Host-Microbe Interactions: Shaping the Evolution of the Plant Immune Response

The evolution of the plant immune response has culminated in a highly effective defense system that is able to resist potential attack by microbial pathogens. The primary immune response is referred to as PAMP-triggered immunity (PTI) and has evolved to recognize common features of microbial pathogens. In the coevolution of host-microbe interactions, pathogens acquired the ability to deliver effector proteins to the plant cell to suppress PTI, allowing pathogen growth and disease. In response to the delivery of pathogen effector proteins, plants acquired surveillance proteins (R proteins) to either directly or indirectly monitor the presence of the pathogen effector proteins. In this review, taking an evolutionary perspective, we highlight important discoveries over the last decade about the plant immune response.

Networks of WRKY transcription factors in defense signaling

Members of the complex family of WRKY transcription factors have been implicated in the regulation of transcriptional reprogramming associated with plant immune responses. Recently genetic evidence directly proving their significance as positive and negative regulators of disease resistance has accumulated. WRKY genes were shown to be functionally connected forming a transcriptional network composed of positive and negative feedback loops and feed-forward modules. Within a web of partially redundant elements some WRKY factors hold central positions mediating fast and efficient activation of defense programs. A key mechanism triggering strong immune responses appears to be based on the inactivation of defense-suppressing WRKY proteins.

Independently evolved virulence effectors converge onto hubs in a plant immune system network

Plants generate effective responses to infection by recognizing both conserved and variable pathogen-encoded molecules. Pathogens deploy virulence effector proteins into host cells, where they interact physically with host proteins to modulate defense. We generated an interaction network of plant-pathogen effectors from two pathogens spanning the eukaryote-eubacteria divergence, three classes of Arabidopsis immune system proteins, and ~8000 other Arabidopsis proteins. We noted convergence of effectors onto highly interconnected host proteins and indirect, rather than direct, connections between effectors and plant immune receptors. We demonstrated plant immune system functions for 15 of 17 tested host proteins that interact with effectors from both pathogens. Thus, pathogens from different kingdoms deploy independently evolved virulence proteins that interact with a limited set of highly connected cellular hubs to facilitate their diverse life-cycle strategies.

The Arabidopsis leucine-rich repeat receptor-like kinases BAK1/SERK3 and BKK1/SERK4 are required for innate immunity to hemibiotrophic and biotrophic pathogens

Recognition of pathogen-associated molecular patterns (PAMPs) by surface-localized pattern recognition receptors (PRRs) constitutes an important layer of innate immunity in plants. The leucine-rich repeat (LRR) receptor kinases EF-TU RECEPTOR (EFR) and FLAGELLIN SENSING2 (FLS2) are the PRRs for the peptide PAMPs elf18 and flg22, which are derived from bacterial EF-Tu and flagellin, respectively. Using coimmunoprecipitation and mass spectrometry analyses, we demonstrated that EFR and FLS2 undergo ligand-induced heteromerization in planta with several LRR receptor-like kinases that belong to the SOMATIC-EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) family, including BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1/SERK3 (BAK1/SERK3) and BAK1-LIKE1/SERK4 (BKK1/SERK4). Using a novel bak1 allele that does not exhibit pleiotropic defects in brassinosteroid and cell death responses, we determined that BAK1 and BKK1 cooperate genetically to achieve full signaling capability in response to elf18 and flg22 and to the damage-associated molecular pattern AtPep1. Furthermore, we demonstrated that BAK1 and BKK1 contribute to disease resistance against the hemibiotrophic bacterium Pseudomonas syringae and the obligate biotrophic oomycete Hyaloperonospora arabidopsidis. Our work reveals that the establishment of PAMP-triggered immunity (PTI) relies on the rapid ligand-induced recruitment of multiple SERKs within PRR complexes and provides insight into the early PTI signaling events underlying this important layer of plant innate immunity.

