The disease-resistance gene Pto and the fenthion-sensitivity gene fen encode closely related functional protein kinases

Characterization, identification and expression profiling of genome-wide R-genes in melon and their putative roles in bacterial fruit blotch resistance

Background

Bacterial fruit blotch (BFB), a disease caused by Acidovorax citrulli, results in significant economic losses in melon. The causal QTLs and genes for resistance to this disease have yet to be identified. Resistance (R)-genes play vital roles in resistance to plant diseases. Since the complete genome sequence of melon is available and genome-wide identification of R-genes has been performed for this important crop, comprehensive expression profiling may lead to the identification of putative candidate genes that function in the response to BFB.

Results

We identified melon accessions that are resistant and susceptible to BFB through repeated bioassays and characterized all 70 R-genes in melon, including their gene structures, chromosomal locations, domain organizations, motif distributions, and syntenic relationships. Several disease resistance-related domains were identified, including NBS, TIR, LRR, CC, RLK, and DUF domains, and the genes were categorized based on the domains of their encoded proteins. In addition, we profiled the expression patterns of the genes in melon accessions with contrasting levels of BFB resistance at 12 h, 1 d, 3 d, and 6 d after inoculation with A. citrulli. Six R-genes exhibited consistent expression patterns (MELO3C023441, MELO3C016529, MELO3C022157, MELO3C022146, MELO3C025518, and MELO3C004303), with higher expression levels in the resistant vs. susceptible accession.

Conclusion

We identified six putative candidate R-genes against BFB in melon. Upon functional validation, these genes could be targeted for manipulation via breeding and biotechnological approaches to improve BFB resistance in melon in the future.

Origin and Diversity of Plant Receptor-Like Kinases

Because of their high level of diversity and complex evolutionary histories, most studies on plant receptor-like kinase subfamilies have focused on their kinase domains. With the large amount of genome sequence data available today, particularly on basal land plants and Charophyta, more attention should be paid to primary events that shaped the diversity of the RLK gene family. We thus focus on the motifs and domains found in association with kinase domains to illustrate their origin, organization, and evolutionary dynamics. We discuss when these different domain associations first occurred and how they evolved, based on a literature review complemented by some of our unpublished results.

PK-profiling method for identifying the expression of resistance-associated genes in partially resistant oats to crown rust

BACKGROUND

Protein kinases play a key role in plant cell homeostasis and the activation of defense mechanisms. Partial resistance to fungi in plants is interesting because of its durability. However, the variable number of minor loci associated with this type of resistance hampers the reliable identification of the full range of genes involved. The present work reports the technique of protein kinase (PK)-profiling for the identification of the PK genes induced in the partially resistant oats line MN841801-1 following exposure to the fungus Puccinia coronata. This is the first time this technique has been used with cDNA (complementary DNA) from a suppression subtractive hybridization library obtained after the hybridization of cDNAs from inoculated and mock-inoculated plants.

RESULTS

Six degenerate primers based on the conserved domains of protein kinases were used in a PK-profiling assay including cDNA from mock-inoculated leaves and subtracted cDNA. Of the 75.7% of sequences cloned and sequenced that showed significant similarity to resistance genes, 76% were found to code for PKs. Translation and ClustalW2 alignment of each sequence cloned with the complete sequences of the most similar B. distachyon PKs allowed those of the partially resistant oat line to be deduced and characterized. Further, a phylogenetic study carried out after alignment of these B. distachyon PK sequences with the most similar protein sequences of related species also allowed to deduce different functions for the PK cloned. RT-qPCR (Reverse Transcription-quantitative PCR) was analyzed on nine representative sequences to validate the reliability of the employed PK-profiling method as a tool for identifying the expression of resistance-associated genes.

CONCLUSIONS

PK-profiling would appear to be a useful tool for the identification of the PKs expressed in oats after challenge by P. coronata, and perhaps other pathogens. Most of the PKs studied are related to receptor-like protein kinases expressed shortly after infection. This is in agreement with previous studies indicating a close relationship between partial resistance and the first layer of defense against pathogen used by plants.

