago group 1 CEP (MtCEP1) to wild-type Medicago or Arabidopsis vascular tissue cells, but not to the CEP receptor mutants, cra2 or cepr1. Binding competition showed that unlabelled MtCEP1 displaces FITC-MtCEP1 from CRA2. In contrast, the group 2 CEP, FITC-AtCEP14, bound to vascular tissue independently of CEPR1 or CRA2, and AtCEP14 did not complete with FITC-MtCEP1 to bind CEP receptors. The binding of a photoactivatable FITC-MtCEP1 to the periphery of Medicago vascular cells suggested that CRA2 localizes to the plasma membrane. We separated and visualized a fluorescent 105 kDa protein corresponding to the photo-cross-linked FITC-MtCEP1-CRA2 complex using SDS-PAGE. Mass spectrometry identified CRA2-specific peptides in this protein band. The results indicate that FITC-MtCEP1 binds to CRA2, MtCRA2 and AtCEPR1 are functionally equivalent, and the binding specificities of group 1 and group 2 CEPs are distinct. Using formaldehyde or photoactivated cross-linking of biologically active, fluorescently tagged ligands may find wider utility by identifying CEP-CEP receptor pairings in diverse plants.

The First Line of Defense: Receptor-like Protein Kinase-Mediated Stomatal Immunity.

Stomata regulate gas and water exchange between the plant and external atmosphere, which are vital for photosynthesis and transpiration. Stomata are also the natural entrance for pathogens invading into the apoplast. Therefore, stomata play an important role in plants against pathogens. The pattern recognition receptors (PRRs) locate in guard cells to perceive pathogen/microbe-associated molecular patterns (PAMPs) and trigger a series of plant innate immune responses, including rapid closure of stomata to limit bacterial invasion, which is termed stomatal immunity. Many PRRs involved in stomatal immunity are plasma membrane-located receptor-like protein kinases (RLKs). This review focuses on the current research progress of RLK-mediated signaling pathways involved in stomatal immunity, and discusses questions that need to be addressed in future research.

Protein kinases in plant responses to drought, salt, and cold stress.

Protein kinases are major players in various signal transduction pathways. Understanding the molecular mechanisms behind plant responses to biotic and abiotic stresses has become critical for developing and breeding climate-resilient crops. In this review, we summarize recent progress on understanding plant drought, salt, and cold stress responses, with a focus on signal perception and transduction by different protein kinases, especially sucrose nonfermenting1 (SNF1)-related protein kinases (SnRKs), mitogen-activated protein kinase (MAPK) cascades, calcium-dependent protein kinases (CDPKs/CPKs), and receptor-like kinases (RLKs). We also discuss future challenges in these research fields.

The wild strawberry kinome: identification, classification and transcript profiling of protein kinases during development and in response to gray mold infection.

BACKGROUND: Protein kinases (PKs) play an important role in signaling cascades and are one of the largest and most conserved protein super families in plants. Despite their importance, the woodland strawberry (Fragaria vesca) kinome and expression patterns of PK genes remain to be characterized. RESULTS: Here, we report on the identification and classification of 954 Fragaria vesca PK genes, which were classified into nine groups and 124 gene families. These genes were distributed unevenly among the seven chromosomes, and the number of introns per gene varied from 0 to 47. Almost half of the putative PKs were predicted to localize to the nucleus and 24.6% were predicted to localize to the cell membrane. The expansion of the woodland strawberry PK gene family occurred via different duplication mechanisms and tandem duplicates occurred relatively late as compared to other duplication types. Moreover, we found that tandem and transposed duplicated PK gene pairs had undergone stronger diversifying selection and evolved relatively faster than WGD genes. The GO enrichment and transcriptome analysis implicates the involvement of strawberry PK genes in multiple biological processes and molecular functions in differential tissues, especially in pollens. Finally, 109 PKs, mostly the receptor-like kinases (RLKs), were found transcriptionally responsive to Botrytis cinerea infection. CONCLUSIONS: The findings of this research expand the understanding of the evolutionary dynamics of PK genes in plant species and provide a potential link between cell signaling pathways and pathogen attack.

Multiple N-glycans cooperate in balancing misfolded BRI1 secretion and ER retention.