The Top 10 oomycete pathogens in molecular plant pathology

Oomycetes form a deep lineage of eukaryotic organisms that includes a large number of plant pathogens which threaten natural and managed ecosystems. We undertook a survey to query the community for their ranking of plant-pathogenic oomycete species based on scientific and economic importance. In total, we received 263 votes from 62 scientists in 15 countries for a total of 33 species. The Top 10 species and their ranking are: (1) Phytophthora infestans; (2, tied) Hyaloperonospora arabidopsidis; (2, tied) Phytophthora ramorum; (4) Phytophthora sojae; (5) Phytophthora capsici; (6) Plasmopara viticola; (7) Phytophthora cinnamomi; (8, tied) Phytophthora parasitica; (8, tied) Pythium ultimum; and (10) Albugo candida. This article provides an introduction to these 10 taxa and a snapshot of current research. We hope that the list will serve as a benchmark for future trends in oomycete research.

Arabidopsis PEN3/PDR8, an ATP binding cassette transporter, contributes to nonhost resistance to inappropriate pathogens that enter by direct penetration

Arabidopsis thaliana is a host to the powdery mildew Erysiphe cichoracearum and nonhost to Blumeria graminis f. sp hordei, the powdery mildew pathogenic on barley (Hordeum vulgare). Screening for Arabidopsis mutants deficient in resistance to barley powdery mildew identified PENETRATION3 (PEN3). pen3 plants permitted both increased invasion into epidermal cells and initiation of hyphae by B. g. hordei, suggesting that PEN3 contributes to defenses at the cell wall and intracellularly. pen3 mutants were compromised in resistance to the necrotroph Plectosphaerella cucumerina and to two additional inappropriate biotrophs, pea powdery mildew (Erysiphe pisi) and potato late blight (Phytophthora infestans). Unexpectedly, pen3 mutants were resistant to E. cichoracearum. This resistance was salicylic acid-dependent and correlated with chlorotic patches. Consistent with this observation, salicylic acid pathway genes were hyperinduced in pen3 relative to the wild type. The phenotypes conferred by pen3 result from the loss of function of PLEIOTROPIC DRUG RESISTANCE8 (PDR8), a highly expressed putative ATP binding cassette transporter. PEN3/PDR8 tagged with green fluorescent protein localized to the plasma membrane in uninfected cells. In infected leaves, the protein concentrated at infection sites. PEN3/PDR8 may be involved in exporting toxic materials to attempted invasion sites, and intracellular accumulation of these toxins in pen3 may secondarily activate the salicylic acid pathway.

Arabidopsis is susceptible to infection by a downy mildew fungus

A population of Arabidopsis thaliana growing locally in a suburb of Zürich called Weiningen was observed to be infected with downy mildew. Plants were collected and the progress of infection was investigated in artificial inoculations in the laboratory. The plants proved to be highly susceptible, and pronounced intercellular mycelial growth, haustoria formation, conidiophore production, and sporulation of the causal organism Peronospora parasitica were all observed. The formation of oogonia, antheridia, and oospores also occurred. In contrast, Arabidopsis strain RLD was resistant to infection and none of the above structures was formed. The fungus was localized very soon after penetration of RLD leaf cells, which responded with a typical hypersensitive reaction. The differential interaction of an isolate of P. parasitica with two strains of Arabidopsis opens up the possibility of cloning resistance determinants from a host that is very amenable to genetic and molecular analysis.

Salicylic acid induction-deficient mutants of Arabidopsis express PR-2 and PR-5 and accumulate high levels of camalexin after pathogen inoculation

In Arabidopsis, systemic acquired resistance against pathogens has been associated with the accumulation of salicylic acid (SA) and the expression of the pathogenesis-related proteins PR-1, PR-2, and PR-5. We report here the isolation of two nonallelic mutants impaired in the pathway leading to SA biosynthesis. These SA induction-deficient (sid) mutants do not accumulate SA after pathogen inoculation and are more susceptible to both virulent and avirulent forms of Pseudomonas syringae and Peronospora parasitica. However, sid mutants are not as susceptible to these pathogens as are transgenic plants expressing the nahG gene encoding an SA hydroxylase that degrades SA to catechol. In contrast to NahG plants, only the expression of PR-1 is strongly reduced in sid mutants, whereas PR-2 and PR-5 are still expressed after pathogen attack. Furthermore, the accumulation of the phytoalexin camalexin is normal. These results indicate that SA-independent compensation pathways that do not operate in NahG plants are active in sid mutants. One of the mutants is allelic to eds5 (for enhanced disease susceptibility), whereas the other mutant has not been described previously.

