Signal recognition and transduction mediated by the tomato Pto kinase: a paradigm of innate immunity in plants

Germplasm Screening Using DNA Markers and Genome-Wide Association Study for the Identification of Powdery Mildew Resistance Loci in Tomato

Powdery mildew (PM), caused by Oidium spp. in tomato, is a global concern that leads to diminished yield. We aimed to evaluate previously reported DNA markers linked to powdery mildew resistance (PMR) and identify novel quantitative trait loci (QTLs) for PMR through a genome-wide association study in tomato. Sequencing analysis of the internal transcribed spacer (ITS) of a PM strain (PNU_PM) isolated from Miryang, Gyeongnam, led to its identification as Oidium neolycopersici. Thereafter, a PM bioassay was conducted for a total of 295 tomato accessions, among which 24 accessions (4 S. lycopersicum accessions and 20 accessions of seven wild species) showed high levels of resistance to PNU_PM. Subsequently, we genotyped 11 markers previously linked to PMR in 56 accessions. PMR-specific banding patterns were detected in 15/22 PMR accessions, while no such bands were observed in the powdery mildew-susceptible accessions. The genome-wide association study was performed using TASSEL and GAPIT, based on the phenotypic data of 290 accessions and 11,912 single nucleotide polymorphisms (SNPs) obtained from the Axiom® Tomato SNP Chip Array. Nine significant SNPs in chromosomes 1, 4, 6, 8, and 12, were selected and five novel QTL regions distinct from previously known PMR-QTL regions were identified. Of these QTL regions, three putative candidate genes for PMR were selected from chromosomes 4 and 8, including two nucleotide binding site-leucine rich repeat class genes and a receptor-like kinase gene, all of which have been identified previously as causative genes for PMR in several crop species. The SNPs discovered in these genes provide useful information for understanding the molecular basis of PMR and developing DNA markers for marker-assisted selection of PMR in tomato.

Introgression of rpg4/Rpg5 Into Barley Germplasm Provides Insights Into the Genetics of Resistance to Puccinia graminis f. sp. tritici Race TTKSK and Resources for Developing Resistant Cultivars

Stem rust (incited by Puccinia graminis f. sp. tritici) is a devastating disease of wheat and barley in many production areas. The widely virulent African P. graminis f. sp. tritici race TTKSK is of particular concern, because most cultivars are susceptible. To prepare for the possible arrival of race TTKSK in North America, we crossed a range of barley germplasm-representing different growth habits and end uses-with donors of stem rust resistance genes Rpg1 and rpg4/Rpg5. The former confers resistance to prevalent races of P. graminis f. sp. tritici in North America, and the latter confers resistance to TTKSK and other closely related races from Africa. We produced doubled haploids from these crosses and determined their allele type at the Rpg loci and haplotype at 7,864 single-nucleotide polymorphism loci. The doubled haploids were phenotyped for TTKSK resistance at the seedling stage. Integration of genotype and phenotype data revealed that (i) Rpg1 was not associated with TTKSK resistance, (ii) rpg4/Rpg5 was necessary but was not sufficient for resistance, and (iii) specific haplotypes at two quantitative trait loci were required for rpg4/Rpg5 to confer resistance to TTKSK. To confirm whether lines found resistant to TTKSK at the seedling resistance were also resistant at the adult plant stage, a subset of doubled haploids was evaluated in Kenya. Additionally, adult plant resistance to leaf rust and stripe rust (incited by Puccinia hordei and Puccinia striiformis f. sp. hordei, respectively) was also assessed on the doubled haploids in field trials at three locations in the United States over a 2-year period. Doubled haploids were identified with adult plant resistance to all three rusts, and this germplasm is available to the research and breeding communities.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.

