Arabidopsis Receptor-like Kinase Research Articles

Evidence for intermolecular interactions between the intracellular domains of the arabidopsis receptor-like kinase ACR4, its homologs and the Wox5 transcription factor.

Arabidopsis CRINKLY4 (ACR4) is a receptor-like kinase (RLK) involved in the global development of the plant. The Arabidopsis genome encodes four homologs of ACR4 that contain sequence similarity and analogous architectural elements to ACR4, termed Arabidopsis CRINKLY4 Related (AtCRRs) proteins. Additionally, a signaling module has been previously proposed including a postulated peptide ligand, CLE40, the ACR4 RLK, and the WOX5 transcription factor that engage in a possible feedback mechanism controlling stem cell differentiation. However, little biochemical evidence is available to ascertain the molecular aspects of receptor heterodimerization and the role of phosphorylation in these interactions. Therefore, we have undertaken an investigation of the in vitro interactions between the intracellular domains (ICD) of ACR4, the CRRs and WOX5. We demonstrate that interaction can occur between ACR4 and all four CRRs in the unphosphorylated state. However, phosphorylation dependency is observed for the interaction between ACR4 and CRR3. Furthermore, sequence analysis of the ACR4 gene family has revealed a conserved ‘KDSAF’ motif that may be involved in protein-protein interactions among the receptor family. We demonstrate that peptides harboring this conserved motif in CRR3 and CRK1are able to bind to the ACR4 kinase domain. Our investigations also indicate that the ACR4 ICD can interact with and phosphorylate the transcription factor WOX5.

The Arabidopsis ROP-activated receptor-like cytoplasmic kinase RLCK VI_A3 is involved in control of basal resistance to powdery mildew and trichome branching

The Arabidopsis receptor-like cytoplasmic kinase AtRLCK VI_A3 is activated by AtROPs and is involved in trichome branching and pathogen interaction. Receptor-like cytoplasmic kinases (RLCKs) belong to the large superfamily of receptor-like kinases, which are involved in a variety of cellular processes like plant growth, development and immune responses. Recent studies suggest that RLCKs of the VI_A subfamily are possible downstream effectors of the small monomeric G proteins of the plant-specific Rho family, called 'Rho of plants' (RAC/ROPs). Here, we describe Arabidopsis thaliana AtRLCK VI_A3 as a molecular interactor of AtROPs. In Arabidopsis epidermal cells, transient co-expression of plasma membrane located constitutively activated (CA) AtROP4 or CA AtROP6 resulting in the recruitment of green fluorescent protein-tagged AtRLCK VI_A3 to the cell periphery. Intrinsic kinase activity of AtRLCK VI_A3 was enhanced in the presence of CA AtROP6 in vitro and further suggested a functional interaction between the proteins. In the interaction of the biotrophic powdery mildew fungus Erysiphe cruciferarum (E. cruciferarum) and its host plant Arabidopsis, Atrlck VI_A3 mutant lines supported enhanced fungal reproduction. Furthermore Atrlck VI_A3 mutant lines showed slightly reduced size and an increase in trichome branch number compared to wild-type plants. In summary, our data suggest a role of the AtROP-regulated AtRLCK VI_A3 in basal resistance to E. cruciferarum as well as in plant growth and cellular differentiation during trichome morphogenesis. Results are discussed in the context of literature suggesting a function of RAC/ROPs in both resistance and susceptibility to pathogen infection.

In silico characterization and expression analysis of selected Arabidopsis receptor-like kinase genes responsive to different MAMP inducers

Candidate Arabidopsis thaliana S-domain receptor-like kinase (SD-RLK) (At1g11330, At1g61430, and At1g61610) and leucine-rich repeat receptor-like kinase (LRR-RLK) (At1g51850, At2g19190, and At5g45840) genes were characterized utilizing PlantPAN, AGRIS, and AthaMap databases. Following determination of the main conserved domains, both classes of RLKs were found to be structurally similar with extracellular, transmembrane, and intracellular domains including a serine/threonine kinase domain, which might suggest a functional role in intracellular signal transduction. This established that the RLK genes had a superficially similar structure but distinct ligand binding domains. The expressions of these genes in response to a treatment with microbe-associated molecular pattern molecules (MAMPs), namely lipopolysaccharides, flg22 peptide from flagellin, peptidoglycan, chitosan, and ergosterol were compared. The candidate RLKs, potentially involved in surveillance, were found to be responsive to the elicitation treatments. Furthermore, differential regulation that was observed at the transcriptional level as well as the intensity of responses could possibly be correlated to the promoter architecture. With the use of in silico analyses, the architectures of 1 000 bp promoter regions upstream from the transcription start sites were determined. The analyses also revealed putative defense-related cis-regulatory elements that included W-boxes, MYB factor, AP2/ERF-, GT1- and ATHB-5 binding sites. The frequency at which these cis-elements occurred in each promoter differed, and the number or clusters within the core-regulatory region of individual promoters might be indicative of the responsiveness of each gene to the MAMP elicitation.