Pto kinase binds two domains of AvrPtoB and its proximity to the effector E3 ligase determines if it evades degradation and activates plant immunity

The tomato--Pseudomonas syringae pv. tomato (Pst)--pathosystem is one of the best understood models for plant-pathogen interactions. Certain wild relatives of tomato express two closely related members of the same kinase family, Pto and Fen, which recognize the Pst virulence protein AvrPtoB and activate effector-triggered immunity (ETI). AvrPtoB, however, contains an E3 ubiquitin ligase domain in its carboxyl terminus which causes degradation of Fen and undermines its ability to activate ETI. In contrast, Pto evades AvrPtoB-mediated degradation and triggers ETI in response to the effector. It has been reported recently that Pto has higher kinase activity than Fen and that this difference allows Pto to inactivate the E3 ligase through phosphorylation of threonine-450 (T450) in AvrPtoB. Here we show that, in contrast to Fen which can only interact with a single domain proximal to the E3 ligase of AvrPtoB, Pto binds two distinct domains of the effector, the same site as Fen and another N-terminal domain. In the absence of E3 ligase activity Pto binds to either domain of AvrPtoB to activate ETI. However, the presence of an active E3 ligase domain causes ubiquitination of Pto that interacts with the domain proximal to the E3 ligase, identical to ubiquitination of Fen. Only when Pto binds its unique distal domain can it resist AvrPtoB-mediated degradation and activate ETI. We show that phosphorylation of T450 is not required for Pto-mediated resistance in vivo and that a kinase-inactive version of Pto is still capable of activating ETI in response to AvrPtoB. Our results demonstrate that the ability of Pto to interact with a second site distal to the E3 ligase domain in AvrPtoB, and not a higher kinase activity or T450 phosphorylation, allows Pto to evade ubiquitination and to confer immunity to Pst.

The American cranberry: first insights into the whole genome of a species adapted to bog habitat

BACKGROUND

The American cranberry (Vaccinium macrocarpon Ait.) is one of only three widely-cultivated fruit crops native to North America- the other two are blueberry (Vaccinium spp.) and native grape (Vitis spp.). In terms of taxonomy, cranberries are in the core Ericales, an order for which genome sequence data are currently lacking. In addition, cranberries produce a host of important polyphenolic secondary compounds, some of which are beneficial to human health. Whereas next-generation sequencing technology is allowing the advancement of whole-genome sequencing, one major obstacle to the successful assembly from short-read sequence data of complex diploid (and higher ploidy) organisms is heterozygosity. Cranberry has the advantage of being diploid (2n = 2x = 24) and self-fertile. To minimize the issue of heterozygosity, we sequenced the genome of a fifth-generation inbred genotype (F ≥ 0.97) derived from five generations of selfing originating from the cultivar Ben Lear.

RESULTS

The genome size of V. macrocarpon has been estimated to be about 470 Mb. Genomic sequences were assembled into 229,745 scaffolds representing 420 Mbp (N50 = 4,237 bp) with 20X average coverage. The number of predicted genes was 36,364 and represents 17.7% of the assembled genome. Of the predicted genes, 30,090 were assigned to candidate genes based on homology. Genes supported by transcriptome data totaled 13,170 (36%).

CONCLUSIONS

Shotgun sequencing of the cranberry genome, with an average sequencing coverage of 20X, allowed efficient assembly and gene calling. The candidate genes identified represent a useful collection to further study important biochemical pathways and cellular processes and to use for marker development for breeding and the study of horticultural characteristics, such as disease resistance.

Novel applications of motif-directed profiling to identify disease resistance genes in plants

BACKGROUND

Molecular profiling of gene families is a versatile tool to study diversity between individual genomes in sexual crosses and germplasm. Nucleotide binding site (NBS) profiling, in particular, targets conserved nucleotide binding site-encoding sequences of resistance gene analogs (RGAs), and is widely used to identify molecular markers for disease resistance (R) genes.