De Novo Assembly, Annotation, and Characterization of Root Transcriptomes of Three Caladium Cultivars with a Focus on Necrotrophic Pathogen Resistance/Defense-Related Genes

Roots are vital to plant survival and crop yield, yet few efforts have been made to characterize the expressed genes in the roots of non-model plants (root transcriptomes). This study was conducted to sequence, assemble, annotate, and characterize the root transcriptomes of three caladium cultivars (Caladium × hortulanum) using RNA-Seq. The caladium cultivars used in this study have different levels of resistance to Pythiummyriotylum, the most damaging necrotrophic pathogen to caladium roots. Forty-six to 61 million clean reads were obtained for each caladium root transcriptome. De novo assembly of the reads resulted in approximately 130,000 unigenes. Based on bioinformatic analysis, 71,825 (52.3%) caladium unigenes were annotated for putative functions, 48,417 (67.4%) and 31,417 (72.7%) were assigned to Gene Ontology (GO) and Clusters of Orthologous Groups (COG), respectively, and 46,406 (64.6%) unigenes were assigned to 128 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. A total of 4518 distinct unigenes were observed only in Pythium-resistant "Candidum" roots, of which 98 seemed to be involved in disease resistance and defense responses. In addition, 28,837 simple sequence repeat sites and 44,628 single nucleotide polymorphism sites were identified among the three caladium cultivars. These root transcriptome data will be valuable for further genetic improvement of caladium and related aroids.

Riboflavin-Induced Disease Resistance Requires the Mitogen-Activated Protein Kinases 3 and 6 in Arabidopsis thaliana

As a resistance elicitor, riboflavin (vitamin B₂) protects plants against a wide range of pathogens. At molecular biological levels, it is important to elucidate the signaling pathways underlying the disease resistance induced by riboflavin. Here, riboflavin was tested to induce resistance against virulent Pseudomonas syringae pv. Tomato DC3000 (Pst DC3000) in Arabidopsis. Results showed that riboflavin induced disease resistance based on MAPK-dependent priming for the expression of PR1 gene. Riboflavin induced transient expression of PR1 gene. However, following Pst DC3000 inoculation, riboflavin potentiated stronger PR1 gene transcription. Further was suggested that the transcript levels of mitogen-activated protein kinases, MPK3 and MPK6, were primed under riboflavin. Upon infection by Pst DC3000, these two enzymes were more strongly activated. The elevated activation of both MPK3 and MPK6 was responsible for enhanced defense gene expression and resistance after riboflavin treatment. Moreover, riboflavin significantly reduced the transcript levels of MPK3 and MPK6 by application of AsA and BAPTA, an H₂O₂ scavenger and a calcium (Ca²⁺) scavenger, respectively. In conclusion, MPK3 and MPK6 were responsible for riboflavin-induced resistance, and played an important role in H₂O₂- and Ca²⁺-related signaling pathways, and this study could provide a new insight into the mechanistic study of riboflavin-induced defense responses.

The Co-4 locus on chromosome Pv08 contains a unique cluster of 18 COK-4 genes and is regulated by immune response in common bean

The common bean locus Co - 4, traditionally referred to as an anthracnose-resistant gene, contains a cluster of predicted receptor-like kinases (COK-4 and CrRLK1-like), and at least two of these kinases are co-regulated with the plant's basal immunity. Genetic resistance to anthracnose, caused by the fungus Colletotrichum lindemuthianum (Sacc. and Magnus) Briosi and Cavara, is conferred by major loci throughout the Phaseolus vulgaris genome, named Co. The complex Co-4 locus was previously reported to have several copies of the COK-4 gene that is predicted to code for a receptor-like kinase (RLK). In general, plant RLKs are involved in pathogen perception and signal transduction; however, the molecular function of COK-4 remains elusive. Using newly identified molecular markers (PvTA25 and PvSNPCOK-4), the SAS13 marker, COK-4 sequences and phylogeny, and the recently released bean genome sequence, we determined the most probable boundaries of the Co-4 locus: a 325-Kbp region on chromosome Pv08. Out of the 49 predicted transcripts in that region, 24 encode for putative RLKs (including 18 COK-4 copies) with high similarity to members of the Catharanthus roseus RLK1-like (CrRLK1L) protein family from different plant species, including the well-described FERONIA (FER) and ANXUR. We also determined that two RLK-coding genes in the Co-4 locus (COK-4-3 and FER-like) are transcriptionally regulated when bean plants are challenged with the flg22 peptide, a commonly used elicitor of plant immunity, or the bacterium Pseudomonas syringae pv. phaseolicola, the causal agent of halo blight. While COK-4-3 is activated during immune response, FER-like is downregulated suggesting that these genes could play a role in plant responses to biotic stress. These results highlight the importance of dissecting the regulation and molecular function of individual genes within each locus, traditionally referred to as resistance gene based on genetic segregation analysis.