Direct interaction between the Arabidopsis disease resistance signaling proteins, EDS1 and PAD4

The Arabidopsis EDS1 and PAD4 genes encode lipase-like proteins that function in resistance (R) gene-mediated and basal plant disease resistance. Phenotypic analysis of eds1 and pad4 null mutants shows that EDS1 and PAD4 are required for resistance conditioned by the same spectrum of R genes but fulfil distinct roles within the defence pathway. EDS1 is essential for elaboration of the plant hypersensitive response, whereas EDS1 and PAD4 are both required for accumulation of the plant defence-potentiating molecule, salicylic acid. EDS1 is necessary for pathogen-induced PAD4 mRNA accumulation, whereas mutations in PAD4 or depletion of salicylic acid only partially compromise EDS1 expression. Yeast two-hybrid analysis reveals that EDS1 can dimerize and interact with PAD4. However, EDS1 dimerization is mediated by different domains to those involved in EDS1-PAD4 association. Co-immunoprecipitation experiments show that EDS1 and PAD4 proteins interact in healthy and pathogen-challenged plant cells. We propose two functions for EDS1. The first is required early in plant defence, independently of PAD4. The second recruits PAD4 in the amplification of defences, possibly by direct EDS1-PAD4 association.

Arabidopsis mutants simulating disease resistance response

We describe six Arabidopsis mutants, defining at least four loci, that spontaneously form necrotic lesions on leaves. Lesions resemble those resulting from disease, but occur in the absence of pathogen. In five mutants, lesion formation correlates with expression of histochemical and molecular markers of plant disease resistance responses and with expression of genes activated during development of broad disease resistance in plants (systemic acquired resistance [SAR]). We designate this novel mutant class Isd (for lesions simulating disease resistance response). Strikingly, four Isd mutants express substantial resistance to virulent fungal pathogen isolates. Isd mutants vary in cell type preferences for lesion onset and spread. Lesion formation can be conditional and can be induced specifically by biotic and chemical activators of SAR in Isd1 mutants.

Signatures of adaptation to obligate biotrophy in the Hyaloperonospora arabidopsidis genome

Many oomycete and fungal plant pathogens are obligate biotrophs, which extract nutrients only from living plant tissue and cannot grow apart from their hosts. Although these pathogens cause substantial crop losses, little is known about the molecular basis or evolution of obligate biotrophy. Here, we report the genome sequence of the oomycete Hyaloperonospora arabidopsidis (Hpa), an obligate biotroph and natural pathogen of Arabidopsis thaliana. In comparison with genomes of related, hemibiotrophic Phytophthora species, the Hpa genome exhibits dramatic reductions in genes encoding (i) RXLR effectors and other secreted pathogenicity proteins, (ii) enzymes for assimilation of inorganic nitrogen and sulfur, and (iii) proteins associated with zoospore formation and motility. These attributes comprise a genomic signature of evolution toward obligate biotrophy.