RESULTS

In this study, we used NBS profiling to identify genome-wide locations of RGA clusters in the genome of potato clone RH. Positions of RGAs in the potato RH and DM genomes that were generated using profiling and genome sequencing, respectively, were compared. Largely overlapping results, but also interesting discrepancies, were found. Due to the clustering of RGAs, several parts of the genome are overexposed while others remain underexposed using NBS profiling. It is shown how the profiling of other gene families, i.e. protein kinases and different protein domain-coding sequences (i.e., TIR), can be used to achieve a better marker distribution. The power of profiling techniques is further illustrated using RGA cluster-directed profiling in a population of Solanum berthaultii. Multiple different paralogous RGAs within the Rpi-ber cluster could be genetically distinguished. Finally, an adaptation of the profiling protocol was made that allowed the parallel sequencing of profiling fragments using next generation sequencing. The types of RGAs that were tagged in this next-generation profiling approach largely overlapped with classical gel-based profiling. As a potential application of next-generation profiling, we showed how the R gene family associated with late blight resistance in the SH*RH population could be identified using a bulked segregant approach.

CONCLUSIONS

In this study, we provide a comprehensive overview of previously described and novel profiling primers and their genomic targets in potato through genetic mapping and comparative genomics. Furthermore, it is shown how genome-wide or fine mapping can be pursued by choosing different sets of profiling primers. A protocol for next-generation profiling is provided and will form the basis for novel applications. Using the current overview of genomic targets, a rational choice can be made for profiling primers to be employed.

The genome of melon (Cucumis melo L.)

We report the genome sequence of melon, an important horticultural crop worldwide. We assembled 375 Mb of the double-haploid line DHL92, representing 83.3% of the estimated melon genome. We predicted 27,427 protein-coding genes, which we analyzed by reconstructing 22,218 phylogenetic trees, allowing mapping of the orthology and paralogy relationships of sequenced plant genomes. We observed the absence of recent whole-genome duplications in the melon lineage since the ancient eudicot triplication, and our data suggest that transposon amplification may in part explain the increased size of the melon genome compared with the close relative cucumber. A low number of nucleotide-binding site-leucine-rich repeat disease resistance genes were annotated, suggesting the existence of specific defense mechanisms in this species. The DHL92 genome was compared with that of its parental lines allowing the quantification of sequence variability in the species. The use of the genome sequence in future investigations will facilitate the understanding of evolution of cucurbits and the improvement of breeding strategies.

The Arabidopsis Protein Kinase PTI1-2 Is Activated by Convergent Phosphatidic Acid and Oxidative Stress Signaling Pathways Downstream of PDK1 and OXI1

Arabidopsis PDK1 activity is regulated by binding to the lipid phosphatidic acid (PA) resulting in activation of the oxidative stress-response protein kinase OXI1/AGC2-1. Thus there is an inferred link between lipid signaling and oxidative stress signaling modules. Among a panel of hormones and stresses tested, we found that, in addition to PA, the fungal elicitor xylanase activated PDK1, suggesting that PDK1 has a role in plant pathogen defense mechanisms. The downstream OXI1 was activated by additional stress factors, including PA, H(2)O(2), and partially by xylanase. We have isolated an interacting partner of OXI1, a Ser/Thr kinase (PTI1-2), which is downstream of OXI1. Its sequence closely resembles the tomato Pti kinase, which has been implicated in the hypersensitive response, a localized programmed cell death that occurs at the site of pathogen infection. PTI1-2 is activated by the same stresses/elicitors as OXI1 and additionally flagellin. We have used RNA interference to knock out the expression of PDK1 and OXI1 and to study the effects on PTI1-2 activity. We show that specific lipid signaling pathways converge on PTI1-2 via the PDK1-OXI1 axis, whereas H(2)O(2) and flagellin signals to OXI1-PTI1-2 via a PDK1-independent pathway. PTI1-2 represents a new downstream component that integrates diverse lipid and reactive oxygen stress signals and functions closely with OXI1.

AvrPtoB: a bacterial type III effector that both elicits and suppresses programmed cell death associated with plant immunity

Pseudomonas syringae pv. tomato DC3000 is a model pathogen for studying the molecular basis of plant immunity and disease susceptibility in tomato and Arabidopsis. DC3000 uses a type III secretion system to inject effector proteins into the plant cell. Type III effectors are thought to promote bacterial virulence by suppressing plant defenses and enhancing access to nutrients trapped in the plant cell. The AvrPtoB type III effector elicits immunity-associated programmed cell death (PCD) when expressed in tomato plants carrying the Pto resistance protein. However, in the absence of Pto, AvrPtoB functions to suppress PCD and immunity in tomato. Here, we review current research examining the molecular basis of AvrPtoB-mediated elicitation and suppression of plant PCD. In addition, the "trump model" is proposed to explain how resistance proteins successfully elicit immunity-associated PCD in response to effectors that suppress PCD.