KEY MESSAGE: N-glycans play a protective or monitoring role according to the folding state of associated protein or the distance from structural defects. Asparagine-linked (Asn/N-) glycosylation is one of the most prevalent and complex protein modifications and the associated N-glycans play crucial roles on protein folding and secretion. The studies have shown that many glycoproteins hold multiple N-glycans, yet little is known about the redundancy of N-glycans on a protein. In this study, we used BRI1 to decipher the roles of N-glycans on protein secretion and function. We found that all 14 potential N-glycosylation sites on BRI1 were occupied with oligosaccharides. The elimination of single N-glycan had no obvious effect on BRI1 secretion or function except N154-glycan, which resulted in the retention of BRI1 in the endoplasmic reticulum (ER), similar to the loss of multiple highly conserved N-glycans. To misfolded bri1, the absence of N-glycans next to local structural defects enhanced the ER retention and the artificial addition of N-glycan could help the misfolded bri1-GFPs exiting from the ER, indicating that the N-glycans might serve as steric hindrance to protect the structure defects from ER recognition. We also found that the retention of misfolded bri1-9 by lectins and chaperones in the ER relied on the presence of multiple N-glycans distal to the local defects. Our findings revealed that the N-glycans might play a protective or monitoring role according to the folding state of associated protein or the distance from structural defects.

Differential expression of IDA (INFLORESCENCE DEFICIENT IN ABSCISSION)-like genes in Nicotiana benthamiana during corolla abscission, stem growth and water stress.

BACKGROUND: IDA (INFLORESCENCE DEFICIENT IN ABSCISSION)-like signaling peptides and the associated HAE (HAESA)-like family of receptor kinases were originally reported in the model plant Arabidopsis thaliana (Arabidopsis) to be deeply involved in the regulation of abscission. Actually, IDA peptides, as cell-to-cell communication elements, appear to be implicated in many developmental processes that rely on cell separation events, and even in the responses to abiotic stresses. However, the knowledge related to the molecular machinery regulating abscission in economically important crops is scarce. In this work, we determined the conservation and phylogeny of the IDA-like and HAE-like gene families in relevant species of the Solanaceae family and analyzed the expression of these genes in the allopolyploid Nicotiana benthamiana, in order to identify members involved in abscission, stem growth and in the response to drought conditions. RESULTS: The phylogenetic relationships among the IDA-like members of the Solanaceae studied, grouped the two pairs of NbenIDA1 and NbenIDA2 protein homeologs with the Arabidopsis prepropeptides related to abscission. Analysis of promoter regions searching for regulatory elements showed that these two pairs of homeologs contained both hormonal and drought response elements, although NbenIDA2A lacked the hormonal regulatory elements. Expression analyses showed that the pair of NbenIDA1 homeologs were upregulated during corolla abscission. NbenIDA1 and NbenIDA2 pairs showed tissue differential expression under water stress conditions, since NbenIDA1 homeologs were highly expressed in stressed leaves while NbenIDA2 homeologs, especially NbenIDA2B, were highly expressed in stressed roots. In non-stressed active growing plants, nodes and internodes were the tissues with the highest expression levels of all members of the IDA-like family and their putative HAE-like receptors. CONCLUSION: Our results suggest that the pair of NbenIDA1 homeologs are involved in the natural process of corolla abscission while both pairs of NbenIDA1 and NbenIDA2 homeologs are implicated in the response to water stress. The data also suggest that IDA peptides may be important during stem growth and development. These results provide additional evidence that the functional module formed by IDA peptides and its receptor kinases, as defined in Arabidopsis, may also be conserved in Solanaceae.

Receptor-Like Protein Kinases Function Upstream of MAPKs in Regulating Plant Development.

Mitogen-activated protein kinases (MAPKs) are a group of protein kinase broadly involved in various signal pathways in eukaryotes. In plants, MAPK cascades regulate growth, development, stress responses and immunity by perceiving signals from the upstream regulators and transmitting the phosphorylation signals to the downstream signaling components. To reveal the interactions between MAPK cascades and their upstream regulators is important for understanding the functional mechanisms of MAPKs in the life span of higher plants. Typical receptor-like protein kinases (RLKs) are plasma membrane-located to perceive endogenous or exogenous signal molecules in regulating plant growth, development and immunity. MAPK cascades bridge the extracellular signals and intracellular transcription factors in many RLK-mediated signaling pathways. This review focuses on the current findings that RLKs regulate plant development through MAPK cascades and discusses questions that are worth investigating in the near future.

Receptor-Like Kinase Phosphorylation of Arabidopsis Heterotrimeric G-Protein Gα -Subunit AtGPA1.

As molecular on-off switches, heterotrimeric G protein complexes, comprised of a Gα subunit and an obligate Gßγ dimer, transmit extracellular signals received by G protein-coupled receptors (GPCRs) to cytoplasmic targets that respond to biotic and abiotic stimuli. Signal transduction is modulated by phosphorylation of GPCRs and G protein complexes. In Arabidopsis thaliana, the Gα subunit AtGPA1 is phosphorylated by the receptor-like kinase (RLK) BRI1-associated Kinase 1 (BAK1), but the extent that other RLKs phosphorylates AtGPA1 is unknown. Twenty-two trans-phosphorylation sites on AtGPA1 are mapped by 12 RLKs hypothesized to act in the Arabidopsis G protein signaling pathway. Cis-phosphorylation sites are also identified on these RLKs, some newly shown to be dual specific kinases. Multiple sites are present in the core AtGPA1 functional units, including pSer52 and/or pThr53 of the conserved P-loop that directly binds nucleotide/phosphate, pThr164, and pSer175 from αE helix in the intramolecular domain interface for nucleotide exchange and GTP hydrolysis, and pThr193 and/or pThr194 in Switch I (SwI) that coordinates nucleotide exchange and protein partner binding. Several AtGPA1 S/T phosphorylation sites are potentially nucleotide-dependent phosphorylation patterns, such as Ser52/Thr53 in the P-loop and Thr193 and/or Thr194 in SwI.