Epistatic selection and coadaptation in the Prf resistance complex of wild tomato

Natural selection imposed by pathogens is a strong and pervasive evolutionary force structuring genetic diversity within their hosts' genomes and populations. As a model system for understanding the genomic impact of host-parasite coevolution, we have been studying the evolutionary dynamics of disease resistance genes in wild relatives of the cultivated tomato species. In this study, we investigated the sequence variation and evolutionary history of three linked genes involved in pathogen resistance in populations of Solanum peruvianum (Pto, Fen, and Prf). These genes encode proteins, which form a multimeric complex and together activate defense responses. We used standard linkage disequilibrium, as well as partitioning of linkage disequilibrium components across populations and correlated substitution analysis to identify amino acid positions that are candidates for coevolving sites between Pto/Fen and Prf. These candidates were mapped onto known and predicted structures of Pto, Fen and Prf to visualize putative coevolving regions between proteins. We discuss the functional significance of these coevolving pairs in the context of what is known from previous structure-function studies of Pto, Fen and Prf.

Expression of a metacaspase gene of Nicotiana benthamiana after inoculation with Colletotrichum destructivum or Pseudomonas syringae pv. tomato, and the effect of silencing the gene on the host response

Metacaspases are cysteine proteinases that have homology to caspases, which play a central role in signaling and executing programmed cell death in animals. A type II metacaspase cDNA, NbMCA1, was amplified from Nicotiana benthamiana infected with Colletotrichum destructivum. It showed a peak in expression at 72 h post-inoculation corresponding with the switch to necrotrophy by C. destructivum. Inoculation of N. benthamiana with an incompatible bacterium, Pseudomonas syringae pv. tomato, which should induce a non-host hypersensitive response (HR), did not result in an increase in NbMCA1 expression at the time of necrosis development at 20-24 h postinoculation. Virus-induced silencing of NbMCA1 resulted in three to four times more lesions due to C. destructivum compared with leaves inoculated with the PVX vector without the cloned metacaspase gene or inoculated with water only. However, virus-induced silencing of NbMCA1 did not affect the HR necrosis or population levels of P. syringae pv. tomato. Although this metacaspase gene does not appear to be involved in the programmed cell death of non-host HR resistance to P. syringae, it does affect the susceptibility of N. benthamiana to C. destructivum indicating a function in a basal defense response. Possible roles of NbMCA1could be in degrading virulence factors of the pathogen, processing pro-proteins involved in stress responses, eliminating damaged proteins created during stress, and/or degrading proteins to remobilize amino acids to fuel de novo synthesis of proteins involved in stress adaptations.

Early events in the pathogenicity of Pseudomonas syringae on Nicotiana benthamiana

Conserved microbial molecules known as PAMPs (pathogen-associated molecular patterns) elicit defence responses in plants through extracellular receptor proteins. One important PAMP is the flagellin protein derived from motile bacteria. We show here that the solanaceous species Nicotiana benthamiana perceives the flagellin proteins of both pathogenic and non-host species of Pseudomonas syringae. The response to flagellin required a gene closely related to that encoding the Arabidopsis thaliana flagellin receptor that we designated NbFls2. In addition, silencing of NbFls2 led to increased growth of compatible, non-host and non-pathogenic strains of P. syringae. Thus, flagellin perception restricts growth of P. syringae strains on N. benthamiana. Pathogenic bacteria secrete effector proteins into the plant cell to enhance virulence. We tested the ability of several unrelated effectors to suppress PAMP-mediated defences. The effector proteins AvrPto and AvrPtoB, but not AvrRps4, suppressed all responses tested including the hypersensitive response induced by non-host flagellins and the oomycete elicitor INF1. Strikingly, transient expression of avrPto or avrPtoB stimulated the growth of non-pathogenic Agrobacterium tumefaciensin planta, suggesting that multiplication of this species is also restricted by PAMP perception. Unexpectedly, AvrPtoB but not AvrPto required the defence-associated genes Rar1, Sgt1 and Eds1 for suppression. This observation separates the respective mechanisms of the two effectors, and suggests that AvrPtoB may target the defence machinery directly for its suppressive effect.