The F-box protein AhSLF-S2 controls the pollen function of S-RNase-based self-incompatibility

Recently, we have provided evidence that the polymorphic self-incompatibility (S) locus-encoded F-box (SLF) protein AhSLF-S(2) plays a role in mediating a selective S-RNase destruction during the self-incompatible response in Antirrhinum hispanicum. To investigate its role further, we first transformed a transformation-competent artificial chromosome clone (TAC26) containing both AhSLF-S(2) and AhS(2)-RNase into a self-incompatible (SI) line of Petunia hybrida. Molecular analyses showed that both genes are correctly expressed in pollen and pistil in four independent transgenic lines of petunia. Pollination tests indicated that all four lines became self-compatible because of the specific loss of the pollen function of SI. This alteration was transmitted stably into the T1 progeny. We then transformed AhSLF-S(2) cDNA under the control of a tomato (Lycopersicon esculentum) pollen-specific promoter LAT52 into the self-incompatible petunia line. Molecular studies revealed that AhSLF-S(2) is specifically expressed in pollen of five independent transgenic plants. Pollination tests showed that they also had lost the pollen function of SI. Importantly, expression of endogenous SLF or SLF-like genes was not altered in these transgenic plants. These results phenocopy a well-known phenomenon called competitive interaction whereby the presence of two different pollen S alleles within pollen leads to the breakdown of the pollen function of SI in several solanaceaous species. Furthermore, we demonstrated that AhSLF-S(2) physically interacts with PhS(3)-RNase from the P. hybrida line used for transformation. Together with the recent demonstration of PiSLF as the pollen determinant in P. inflata, these results provide direct evidence that the polymorphic SLF including AhSLF-S(2) controls the pollen function of S-RNase-based self-incompatibility.

Molecular interactions between the specialist herbivore Manduca sexta (lepidoptera, sphingidae) and its natural host Nicotiana attenuata: V. microarray analysis and further characterization of large-scale changes in herbivore-induced mRNAs

We extend our analysis of the transcriptional reorganization that occurs when the native tobacco, Nicotiana attenuata, is attacked by Manduca sexta larvae by cloning 115 transcripts by mRNA differential display reverse transcription-polymerase chain reaction and subtractive hybridization using magnetic beads (SHMB) from the M. sexta-responsive transcriptome. These transcripts were spotted as cDNA with eight others, previously confirmed to be differentially regulated by northern analysis on glass slide microarrays, and hybridized with Cy3- and Cy5-labeled probes derived from plants after 2, 6, 12, and 24 h of continuous attack. Microarray analysis proved to be a powerful means of verifying differential expression; 73 of the cloned genes (63%) were differentially regulated (in equal proportions from differential display reverse transcription-polymerase chain reaction and SHMB procedures), and of these, 24 (32%) had similarity to known genes or putative proteins (more from SHMB). The analysis provided insights into the signaling and transcriptional basis of direct and indirect defenses used against herbivores, suggesting simultaneous activation of salicylic acid-, ethylene-, cytokinin-, WRKY-, MYB-, and oxylipin-signaling pathways and implicating terpenoid-, pathogen-, and cell wall-related transcripts in defense responses. These defense responses require resources that could be made available by decreases in four photosynthetic-related transcripts, increases in transcripts associated with protein and nucleotide turnover, and increases in transcripts associated with carbohydrate metabolism. This putative up-regulation of defense-associated and down-regulation of growth-associated transcripts occur against a backdrop of altered transcripts for RNA-binding proteins, putative ATP/ADP translocators, chaperonins, histones, and water channel proteins, responses consistent with a major metabolic reconfiguration that underscores the complexity of response to herbivore attack.