Distinct Signaling Mechanisms in Multiple Developmental Pathways by the SCRAMBLED Receptor of Arabidopsis

SCRAMBLED (SCM), a leucine-rich repeat receptor-like kinase in Arabidopsis (Arabidopsis thaliana), is required for positional signaling in the root epidermis and for tissue/organ development in the shoot. To further understand SCM action, we generated a series of kinase domain variants and analyzed their ability to complement scm mutant defects. We found that the SCM kinase domain, but not kinase activity, is required for its role in root epidermal patterning, supporting the view that SCM is an atypical receptor kinase. We also describe a previously uncharacterized role for SCM in fruit dehiscence, because mature siliques from scm mutants fail to open properly. Interestingly, the kinase domain of SCM appears to be dispensable for this developmental process. Furthermore, we found that most of the SCM kinase domain mutations dramatically inhibit inflorescence development. Because this process is not affected in scm null mutants, it is likely that SCM acts redundantly to regulate inflorescence size. The importance of distinct kinase residues for these three developmental processes provides an explanation for the maintenance of the conserved kinase domain in the SCM protein, and it may generally explain its conservation in other atypical kinases. Furthermore, these results indicate that individual leucine-rich repeat receptor-like kinases may participate in multiple pathways using distinct signaling mechanisms to mediate diverse cellular communication events.

The Arabidopsis thaliana cysteine-rich receptor-like kinases CRK6 and CRK7 protect against apoplastic oxidative stress

Receptor-like kinases are important regulators of many different processes in plants. Despite their large number only a few have been functionally characterized. One of the largest subgroups of receptor-like kinases in Arabidopsis is the cysteine-rich receptor like kinases (CRKs). High sequence similarity among the CRKs has been suggested as major cause for functional redundancy. The genomic localization of CRK genes in back-to-back repeats has made their characterization through mutant analysis unpractical. Expression profiling has linked the CRKs with reactive oxygen species, important signaling molecules in plants. Here we have investigated the role of two CRKs, CRK6 and CRK7, and analyzed their role in extracellular ROS signaling. CRK6 and CRK7 are active protein kinases with differential preference for divalent cations. Our results suggest that CRK7 is involved in mediating the responses to extracellular but not chloroplastic ROS production.

XopAC-triggered Immunity against Xanthomonas Depends on Arabidopsis Receptor-Like Cytoplasmic Kinase Genes PBL2 and RIPK

Xanthomonas campestris pv. campestris (Xcc) colonizes the vascular system of Brassicaceae and ultimately causes black rot. In susceptible Arabidopsis plants, XopAC type III effector inhibits by uridylylation positive regulators of the PAMP-triggered immunity such as the receptor-like cytoplasmic kinases (RLCK) BIK1 and PBL1. In the resistant ecotype Col-0, xopAC is a major avirulence gene of Xcc. In this study, we show that both the RLCK interaction domain and the uridylyl transferase domain of XopAC are required for avirulence. Furthermore, xopAC can also confer avirulence to both the vascular pathogen Ralstonia solanacearum and the mesophyll-colonizing pathogen Pseudomonas syringae indicating that xopAC-specified effector-triggered immunity is not specific to the vascular system. In planta, XopAC-YFP fusions are localized at the plasma membrane suggesting that XopAC might interact with membrane-localized proteins. Eight RLCK of subfamily VII predicted to be localized at the plasma membrane and interacting with XopAC in yeast two-hybrid assays have been isolated. Within this subfamily, PBL2 and RIPK RLCK genes but not BIK1 are important for xopAC-specified effector-triggered immunity and Arabidopsis resistance to Xcc.

The TMK subfamily of receptor-like kinases in Arabidopsis display an essential role in growth and a reduced sensitivity to auxin.