Diversity in domain architectures of Ser/Thr kinases and their homologues in prokaryotes

Background

Ser/Thr/Tyr kinases (STYKs) commonly found in eukaryotes have been recently reported in many bacterial species. Recent studies elucidating their cellular functions have established their roles in bacterial growth and development. However functions of a large number of bacterial STYKs still remain elusive. The organisation of domains in a large dataset of bacterial STYKs has been investigated here in order to recognise variety in domain combinations which determine functions of bacterial STYKs.

Results

Using sensitive sequence and profile search methods, domain organisation of over 600 STYKs from 125 prokaryotic genomes have been examined. Kinase catalytic domains of STYKs tethered to a wide range of enzymatic domains such as phosphatases, HSP70, peptidyl prolyl isomerases, pectin esterases and glycoproteases have been identified. Such distinct preferences for domain combinations are not known to be present in either the Histidine kinase or the eukaryotic STYK families. Domain organisation of STYKs specific to certain groups of bacteria has also been noted in the current anlaysis. For example, Hydrophobin like domains in Mycobacterial STYK and penicillin binding domains in few STYKs of Gram-positive organisms and FHA domains in cyanobacterial STYKs. Homologues of characterised substrates of prokaryotic STYKs have also been identified.

Conclusion

The domains and domain architectures of most of the bacterial STYKs identified are very different from the known domain organisation in STYKs of eukaryotes. This observation highlights distinct biological roles of bacterial STYKs compared to eukaryotic STYKs. Bacterial STYKs reveal high diversity in domain organisation. Some of the modular organisations conserved across diverse bacterial species suggests their central role in bacterial physiology. Unique domain architectures of few other groups of STYKs reveal recruitment of functions specific to the species.

Natural variation in the Pto pathogen resistance gene within species of wild tomato (Lycopersicon). I. Functional analysis of Pto alleles

Disease resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) in the cultivated tomato, Lycopersicon esculentum, and the closely related L. pimpinellifolium is triggered by the physical interaction between plant disease resistance protein, Pto, and the pathogen avirulence protein, AvrPto. To investigate the extent to which variation in the Pto gene is responsible for naturally occurring variation in resistance to Pst, we determined the resistance phenotype of 51 accessions from seven species of Lycopersicon to isogenic strains of Pst differing in the presence of avrPto. One-third of the plants displayed resistance specifically when the pathogen expressed AvrPto, consistent with a gene-for-gene interaction. To test whether this resistance in these species was conferred specifically by the Pto gene, alleles of Pto were amplified and sequenced from 49 individuals and a subset (16) of these alleles was tested in planta using Agrobacterium-mediated transient assays. Eleven alleles conferred a hypersensitive resistance response (HR) in the presence of AvrPto, while 5 did not. Ten amino acid substitutions associated with the absence of AvrPto recognition and HR were identified, none of which had been identified in previous structure-function studies. Additionally, 3 alleles encoding putative pseudogenes of Pto were isolated from two species of Lycopersicon. Therefore, a large proportion, but not all, of the natural variation in the reaction to strains of Pst expressing AvrPto can be attributed to sequence variation in the Pto gene.

Functional analysis of the plant disease resistance gene Pto using DNA shuffling

Pto is a serine/threonine kinase that mediates resistance in tomato to strains of Pseudomonas syringae pv. tomato expressing the (a)virulence proteins AvrPto or AvrPtoB. DNA shuffling was used as a combinatorial in vitro genetic approach to dissect the functional regions of Pto. The Pto gene was shuffled with four of its paralogs from a resistant haplotype to create a library of recombinant products that was screened for interaction with AvrPto in yeast. All interacting clones and a representative sample of noninteracting clones were sequenced, and their ability to signal downstream was tested by the elicitation of a hypersensitive response in an AvrPto-dependent or -independent manner in planta. Eight candidate regions important for binding to AvrPto or for downstream signaling were identified by statistical correlations between individual amino acid positions and phenotype. A subset of the regions had previously been identified as important for recognition, confirming the validity of the shuffling approach. Three novel regions important for Pto function were validated by site-directed mutagenesis. Several chimeras and point mutants exhibited a differential interaction with (a)virulence proteins in the AvrPto and VirPphA family, demonstrating distinct binding requirements for different ligands. Additionally, the identification of chimeras that are both constitutively active as well as capable of binding AvrPto indicates that elicitation of downstream signaling does not involve a conformational change that precludes binding of AvrPto, as previously hypothesized. The correlations between phenotypes and variation generated by DNA shuffling paralleled natural variation observed between orthologs of Pto from Lycopersicon spp.