Functional analyses of the Pto resistance gene family in tomato and the identification of a minor resistance determinant in a susceptible haplotype

Pto is a member of a multigene family and encodes a serine/threonine kinase that mediates gene-for-gene resistance to strains of Pseudomonas syringae pv. tomato expressing avrPto. The inferred amino acid sequence of the Pto homologs from both resistant (LpimPth2 to LpimPth4) and susceptible (LescFen, LescPth2 to LescPth5) haplotypes suggested that most could encode functional serine/threonine kinases. In addition, the activation segments of the homologs are similar in sequence to that of Pto, and some have residues previously identified as required for binding of AvrPto by Pto in the yeast two-hybrid system. The Pto homologs were therefore characterized for transcription, for the ability of their products to interact with AvrPto in the yeast two-hybrid system, for their autophosphorylation activity, and for their potential to elicit cell death in the presence of and absence of a ligand, as well as their dependence on Prf. LpimPth5, LpimPth4, and LescPth4 were not transcribed at levels detectable by reverse transcription-polymerase chain reaction. The interaction with AvrPto was unique to Pto in the yeast two-hybrid system. LescPth2 autophosphorylated in vitro as a fusion protein. LpimPth2, LpimPth3, LpimPth4, LescPth3, and LescPth4 did not autophosphorylate in vitro. Transient expression of wild-type Fen and wild-type LpimPth3, as well as LescFen, LescPth3, and LescPth5 with perturbations in their P+1 loop caused cell death in Nicotiana benthamiana. LpimPth3 and LescPth3 with amino acid substitutions in the P+1 loop also elicited cell death in tomato; this was dependent on the presence of wild-type Prf. Consequently, some homologs could potentially encode functional resistance proteins. LescPth5 induced cell death specifically in response to expression of AvrPto in tobacco in a Prf-dependent manner; this is consistent with a homolog from a 'susceptible' haplotype encoding a minor recognition determinant.

Bacterial lipopolysaccharides and innate immunity

Bacterial lipopolysaccharides (LPS) are the major outer surface membrane components present in almost all Gram-negative bacteria and act as extremely strong stimulators of innate or natural immunity in diverse eukaryotic species ranging from insects to humans. LPS consist of a poly- or oligosaccharide region that is anchored in the outer bacterial membrane by a specific carbohydrate lipid moiety termed lipid A. The lipid A component is the primary immunostimulatory centre of LPS. With respect to immunoactivation in mammalian systems, the classical group of strongly agonistic (highly endotoxic) forms of LPS has been shown to be comprised of a rather similar set of lipid A types. In addition, several natural or derivatised lipid A structures have been identified that display comparatively low or even no immunostimulation for a given mammalian species. Some members of the latter more heterogeneous group are capable of antagonizing the effects of strongly stimulatory LPS/lipid A forms. Agonistic forms of LPS or lipid A trigger numerous physiological immunostimulatory effects in mammalian organisms, but--in higher doses--can also lead to pathological reactions such as the induction of septic shock. Cells of the myeloid lineage have been shown to be the primary cellular sensors for LPS in the mammalian immune system. During the past decade, enormous progress has been obtained in the elucidation of the central LPS/lipid A recognition and signaling system in mammalian phagocytes. According to the current model, the specific cellular recognition of agonistic LPS/lipid A is initialized by the combined extracellular actions of LPS binding protein (LBP), the membrane-bound or soluble forms of CD14 and the newly identified Toll-like receptor 4 (TLR4)*MD-2 complex, leading to the rapid activation of an intracellular signaling network that is highly homologous to the signaling systems of IL-1 and IL-18. The elucidation of structure-activity correlations in LPS and lipid A has not only contributed to a molecular understanding of both immunostimulatory and toxic septic processes, but has also re-animated the development of new pharmacological and immunostimulatory strategies for the prevention and therapy of infectious and malignant diseases.