NtRLK5, a novel RLK-like protein kinase from Nitotiana tobacum, positively regulates drought tolerance in transgenic Arabidopsis.

Receptor-like protein kinase (RLKs) plays pivotal roles in plant growth and development as well as stress responses. However, little is known about the function of RLKs in Nitotiana tobacum. In the present study, we present data on NtRLK5, a novel RLK-like gene isolated from Hongda (Nitotiana tobacum L.). Expression profile analysis revealed that NtRLK5 was strongly induced by drought and salt stresses. Transient expression of NtRLK5-GFP fusion protein in protoplast showed that NtRLK5 was localized to plasma membrane. Overexpression of NtRLK5 conferred enhanced drought tolerance in transgenic Arabidopsis plants, which was attributed to not only the lower malondialdehyde (MDA) and H2O2 contents, but also the higher antioxidant enzymes activities. Moreover, the expression of several antioxidation- and stress-related genes was also significantly up-regulated in NtRLK5 transgenic plants under drought condition. Taken together, the results suggest that NtRLK5 functions as a positive regulator in drought tolerance.

Genome-wide identification and characterization of LRR-RLKs reveal functional conservation of the SIF subfamily in cotton (Gossypium hirsutum).

BACKGROUND: As one of the largest subfamilies of the receptor-like protein kinases (RLKs) in plants, Leucine Rich Repeats-RLKs (LRR-RLKs) are involved in many critical biological processes including growth, development and stress responses in addition to various physiological roles. Arabidopsis contains 234 LRR-RLKs, and four members of Stress Induced Factor (SIF) subfamily (AtSIF1-AtSIF4) which are involved in abiotic and biotic stress responses. Herein, we aimed at identification and functional characterization of SIF subfamily in cultivated tetraploid cotton Gossypium hirsutum. RESULTS: Genome-wide analysis of cotton LRR-RLK gene family identified 543 members and phylogenetic analysis led to the identification of 6 cotton LRR-RLKs with high homology to Arabidopsis SIFs. Of the six SIF homologs, GhSIF1 is highly conserved exhibiting 46-47% of homology with AtSIF subfamily in amino acid sequence. The GhSIF1 was transiently silenced using Virus-Induced Gene Silencing system specifically targeting the 3' Untranslated Region. The transiently silenced cotton seedlings showed enhanced salt tolerance compared to the control plants. Further, the transiently silenced plants showed better growth, lower electrolyte leakage, and higher chlorophyll and biomass contents. CONCLUSIONS: Overall, 543 LRR-RLK genes were identified using genome-wide analysis in cultivated tetraploid cotton G. hirsutum. The present investigation also demonstrated the conserved salt tolerance function of SIF family member in cotton. The GhSIF1 gene can be knocked out using genome editing technologies to improve salt tolerance in cotton.

BAM 1 and RECEPTOR-LIKE PROTEIN KINASE 2 constitute a signaling pathway and modulate CLE peptide-triggered growth inhibition in Arabidopsis root.

Ligand receptor-based signaling is a means of cell-to-cell communication for coordinating developmental and physiological processes in multicellular organisms. In plants, cell-producing meristems utilize this signaling to regulate their activities and ensure for proper development. Shoot and root systems share common requirements for carrying out this process; however, its molecular basis is largely unclear. It has been suggested that synthetic CLV3/EMBRYO SURROUNDING REGION (CLE) peptide shrinks the root meristem through the actions of CLAVATA2 (CLV2) and the RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2) pathway in Arabidopsis thaliana. Our genetic screening for mutations that resist CLE peptide signaling in roots determined that BAM1, which is a member of the leucine-rich repeat receptor-like kinase (LRR-RLK) family, is also involved in this pathway. BAM1 is preferentially expressed in the root tip, including the quiescent center and its surrounding stem cells. Our genetic analysis revealed that BAM1 functions together with RPK2. Using coimmunoprecipitation assay, we showed that BAM1 is capable of forming heteromeric complexes with RPK2. These findings suggest that the BAM1 and RPK2 receptors constitute a signaling pathway that modulates cell proliferation in the root meristem and that related molecules are employed in root and shoot meristems.