Pseudomonas Type III effector AvrPto suppresses the programmed cell death induced by two nonhost pathogens in Nicotiana benthamiana and tomato

Many gram-negative bacterial pathogens rely on a type III secretion system to deliver a number of effector proteins into the host cell. Though a number of these effectors have been shown to contribute to bacterial pathogenicity, their functions remain elusive. Here we report that AvrPto, an effector known for its ability to interact with Pto and induce Pto-mediated disease resistance, inhibited the hypersensitive response (HR) induced by nonhost pathogen interactions. Pseudomonas syringae pv. tomato T1 causes an HR-like cell death on Nicotiana benthamiana. This rapid cell death was delayed significantly in plants inoculated with P. syringae pv. tomato expressing avrPto. In addition, P. syringae pv. tabaci expressing avrPto suppressed nonhost HR on tomato prf3 and ptoS lines. Transient expression of avrPto in both N. benthamiana and tomato prf3 plants also was able to suppress nonhost HR. Interestingly, AvrPto failed to suppress cell death caused by other elicitors and nonhost pathogens. AvrPto also failed to suppress cell death caused by certain gene-for-gene disease resistance interactions. Experiments with avrPto mutants revealed several residues important for the suppression effects. AvrPto mutants G2A, G99V, P146L, and a 12-amino-acid C-terminal deletion mutant partially lost the suppression ability, whereas S94P and 196T enhanced suppression of cell death in N. benthamiana. These results, together with other discoveries, demonstrated that suppression of host-programmed cell death may serve as one of the strategies bacterial pathoens use for successful invasion.

Drought tolerance established by enhanced expression of the CC-NBS-LRR gene, ADR1, requires salicylic acid, EDS1 and ABI1

An activation-tagged allele of activated disease resistance 1 (ADR1) has previously been shown to convey broad spectrum disease resistance. ADR1 was found to encode a coiled-coil (CC)-nucleotide-binding site (NBS)-leucine-rich repeat (LRR) protein, which possessed domains of homology with serine/threonine protein kinases. Here, we show that either constitutive or conditional enhanced expression of ADR1 conferred significant drought tolerance. This was not a general feature of defence-related mutants because cir (constitutive induced resistance)1, cir2 and cpr (constitutive expressor of PR genes)1, which constitutively express systemic acquired resistance (SAR), failed to exhibit this phenotype. Cross-tolerance was not a characteristic of adr1 plants, rather they showed increased sensitivity to thermal and salinity stress. Hence, adr1-activated signalling may antagonise some stress responses. Northern analysis of abiotic marker genes revealed that dehydration-responsive element (DRE)B2A but not DREB1A, RD (response to dehydration)29A or RD22 was expressed in adr1 plant lines. Furthermore, DREB2A expression was salicylic acid (SA) dependent but NPR (non-expressor of PR genes)1 independent. In adr1/ADR1 nahG (naphthalene hydroxylase G), adr1/ADR1 eds (enhanced disease susceptibility)1 and adr1/ADR1 abi1 double mutants, drought tolerance was significantly reduced. Microarray analyses of plants containing a conditional adr1 allele demonstrated that a significant number of the upregulated genes had been previously implicated in responses to dehydration. Therefore, biotic and abiotic signalling pathways may share multiple nodes and their outputs may have significant functional overlap.

Plants as models for the study of human pathogenesis

There are many common disease mechanisms used by bacterial pathogens of plants and humans. They use common means of attachment, secretion and genetic regulation. They share many virulence factors, such as extracellular polysaccharides and some type III secreted effectors. Plant and human innate immune systems also share many similarities. Many of these shared bacterial virulence mechanisms are homologous, but even more appear to have independently converged on a common function. This combination of homologous and analogous systems reveals conserved and critical steps in the disease process. Given these similarities, and the many experimental advantages of plant biology, including ease of replication, stringent genetic and reproductive control, and high throughput with low cost, it is proposed that plants would make excellent models for the study of human pathogenesis.