Intragenic recombination and diversifying selection contribute to the evolution of downy mildew resistance at the RPP8 locus of Arabidopsis

Pathogen resistance (R) genes of the NBS-LRR class (for nucleotide binding site and leucine-rich repeat) are found in many plant species and confer resistance to a diverse spectrum of pathogens. Little is known about the mechanisms that drive NBS-LRR gene evolution in the host-pathogen arms race. We cloned the RPP8 gene (for resistance to Peronospora parasitica) and compared the structure of alleles at this locus in resistant Landsberg erecta (Ler-0) and susceptible Columbia (Col-0) accessions. RPP8-Ler encodes an NBS-LRR protein with a putative N-terminal leucine zipper and is more closely related to previously cloned R genes that confer resistance to bacterial pathogens than it is to other known RPP genes. The RPP8 haplotype in Ler-0 contains the functional RPP8-Ler gene and a nonfunctional homolog, RPH8A. In contrast, the rpp8 locus in Col-0 contains a single chimeric gene, which was likely derived from unequal crossing over between RPP8-Ler and RPH8A ancestors within a Ler-like haplotype. Sequence divergence among RPP8 family members has been accelerated by positive selection on the putative ligand binding region in the LRRs. These observations indicate that NBS-LRR molecular evolution is driven by the same mechanisms that promote rapid sequence diversification among other genes involved in non-self-recognition.

Regulatory role of SGT1 in early R gene-mediated plant defenses

Animal SGT1 is a component of Skp1-Cullin-F-box protein (SCF) ubiquitin ligases that target regulatory proteins for degradation. Mutations in one (SGT1b) of two highly homologous Arabidopsis SGT1 genes disable early plant defenses conferred by multiple resistance (R) genes. Loss of SGT1b function in resistance is not compensated for by SGT1a. R genes differ in their requirements for SGT1b and a second resistance signaling gene, RAR1, that was previously implicated as an SGT1 interactor. Moreover, SGT1b and RAR1 contribute additively to RPP5-mediated pathogen recognition. These data imply both operationally distinct and cooperative functions of SGT1 and RAR1 in plant disease resistance.

Increased tolerance to two oomycete pathogens in transgenic tobacco expressing pathogenesis-related protein 1a

Expression of pathogenesis-related protein 1a (PR-1a), a protein of unknown biochemical function, is induced to high levels in tobacco in response to pathogen infection. The induction of PR-1a expression is tightly correlated with the onset of systemic acquired resistance (SAR), a defense response effective against a variety of fungal, viral, and bacterial pathogens. While PR-1a has been postulated to be involved in SAR, and is the most highly expressed of the PR proteins, evidence for its role is lacking. In this report, we demonstrate that constitutive high-level expression of PR-1a in transgenic tobacco results in tolerance to infection by two oomycete pathogens, Peronospora tabacina and Phytophthora parasitica var. nicotianae.

Host-Parasite Coevolutionary Conflict Between Arabidopsis and Downy Mildew

Plants are constantly exposed to attack by an array of diverse pathogens but lack a somatically adaptive immune system. In spite of this, natural plant populations do not often suffer destructive disease epidemics. Elucidating how allelic diversity within plant genes that function to detect pathogens (resistance genes) counteracts changing structures of pathogen genes required for host invasion (pathogenicity effectors) is critical to our understanding of the dynamics of natural plant populations. The RPP13 resistance gene is the most polymorphic gene analyzed to date in the model plant Arabidopsis thaliana. Here we report the cloning of the avirulence gene, ATR13, that triggers RPP13-mediated resistance, and we show that it too exhibits extreme levels of amino acid polymorphism. Evidence of diversifying selection visible in both components suggests that the host and pathogen may be locked in a coevolutionary conflict at these loci, where attempts to evade host resistance by the pathogen are matched by the development of new detection capabilities by the host.

Hormone (dis)harmony moulds plant health and disease

Diseased plants often display phenotypes consistent with hormone perturbations. We review recent data that have revealed roles in plant-microbe interactions for cellular components and signaling molecules that previously were associated only with hormone signaling. A better understanding of cross-talk between hormonal and defense signaling pathways should reveal new potential targets for microbial effectors that attenuate host resistance mechanisms.