Mechanisms that govern the size of plant organs are not well understood but believed to involve both sensing and signaling at the cellular level. We have isolated loss-of-function mutations in the four genes comprising the transmembrane kinase TMK subfamily of receptor-like kinases (RLKs) in Arabidopsis. These TMKs have an extracellular leucine-rich-repeat motif, a single transmembrane region, and a cytoplasmic kinase domain. While single mutants do not display discernable phenotypes, unique double and triple mutant combinations result in a severe reduction in organ size and a substantial retardation in growth. The quadruple mutant displays even greater severity of all phenotypes and is infertile. The kinematic studies of root, hypocotyl, and stamen filament growth reveal that the TMKs specifically control cell expansion. In leaves, TMKs control both cell expansion and cell proliferation. In addition, in the tmk double mutants, roots and hypocotyls show reduced sensitivity to applied auxin, lateral root induction and activation of the auxin response reporter DR5: GUS. Thus, taken together with the structural and biochemical evidence, TMKs appear to orchestrate plant growth by regulation of both cell expansion and cell proliferation, and as a component of auxin signaling.

Expression and Functional Analysis of a CLV3-Like Gene in the Model Legume Lotus japonicus

Plant aerial parts are differentiated from stem cells that are located in the shoot apical meristem (SAM). CLAVATA3 (CLV3)-CLV1 is a well-known ligand-receptor pair, which functions in SAM maintenance. In Lotus japonicus, HYPERNODULATION ABERRANT ROOT FORMATION1 (HAR1) shows the highest similarity with CLV1 of all Arabidopsis receptor-like kinases (RLKs). However, HAR1 functions in the systemic regulation of root nodule development, but does not appear to function in SAM maintenance. Therefore, the gene that is responsible for SAM maintenance in L. japonicus is largely unknown. Here, we identified the L. japonicus CLV3-like (LjCLV3) gene as a counterpart of AtCLV3 and performed expression and functional analysis. LjCLV3 transcripts were detected in the central region of the shoot meristems. However, unlike AtCLV3, LjCLV3 expression was not detected in the epidermal layer, but in the inner layers of the shoot meristems. RNA interference (RNAi) of LjCLV3 caused enlargement of not only the SAM, but also the primary and secondary inflorescence meristems (IMs). Furthermore, LjCLV3-silenced plants exhibited fasciated stems and an increased number of flowers per peduncle. These results reveal that LjCLV3 is responsible for the maintenance of the SAM as well as the primary and secondary IMs.

Conserved endoplasmic reticulum-associated degradation system to eliminate mutated receptor-like kinases in Arabidopsis

Endoplasmic reticulum (ER)-associated degradation (ERAD) is an integral part of the ER quality-control system that removes toxic misfolded proteins via ubiquitin/proteasome-mediated degradation. Most of our knowledge on ERAD comes from biochemical and genetic studies in yeast and mammalian cells. Although ERAD is known to operate in plant cells, little is known about its molecular components and its biochemical mechanism. A genetic screen for suppressors of the Arabidopsis bri1-9, a weak dwarf mutant caused by ER retention of a structurally defective yet biochemically competent brassinosteroid (BR) receptor BRI1, resulted in identification of the EMS-mutagenized bri1 suppressor 5 (EBS5) gene that encodes an Arabidopsis homolog of the yeast Hrd3/mammalian Sel1L protein known to be involved in ERAD. Loss-of-function ebs5 mutations block the ERAD of bri1-9 and bri1-5, another ER-retained BR receptor. We showed that EBS5 complemented the ERAD defect of the yeast Δhrd3 mutant and interacted with the two mutated BR receptors in plant cells. Using a reverse genetic approach, we discovered that two Arabidopsis homologs of the yeast/mammalian Hrd1, an ER membrane-localized ubiquitin ligase, function redundantly in the ERAD of bri1-9. Together, our results revealed functional roles of two conserved ERAD components in degrading mutated/misfolded receptor-like kinases in Arabidopsis.