Alginate gene expression by Pseudomonas syringae pv. tomato DC3000 in host and non-host plants

Pseudomonas syringae produces the exopolysaccharide alginate, a copolymer of mannuronic and guluronic acid. Although alginate has been isolated from plants infected by P. syringae, the signals and timing of alginate gene expression in planta have not been described. In this study, an algD : : uidA transcriptional fusion, designated pDCalgDP, was constructed and used to monitor alginate gene expression in host and non-host plants inoculated with P. syringae pv. tomato DC3000. When leaves of susceptible collard plants were spray-inoculated with DC3000(pDCalgDP), algD was activated within 72 h post-inoculation (p.i.) and was associated with the development of water-soaked lesions. In leaves of the susceptible tomato cv. Rio Grande-PtoS, algD activity was lower than in collard and was not associated with water-soaking. The expression of algD was also monitored in leaves of tomato cv. Rio Grande-PtoR, which is resistant to P. syringae pv. tomato DC3000. Within 12 h p.i., a microscopic hypersensitive response (micro-HR) was observed in Rio Grande-PtoR leaves spray-inoculated with P. syringae pv. tomato DC3000(pDCalgDP). As the HR progressed, histochemical staining indicated that individual bacterial cells on the surface of resistant tomato leaves were expressing algD. These results indicate that algD is expressed in both susceptible (e.g. collard, tomato) and resistant (Rio Grande-PtoR) host plants. The expression of algD in an incompatible host-pathogen interaction was further explored by monitoring transcriptional activity in leaves of tobacco, which is not a host for P. syringae pv. tomato. In tobacco inoculated with DC3000(pDCalgDP), an HR was evident within 12 h p.i., and algD expression was evident within 8-12 h p.i. However, when tobacco was inoculated with an hrcC mutant of DC3000, the HR did not occur and algD expression was substantially lower. These results suggest that signals that precede the HR may stimulate alginate gene expression in P. syringae. Histochemical staining with nitro blue tetrazolium indicated that the superoxide anion () is a signal for algD activation in planta. This study indicates that algD is expressed when P. syringae attempts to colonize both susceptible and resistant plant hosts.

Pseudomonas type III effector AvrPtoB induces plant disease susceptibility by inhibition of host programmed cell death

The AvrPtoB type III effector protein is conserved among diverse genera of plant pathogens suggesting it plays an important role in pathogenesis. Here we report that Pseudomonas AvrPtoB acts inside the plant cell to inhibit programmed cell death (PCD) initiated by the Pto and Cf9 disease resistance proteins and, remarkably, the pro-apoptotic mouse protein Bax. AvrPtoB also suppressed PCD in yeast, demonstrating that AvrPtoB functions as a cell death inhibitor across kingdoms. Using truncated AvrPtoB proteins, we identified distinct N- and C-terminal domains of AvrPtoB that are sufficient for host recognition and PCD inhibition, respectively. We also identified a novel resistance phenotype, Rsb, that is triggered by an AvrPtoB truncation disrupted in the anti-PCD domain. A Pseudomonas syringae pv. tomato DC3000 strain with a chromosomal mutation in the AvrPtoB C-terminus elicited Rsb-mediated immunity in previously susceptible tomato plants and disease was restored when full-length AvrPtoB was expressed in trans. Thus, our results indicate that a type III effector can induce plant susceptibility to bacterial infection by inhibiting host PCD.

Understanding the functions of plant disease resistance proteins

Many disease resistance (R) proteins of plants detect the presence of disease-causing bacteria, viruses, or fungi by recognizing specific pathogen effector molecules that are produced during the infection process. Effectors are often pathogen proteins that probably evolved to subvert various host processes for promotion of the pathogen life cycle. Five classes of effector-specific R proteins are known, and their sequences suggest roles in both effector recognition and signal transduction. Although some R proteins may act as primary receptors of pathogen effector proteins, most appear to play indirect roles in this process. The functions of various R proteins require phosphorylation, protein degradation, or specific localization within the host cell. Some signaling components are shared by many R gene pathways whereas others appear to be pathway specific. New technologies arising from the genomics and proteomics revolution will greatly expand our ability to investigate the role of R proteins in plant disease resistance.