Pattern recognition receptors and signaling in plant-microbe interactions

Plants solely rely on innate immunity of each individual cell to deal with a diversity of microbes in the environment. Extracellular recognition of microbe- and host damage-associated molecular patterns leads to the first layer of inducible defenses, termed pattern-triggered immunity (PTI). In plants, pattern recognition receptors (PRRs) described to date are all membrane-associated receptor-like kinases or receptor-like proteins, reflecting the prevalence of apoplastic colonization of plant-infecting microbes. An increasing inventory of elicitor-active patterns and PRRs indicates that a large number of them are limited to a certain range of plant groups/species, pointing to dynamic and convergent evolution of pattern recognition specificities. In addition to common molecular principles of PRR signaling, recent studies have revealed substantial diversification between PRRs in their functions and regulatory mechanisms. This serves to confer robustness and plasticity to the whole PTI system in natural infections, wherein different PRRs are simultaneously engaged and faced with microbial assaults. We review the functional significance and molecular basis of PRR-mediated pathogen recognition and disease resistance, and also an emerging role for PRRs in homeostatic association with beneficial or commensal microbes.

Potentiation of pathogen-specific defense mechanisms in Arabidopsis by beta -aminobutyric acid

The nonprotein amino acids gamma-aminobutyric acid (GABA) and beta-aminobutyric acid (BABA) have known biological effects in animals and plants. Their mode of action has been the object of thorough research in animals but remains unclear in plants. Our objective was to study the mode of action of BABA in the protection of Arabidopis plants against virulent pathogens. BABA protected Arabidopsis against the oomycete pathogen Peronospora parasitica through activation of natural defense mechanisms of the plant such as callose deposition, the hypersensitive response, and the formation of trailing necroses. BABA was still fully protective against P. parasitica in transgenic plants or mutants impaired in the salicylic acid, jasmonic acid, and ethylene signaling pathways. Treatment with BABA did not induce the accumulation of mRNA of the systemic acquired resistance (SAR)-associated PR-1 and the ethylene- and jasmonic acid-dependent PDF1.2 genes. However, BABA potentiated the accumulation of PR-1 mRNA after attack by virulent pathogenic bacteria. As a result, BABA-treated Arabidopsis plants were less diseased compared with the untreated control. In the case of bacteria, BABA protected mutants insensitive to jasmonic acid and ethylene but was not active in plants impaired in the SAR transduction pathway. Thus, BABA protects Arabidopsis against different virulent pathogens by potentiating pathogen-specific plant resistance mechanisms. In addition, we provide evidence that BABA-mediated papilla formation after P. parasitica infection is independent of the SAR signaling pathway.

NDR1, a pathogen-induced component required for Arabidopsis disease resistance

Plant disease resistance (R) genes confer an ability to resist infection by pathogens expressing specific corresponding avirulence genes. In Arabidopsis thaliana, resistance to both bacterial and fungal pathogens, mediated by several R gene products, requires the NDR1 gene. Positional cloning was used to isolate NDR1, which encodes a 660-base pair open reading frame. The predicted 219-amino acid sequence suggests that NDR1 may be associated with a membrane. NDR1 expression is induced in response to pathogen challenge and may function to integrate various pathogen recognition signals.

Three genes of the Arabidopsis RPP1 complex resistance locus recognize distinct Peronospora parasitica avirulence determinants

Plant resistance (R) genes have evolved specific recognition capabilities in defense against pathogens. The evolution of R gene function and maintenance of R gene diversity within a plant species are therefore of great interest. In the Arabidopsis accession Wassilewskija, the RPP1 region on chromosome 3 contains four genetically linked recognition specificities, conditioning resistance to different isolates of the biotrophic oomycete Peronospora parasitica (downy mildew). We show that three of four tightly linked genes in this region, designated RPP1-WsA, RPP1-WsB, and RPP1-WsC, encode functional products of the NBS-LRR (nucleotide binding site-leucine-rich repeat) R protein class. They possess a TIR (Toll, interleukin-1, resistance) domain that is characteristic of certain other NBS-LRR-type R proteins, but in addition, they have unique hydrophilic or hydrophobic N termini. Together, the three RPP1 genes account for the spectrum of resistance previously assigned to the RPP1 region and thus comprise a complex R locus. The distinct but partially overlapping resistance capabilities conferred by these genes are best explained by the hypothesis that each recognizes a different pathogen avirulence determinant. We present evidence suggesting that the RPP genes at this locus are subject to the same selective forces that have been demonstrated for structurally different LRR-type R genes.