Regulatory Network Identification by Genetical Genomics: Signaling Downstream of the Arabidopsis Receptor-Like Kinase ERECTA

Gene expression differences between individuals within a species can be largely explained by differences in genetic background. The effect of genetic variants (alleles) of genes on expression can be studied in a multifactorial way by the application of genetical genomics or expression quantitative trait locus mapping. In this paper, we present a strategy to construct regulatory networks by the application of genetical genomics in combination with transcript profiling of mutants that are disrupted in single genes. We describe the network identification downstream of the receptor-like kinase ERECTA in Arabidopsis (Arabidopsis thaliana). Extending genetical genomics on the Landsberg erecta/Cape Verde Islands (Ler/Cvi) recombinant inbred population with expression profiling of monogenic mutants enabled the identification of regulatory networks in the so far elusive ERECTA signal transduction cascade. We provide evidence that ERECTA is the causal gene for the major hotspot for transcript regulation in the Arabidopsis Ler/Cvi recombinant inbred population. We further propose additional genetic variation between Ler and Cvi in loci of the signaling pathway downstream of ERECTA and suggest candidate genes underlying these loci. Integration of publicly available microarray expression data of other monogenic mutants allowed us to link ERECTA to a downstream mitogen-activated protein kinase signaling cascade. Our study shows that microarray data of monogenic mutants can be effectively used in combination with genetical genomics data to enhance the identification of genetic regulatory networks.

Identification of senescence-related genes by cDNA-AFLP in cotton (Gossypium hirsutum L.)

Premature senescence at the late developmental stage occurs frequently in cotton (Gossypium hirsutum L.) production in North China. It is desirable to develop elite cotton cultivars with non-premature senescence and high photosynthetic capacity. In this study, cDNA-amplified fragment length polymorphism (cDNA-AFLP) analysis was employed to identify the genes that are related to senescence in cotton. Using 64 primer combinations, about 3000 cDNA fragments were generated, and among them 42 had a markedly up-regulated expression pattern with the leaf growth progression. Based on cloning, sequencing, and Blast search analysis, it was determined that 24 TDFs with putative known biologic functions could be classified into several major categories, such as signal transduction, transcription regulation, stress-responsive, primary and secondary metabolism, nutrients recycling, photosynthesis, cell wall biosynthesis, and senescence-related. TDF31, TDF32 and TDF33, with high similarity to the senescent-regulating genes MAP kinase 9 (MKK9) and non-yellowing protein 1(NYE1) from Arabidopsis and bean senescence-associated receptor-like kinase (SARK) could play possible roles in responding or modulating the leaf senescence in cotton. Therefore, leaf senescence in cotton is a complicated network involving many biological processes. Some putative genes with important modulation functions in regulating or responding to the senescence need to be further analyzed.

Receptor-like Cytoplasmic Kinases Integrate Signaling from Multiple Plant Immune Receptors and Are Targeted by a Pseudomonas syringae Effector

Cell-surface-localized plant immune receptors, such as FLS2, detect pathogen-associated molecular patterns (PAMPs) and initiate PAMP-triggered immunity (PTI) through poorly understood signal-transduction pathways. The pathogenic Pseudomonas syringae effector AvrPphB, a cysteine protease, cleaves the Arabidopsis receptor-like cytoplasmic kinase PBS1 to trigger cytoplasmic immune receptor RPS5-specified effector-triggered immunity (ETI). Analyzing the function of AvrPphB in plants lacking RPS5, we find that AvrPphB can inhibit PTI by cleaving additional PBS1-like (PBL) kinases, including BIK1, PBL1, and PBL2. In unstimulated plants, BIK1 and PBL1 interact with FLS2 and are rapidly phosphorylated upon FLS2 activation by its ligand flg22. Genetic and molecular analyses indicate that BIK1, and possibly PBL1, PBL2, and PBS1, integrate immune signaling from multiple immune receptors. Whereas AvrPphB-mediated degradation of one of these kinases, PBS1, is monitored by RPS5 to initiate ETI, this pathogenic effector targets other PBL kinases for PTI inhibition.

Regulation of Cell Death and Innate Immunity by Two Receptor-like Kinases in Arabidopsis

Upon recognition of bacterial flagellin, the plant receptor FLS2 heterodimerizes with brassinosteroid insensitive 1-associated receptor kinase 1 (BAK1) and activates plant defense responses. Because constitutive activation of defense responses is detrimental, plant resistance signaling pathways must be negatively controlled, although the mechanisms involved are unclear. We identified Arabidopsis BIR1 as a BAK1-interacting receptor-like kinase. Knocking out BIR1 leads to extensive cell death, activation of constitutive defense responses, and impairment in the activation of MPK4, a negative regulator of plant resistance (R) protein signaling, by flagellin. sobir1-1, a mutant obtained in a screen for suppressors of the bir1-1 phenotype, rescued cell death observed in bir1-1. SOBIR1 encodes another receptor-like kinase whose overexpression activates cell death and defense responses. Our data suggest that BIR1 negatively regulates multiple plant resistance signaling pathways, one of which is the SOBIR1-dependent pathway identified here.