Ancient origin of pathogen recognition specificity conferred by the tomato disease resistance gene Pto

We have investigated the origin of the Pto disease resistance (R) gene that was previously identified in the wild tomato species Lycopersicon pimpinellifolium and isolated by map-based cloning. Pto encodes a serine-threonine protein kinase that specifically recognizes strains of Pseudomonas syringae pv. tomato (Pst) that express the avirulence gene avrPto. We examined an accession of the distantly related wild species Lycopersicon hirsutum var. glabratum that exhibits avrPto-specific resistance to Pst. The Pst resistance of L. hirsutum was introgressed into a susceptible Lycopersicon esculentum background to create the near-isogenic line 96T133-3. Resistance to Pst(avrPto) in 96T133-3 was inherited as a single dominant locus and cosegregated with a restriction fragment length polymorphism detected by the Pto gene. This observation suggested that a member of the Pto gene family confers Pst(avrPto) resistance in this L. hirsutum line. Here we report the cloning and characterization of four members of the Pto family from 96T133-3. One gene (LhirPto) is 97% identical to Pto and encodes a catalytically active protein kinase that elicits a hypersensitive response when coexpressed with avrPto in leaves of Nicotiana benthamiana. In common with the Pto kinase, the LhirPto protein physically interacts with AvrPto and downstream members of the Pto signaling pathway. Our studies indicate that R genes of the protein kinase class may not evolve rapidly in response to pathogen pressure and rather that their ability to recognize specific Avr proteins can be highly conserved.

Genetic mapping of the Tsw locus for resistance to the Tospovirus Tomato spotted wilt virus in Capsicum spp. and its relationship to the Sw-5 gene for resistance to the same pathogen in tomato

The Tsw gene conferring dominant resistance to the Tospovirus Tomato spotted wilt virus (TSWV) in Capsicum spp. has been tagged with a random amplified polymorphic DNA marker and mapped to the distal portion of chromosome 10. No mapped homologues of Sw-5, a phenotypically similar dominant TSWV resistance gene in tomato, map to this region in C. annuum, although a number of Sw-5 homologues are found at corresponding positions in pepper and tomato. The relationship between Tsw and Sw-5 was also examined through genetic studies of TSWV. The capacity of TSWV-A to overcome the Tsw gene in pepper and the Sw-5 gene in tomato maps to different TSWV genome segments. Therefore, despite phenotypic and genetic similarities of resistance in tomato and pepper, we infer that distinct viral gene products control the outcome of infection in plants carrying Sw-5 and Tsw, and that these loci do not appear to share a recent common evolutionary ancestor.

Comparative genetics of disease resistance within the solanaceae

Genomic positions of phenotypically defined disease resistance genes (R genes) and R gene homologues were analyzed in three solanaceous crop genera, Lycopersicon (tomato), Solanum (potato), and Capsicum (pepper). R genes occurred at corresponding positions in two or more genomes more frequently than expected by chance; however, in only two cases, both involving Phytophthora spp., did genes at corresponding positions have specificity for closely related pathogen taxa. In contrast, resistances to Globodera spp., potato virus Y, tobacco mosaic virus, and tomato spotted wilt virus were mapped in two or more genera and did not occur in corresponding positions. Without exception, pepper homologues of the cloned R genes Sw-5, N, Pto, Prf, and I2 were found in syntenous positions in other solanaceous genomes and in some cases also mapped to additional positions near phenotypically defined solanaceous R genes. This detailed analysis and synthesis of all available data for solanaceous R genes suggests a working hypothesis regarding the evolution of R genes. Specifically, while the taxonomic specificity of host R genes may be evolving rapidly, general functions of R alleles (e.g., initiation of resistance response) may be conserved at homologous loci in related plant genera.

PLANT PROTEIN SERINE/THREONINE KINASES: Classification and Functions

The first plant protein kinase sequences were reported as recently as 1989, but by mid-1998 there were more than 500, including 175 in Arabidopsis thaliana alone. Despite this impressive pace of discovery, progress in understanding the detailed functions of protein kinases in plants has been slower. Protein serine/threonine kinases from A. thaliana can be divided into around a dozen major groups based on their sequence relationships. For each of these groups, studies on animal and fungal homologs are briefly reviewed, and direct studies of their physiological functions in plants are then discussed in more detail. The network of protein-serine/threonine kinases in plant cells appears to act as a "central processor unit" (cpu), accepting input information from receptors that sense environmental conditions, phytohormones, and other external factors, and converting it into appropriate outputs such as changes in metabolism, gene expression, and cell growth and division.