The barley stem rust-resistance gene Rpg1 is a novel disease-resistance gene with homology to receptor kinases

Stem rust caused by Puccinia graminis f. sp. tritici was among the most devastating diseases of barley in the northern Great Plains of the U.S. and Canada before the deployment of the stem rust-resistance gene Rpg1 in 1942. Since then, Rpg1 has provided durable protection against stem rust losses in widely grown barley cultivars (cvs.). Extensive efforts to clone Rpg1 by synteny with rice provided excellent flanking markers but failed to yield the gene because it does not seem to exist in rice. Here we report the map-based cloning and characterization of Rpg1. A high-resolution genetic map constructed with 8,518 gametes and a 330-kb bacterial artificial chromosome contig physical map positioned the gene between two crossovers approximately 0.21 centimorgan and 110 kb apart. The region including Rpg1 was searched for potential candidate genes by sequencing low-copy probes. Two receptor kinase-like genes were identified. The candidate gene alleles were sequenced from resistant and susceptible cvs. Only one of the candidate genes showed a pattern of apparently functional gene structure in the resistant cvs. and defective gene structure in the susceptible cvs. identifying it as the Rpg1 gene. Rpg1 encodes a receptor kinase-like protein with two tandem protein kinase domains, a novel structure for a plant disease-resistance gene. Thus, it may represent a new class of plant resistance genes.

Pto update: recent progress on an ancient plant defence response signalling pathway

Summary The Pto resistance gene in a gene-for-gene interaction with the avrPto avirulence gene governs resistance to bacterial speck of tomato. A member of a small gene family in tomato, Pto encodes a serine/threonine kinase that interacts in the yeast two-hybrid system with the product of avrPto, an 18-kDa hydrophilic protein. Over the past decade, studies of these genes, their products, and the defence response signalling pathway they govern have led to significant advances in our understanding of the biochemistry of Pto, the bacterial delivery and Pto recognition specificity for AvrPto, and candidate components in the pathway and their potential functions. This article provides an update of recent advances, which include the discovery of AvrPto structure-function relationships in disease and resistance, discovery of a second avirulence protein (AvrPtoB) recognized by Pto and its limited similarity to AvrPto, expression analysis and functional characterization of transcription factors Pti4, Pti5, and Pti6 that interact with Pto, analyses of Pto over-expression that activates defence responses independent of AvrPto, and comparisons of Pto gene family members and homologues in tomato and other Solanaceae, as well as other plant species. These comparisons, in particular, have provided exciting new insight into the antiquity of the Pto gene family and of the capacity for specific recognition of AvrPto that activates plant defence.

Plant receptor-like kinase gene family: diversity, function, and signaling

Plant receptor-like kinases (RLKs) are transmembrane proteins with putative amino-terminal extracellular domains and carboxyl-terminal intracellular kinase domains, with striking resemblance in domain organization to the animal receptor tyrosine kinases such as epidermal growth factor receptor. The recently sequenced Arabidopsis genome contains more than 600 RLK homologs, representing nearly 2.5% of the annotated protein-coding genes in Arabidopsis. Although only a handful of these genes have known functions and fewer still have identified ligands or downstream targets, the studies of several RLKs such as CLAVATA1, Brassinosteroid Insensitive 1, Flagellin Insensitive 2, and S-locus receptor kinase provide much-needed information on the functions mediated by members of this large gene family. RLKs control a wide range of processes, including development, disease resistance, hormone perception, and self-incompatibility. Combined with the expression studies and biochemical analysis of other RLKs, more details of RLK function and signaling are emerging.

Arabidopsis genome sequence as a tool for functional genomics in tomato

Tomato is a well-established model organism for studying many biological processes including resistance and susceptibility to pathogens and the development and ripening of fleshy fruits. The availability of the complete Arabidopsis genome sequence will expedite map-based cloning in tomato on the basis of chromosomal synteny between the two species, and will facilitate the functional analysis of tomato genes.