Diverse Transcriptional Programs Associated with Environmental Stress and Hormones in the Arabidopsis Receptor-Like Kinase Gene Family

The genome of Arabidopsis thaliana encodes more than 600 receptor-like kinase (RLK) genes, by far the dominant class of receptors found in land plants. Although similar to the mammalian receptor tyrosine kinases, plant RLKs are serine/threonine kinases that represent a novel signaling innovation unique to plants and, consequently, an excellent opportunity to understand how extracellular signaling evolved and functions in plants as opposed to animals. RLKs are predicted to be major components of the signaling pathways that allow plants to respond to environmental and developmental conditions. However, breakthroughs in identifying these processes have been limited to only a handful of individual RLKs. Here, we used a Syngenta custom Arabidopsis GeneChip array to compile a detailed profile of the transcriptional activity of 604 receptor-like kinase genes after exposure to a cross-section of known signaling factors in plants, including abiotic stresses, biotic stresses, and hormones. In the 68 experiments comprising the study, we found that 582 of the 604 RLK genes displayed a two-fold or greater change in expression to at least one of 12 types of treatments, thereby providing a large body of experimental evidence for targeted functional screens of individual RLK genes. We investigated whether particular subfamilies of RLK genes are responsive to specific types of signals and found that each subfamily displayed broad ranges of expression, as opposed to being targeted towards particular signal classes. Finally, by analyzing the divergence of sequence and gene expression among the RLK subfamilies, we present evidence as to the functional basis for the expansion of the RLKs and how this expansion may have affected conservation and divergences in their function. Taken as a whole, our study represents a preliminary, working model of processes and interactions in which the members of the RLK gene family may be involved, where such information has remained elusive for so many of its members.

The Arabidopsis receptor-like kinase STRUBBELIG mediates inter-cell-layer signaling during floral development

In plants important questions relate to the mechanisms that control signaling between the histogenic cell layers of apical meristems and developing organs. The Arabidopsis putative atypical leucine-rich repeat receptor-like kinase STRUBBELIG (SUB) regulates amongst others floral organ shape, the plane of cell division in cells of the first subepidermal cell layer of floral meristems, ovule integument morphogenesis, and root hair patterning. Reporter assays using a functional translational fusion between SUB and EGFP indicate that SUB expression is largely confined to interior tissues in young flowers, ovules, and roots. In contrast, SUB mRNA expression can be monitored in all cell layers of those tissues. Specifically, SUB protein is not detectable in cells that show a sub mutant phenotype. Rather, SUB is detected in directly neighbouring cells in flower and ovule primordia, or in cells that are separated from mutant cells by two cell diameters in the root. Inhibitor studies corroborate a posttranscriptional regulation of SUB. Phenotypic analysis of sub-1 plants expressing a SUB:EGFP gene under the control of tissue and epidermis-specific promoters support the notion that SUB-dependent signal transduction relies on the production of secondary intercellular signals. The combined results indicate that SUB acts in a non-cell-autonomous fashion, functions in a radial inside-out signaling process, and mediates cell morphogenesis and cell fate across clonally distinct cell layers in floral primordia, developing ovules, and root meristems.

Bacterial Effectors Target the Common Signaling Partner BAK1 to Disrupt Multiple MAMP Receptor-Signaling Complexes and Impede Plant Immunity

Successful pathogens have evolved strategies to interfere with host immune systems. For example, the ubiquitous plant pathogen Pseudomonas syringae injects two sequence-distinct effectors, AvrPto and AvrPtoB, to intercept convergent innate immune responses stimulated by multiple microbe-associated molecular patterns (MAMPs). However, the direct host targets and precise molecular mechanisms of bacterial effectors remain largely obscure. We show that AvrPto and AvrPtoB bind the Arabidopsis receptor-like kinase BAK1, a shared signaling partner of both the flagellin receptor FLS2 and the brassinosteroid receptor BRI1. This targeting interferes with ligand-dependent association of FLS2 with BAK1 during infection. It also impedes BAK1-dependent host immune responses to diverse other MAMPs and brassinosteroid signaling. Significantly, the structural basis of AvrPto-BAK1 interaction appears to be distinct from AvrPto-Pto association required for effector-triggered immunity. These findings uncover a unique strategy of bacterial pathogenesis where virulence effectors block signal transmission through a key common component of multiple MAMP-receptor complexes.