Regulation of expression of avirulence gene avrRxv and identification of a family of host interaction factors by sequence analysis of avrBsT

Resistance in tomato line Hawaii 7998 as well as in several nonhost plants to Xanthomonas campestris pv. vesicatoria tomato strain (XcvT) is mediated in part by the avirulence gene avrRxv. Analysis of growth of wild-type and avrRxv deletion strains indicates that avrRxv plays a crucial role in the ability of XcvT 92-14 to induce resistance on Hawaii 7998. We used avrRxv reporter gene fusions and Northern (RNA) blot analysis to test several growth environments for inductive potential. We found that avrRxv is constitutively expressed at high levels and that growth in planta, in tobacco conditioned medium, and in hrp-inductive medium XVM2 did not affect the high levels of expression. In addition, hrp structural and regulatory mutant backgrounds had no effect. We mutated the bipartite plant inducible promoter (PIP)-box sequence and found that avrRxv activity appears to be independent of an intact PIP-box element. We present the sequence of the avrRxv homologue called avrBsT and align the six AvrRxv host interaction factor family members including mammalian pathogen virulence factors YopJ and YopP from Yersinia spp. and AvrA from Salmonella typhimurium, and open reading frame Y4LO with unknown function from the symbiont Rhizobium sp.

THE OXIDATIVE BURST IN PLANT DISEASE RESISTANCE

Rapid generation of superoxide and accumulation of H2O2 is a characteristic early feature of the hypersensitive response following perception of pathogen avirulence signals. Emerging data indicate that the oxidative burst reflects activation of a membrane-bound NADPH oxidase closely resembling that operating in activated neutrophils. The oxidants are not only direct protective agents, but H2O2 also functions as a substrate for oxidative cross-linking in the cell wall, as a threshold trigger for hypersensitive cell death, and as a diffusible signal for induction of cellular protectant genes in surrounding cells. Activation of the oxidative burst is a central component of a highly amplified and integrated signal system, also involving salicylic acid and perturbations of cytosolic Ca2+, which underlies the expression of disease-resistance mechanisms.

The CLAVATA1 gene encodes a putative receptor kinase that controls shoot and floral meristem size in Arabidopsis

The shoot apical meristem is responsible for above-ground organ initiation in higher plants, accomplishing continuous organogenesis by maintaining a pool of undifferentiated cells and directing descendant cells toward organ formation. Normally, proliferation and differentiation are balanced, so that the structure and size of the shoot meristem is maintained. However, Arabidopsis plants homozygous for mutations at the CLAVATA1 (CLV1) locus accumulate excess undifferentiated cells. We describe the molecular cloning and expression pattern of the CLV1 gene. It encodes a putative receptor kinase, suggesting a role in signal transduction. The extracellular domain is composed of 21 tandem leucine-rich repeats that resemble leucine-rich repeats found in animal hormone receptors. We provide evidence that CLV1 expression in the inflorescence is specifically associated with meristematic activity.

The Pto kinase mediates a signaling pathway leading to the oxidative burst in tomato

The Pto gene encodes a serine/threonine kinase that confers resistance in tomato to Pseudomonas syringae pv. tomato strains that express the avirulence gene avrPto. Partial characterization of the Pto signal transduction pathway and the availability of transgenic tomato lines (+/- Pto) make this an ideal system for exploring the molecular basis of disease resistance. In this paper, we test two transgenic tomato cell suspension cultures (+/-Pto) for production of H2O2 following independent challenge with two strains of P. syringae pv. tomato (+/-avrPto). Only when Pto and avrPto are present in the corresponding organisms are two distinct phases of the oxidative burst seen, a rapid first burst followed by a slower and more prolonged second burst. In the remaining three plant-pathogen interactions, we observe either no burst or only a first burst, indicating that the second burst is correlated with disease resistance. Further support for this observation comes from the finding that both resistant and susceptible tomato lines produce the critical second oxidative burst when challenged with P. syringae pv. tabaci, a nonhost pathogen that elicits a hypersensitive response on both tomato lines. The Pto kinase is not required, however, for the oxidative burst initiated by non-specific elicitors such as oligogalacturonides or osmotic stress. A model describing a possible role for the Pto kinase in the overall scheme of oxidative burst signaling is proposed.