Cellular pattern formation by SCRAMBLED, a leucine-rich repeat receptor-like kinase in Arabidopsis

The appropriate specification of distinct cell types is important for generating the proper tissues and bodies of multicellular organisms. In the root epidermis of Arabidopsis, cell fate determination is accomplished by a transcriptional regulatory circuit that is influenced by positional signaling. A leucine-rich repeat receptor-like kinase, SCRAMBLED (SCM), has been shown to be responsible for the position-dependent aspect of this epidermal pattern. In a recent report, we find that SCM affects the transcriptional regulatory network by down-regulating the WEREWOLF (WER) MYB gene expression in a set of epidermal cells located in a specific position. We also find that SCM and the SCM-related SRF1 and SRF3 are not required for embryonic epidermal patterning and that SRF1 and SRF3 do not act redundantly with SCM. This suggests that distinct positional signaling mechanisms exist for embryonic and post-embryonic epidermal patterning. In this addendum, we discuss the implications of our recent findings and extend our working model for epidermal cell pattering.

Characterization of a family of Arabidopsis receptor-like cytoplasmic kinases (RLCK class VI)

The receptor-like cytoplasmic protein kinases (RLCKs) are plant-specific proteins encoded by almost 200 genes in the Arabidopsis genome. Despite of their high number, the available information on the potential function of RLCKs is very limited. In this report, the sequence analysis and the gene expression pattern of 14 members of one of the Arabidopsis RLCK families (RLCK class VI) are described. Sequence comparison indicated that gene duplication played a significant role in the formation of the kinase family and that several members carry an N-terminal "universal stress protein" (UspA) domain. In order to gain insight into the potential function of the RLCK VI kinases, real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR) was used to determine the relative transcript levels in the various organs of the Arabidopsis plant as well as under a series of abiotic stress/hormone treatments in seedlings. The obtained data revealed the differentially regulated expression of the genes in agreement with a high variability of sequence elements in their promoters. The divergent expression patterns indicate that the encoded kinase proteins may be involved in a wide variety of signal transduction pathways related to plant development and stress responses. The significance of gene duplication and expression divergence in the extension of the Arabidopsis RLCK VI family during evolution is discussed.

Characterization of a leucine-rich repeat receptor-like protein kinase (LRPK) gene from Japanese pear and its possible involvement in scab resistance

A gene encoding a leucine-rich repeat receptor-like protein kinase (LRPK) was identified from Japanese pear (Pyrus pyrifolia var. culta) using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) PCR. The Japanese pear LRPKp protein was composed of a leucine-rich extracellular ligand-binding domain, a transmembrane-spanning segment, and a cytoplasmic kinase domain; it had the highest sequence homology with apple LRPKm and Arabidopsis receptor-like protein kinase (RLK). Southern blot analysis revealed that the LRPKp belongs to a multigenic family, and a polymorphism in restriction fragment length was observed among Asian and European pear cultivars. The LRPKp transcripts were present at a high level in leaves and at a low level in flowers, while no expression was detected in fruits. The expression pattern of LRPKp was also studied by real-time RT-PCR in the scab susceptible cv. Kousui and resistant cvs. Kinchaku and Flemish Beauty after challenge inoculation with Venturia nashicola, the causal agent of Japanese pear scab. Transcripts of LRPKp were transiently enhanced in both the susceptible Kousui and to a great extent in the non-host-resistant Flemish Beauty. However, in resistant Kinchaku, expression of this gene was only weakly enhanced. Moreover, treatment with a resistance inducer, acibenzolar-S-methyl (ASM), which significantly reduces scab severity on potted trees of Kousui, was also able to induce and to potentiate the expression of this gene after inoculation. This suggests that the LRPKp gene is possibly involved in ASM-induced resistance.

Positively Selected Sites in the Arabidopsis Receptor-Like Kinase Gene Family

We analyze members of the receptor-like kinase (RLK) gene family in Arabidopsis thaliana for positive selection. Likelihood analyses find evidence for positive selection in 12 of the 52 RLK family sequences groups. These 12 groups represent 97 of the 403 sequences analyzed. The majority of genes in groups subject to positive selection have not been functionally characterized, but sites under selection are predominantly located in the extracellular region. The pattern of selection in the extracellular leucine-rich repeat (LRR) motif of groups 14 and 51 is similar to previous studies where positively selected positions are located in a solvent exposed beta-strand that may determine disease specificity, raising the possibility that some RLK genes function in a similar role.