Resistance gene analogs are conserved and clustered in soybean

Sequences of cloned resistance genes from a wide range of plant taxa reveal significant similarities in sequence homology and structural motifs. This is observed among genes conferring resistance to viral, bacterial, and fungal pathogens. In this study, oligonucleotide primers designed for conserved sequences from coding regions of disease resistance genes N (tobacco), RPS2 (Arabidopsis) and L6 (flax) were used to amplify related sequences from soybean [Glycine max (L.) Merr.]. Sequencing of amplification products indicated that at least nine classes of resistance gene analogs (RGAs) were detected. Genetic mapping of members of these classes located them to eight different linkage groups. Several RGA loci mapped near known resistance genes. A bacterial artificial chromosome library of soybean DNA was screened using primers and probes specific for eight RGA classes and clones were identified containing sequences unique to seven classes. Individual bacterial artificial chromosomes contained 2-10 members of single RGA classes. Clustering and sequence similarity of members of RGA classes suggests a common process in their evolution. Our data indicate that it may be possible to use sequence homologies from conserved motifs of cloned resistance genes to identify candidate resistance loci from widely diverse plant taxa.

Developmental and tissue-specific expression of mouse pelle-like protein kinase

The NF-kappaB/c-Rel proteins are a family of evolutionarily conserved transcription factors activated during development that in the adult, mediate many processes including the immune response. A high degree of sequence similarity is shared between the NF-kappaB/c-Rel family of transcription factors and the Drosophila Dorsal protein as well as between its cytoplasmic inhibitor, IkappaBalpha, and the Drosophila Cactus protein. Genetic analyses of Dorsal have defined components of a signaling pathway for Dorsal activation, including a serine/threonine kinase, Pelle, placed upstream of Dorsal and Cactus. We demonstrate that this pathway is likely to be conserved in mammals by the isolation of a cDNA that encodes a novel mouse protein highly related to Pelle, mPLK (mouse Pelle-like protein kinase). Expression of mPLK mRNA is developmentally regulated in the mouse and in adult tissue mPLK expression is greatest in the liver, a tissue that expresses a high level of NF-kappaB. Recombinant mPLK produced in bacteria is a protein kinase capable of autophosphorylating and phosphorylating IkappaBalpha.

Signal transduction in plant immunity

Significant recent advances in the understanding of plant defense mechanisms include the isolation and characterization of resistance genes against bacterial, fungal and viral pathogens, the identification of genes involved in cell death, and the demonstration of the involvement of reactive oxygen species and salicylic acid in the signal-transduction pathways for expression of induced resistance.

Introgression into tomato (Lycopersicon esculentum) of the L. chmielewskii sucrose accumulator gene (sucr) controlling fruit sugar composition

High sucrose concentration in fruit of Lycopersicon chmielewskii is governed by the recessive sucrose accumulator gene (sucr) that is situated in the pericentromeric region of chromosome 3. The sucr gene was introgressed into the genetic background of the hexose-accumulating cultivated tomato (L. esculentum cv 'Hunt 100') by marker-assisted selection using tightly linked RFLP markers and a tomato acid invertase cDNA as probes for sucr. RFLP mapping indicated that the segment containing sucr comprised over 43.2 cM in the BC1F2 generation, representing over one-third of the total length of chromosome 3. By selecting for crossovers between sucr and the flanking visual marker r (yellow fruit flesh) and RFLP marker TG288, we were able to reduce the size of the sucr introgression fragment to 0.8-7.1 cM by the BC5 generation. Smaller recombinant fragments were not obtained despite screening a large BC6F2 population. The smallest sucr introgression reduced recombination between the flanking visual markers sy (sunny) and bls (baby lea syndrome) by 38%. To facilitate future introgression and recombination experiments, a PCR-based test for the sucr gene was developed using primers specific to the tomato invertase gene. This assay takes advantage of a small deletion that maps to the second intron of the L. chmielewskii nvertase gene. The assay detected significant allelic variation both within and between hexose- and sucrose-accumulating Lycopersicon spp.