Dissecting the beta-aminobutyric acid-induced priming phenomenon in Arabidopsis

Plants treated with the nonprotein amino acid beta-aminobutyric acid (BABA) develop an enhanced capacity to resist biotic and abiotic stresses. This BABA-induced resistance (BABA-IR) is associated with an augmented capacity to express basal defense responses, a phenomenon known as priming. Based on the observation that high amounts of BABA induce sterility in Arabidopsis thaliana, a mutagenesis screen was performed to select mutants impaired in BABA-induced sterility (ibs). Here, we report the isolation and subsequent characterization of three T-DNA-tagged ibs mutants. Mutant ibs1 is affected in a cyclin-dependent kinase-like protein, and ibs2 is defective in AtSAC1b encoding a polyphosphoinositide phosphatase. Mutant ibs3 is affected in the regulation of the ABA1 gene encoding the abscisic acid (ABA) biosynthetic enzyme zeaxanthin epoxidase. To elucidate the function of the three IBS genes in plant resistance, the mutants were tested for BABA-IR against the bacterium Pseudomonas syringae pv tomato, the oomycete Hyaloperonospora parasitica, and BABA-induced tolerance to salt. All three ibs mutants were compromised in BABA-IR against H. parasitica, although to a different extent. Whereas ibs1 was reduced in priming for salicylate (SA)-dependent trailing necrosis, mutants ibs2 and ibs3 were affected in the priming for callose deposition. Only ibs1 failed to express BABA-IR against P. syringae, which coincided with a defect in priming for SA-inducible PR-1 gene expression. By contrast, ibs2 and ibs3 showed reduced BABA-induced tolerance to salt, which correlated with an affected priming for ABA-inducible gene expression. For all three ibs alleles, the defects in BABA-induced sterility and BABA-induced protection against P. syringae, H. parasitica, and salt could be confirmed in independent mutants. The data presented here introduce three novel regulatory genes involved in priming for different defense responses.

Laminarin elicits defense responses in grapevine and induces protection against Botrytis cinerea and Plasmopara viticola

Grapevine (Vitis vinifera L.) is susceptible to many pathogens, such as Botrytis cinerea, Plasmopara viticola, Uncinula necator, and Eutypa lata. Phytochemicals are used intensively in vineyards to limit pathogen infections, but the appearance of pesticide-resistant pathogen strains and a desire to protect the environment require that alternative strategies be found. In the present study, the beta-1,3-glucan laminarin derived from the brown algae Laminaria digitata was shown both to be an efficient elicitor of defense responses in grapevine cells and plants and to effectively reduce B. cinerea and P. viticola development on infected grapevine plants. Defense reactions elicited by laminarin in grapevine cells include calcium influx, alkalinization of the extracellular medium, an oxidative burst, activation of two mitogen-activated protein kinases, expression of 10 defense-related genes with different kinetics and intensities, increases in chitinase and beta-1,3-glucanase activities, and the production of two phytoalexins (resveratrol and epsilon-viniferin). Several of these effects were checked and confirmed in whole plants. Laminarin did not induce cell death. When applied to grapevine plants, laminarin reduced infection by B. cinerea and P. viticola by approximately 55 and 75%, respectively. Our data describing a large set of defense reactions in grapevine indicate that the activation of defense responses using elicitors could be a valuable strategy to protect plants against pathogens.

PHI-base: a new database for pathogen host interactions

To utilize effectively the growing number of verified genes that mediate an organism's ability to cause disease and/or to trigger host responses, we have developed PHI-base. This is a web-accessible database that currently catalogs 405 experimentally verified pathogenicity, virulence and effector genes from 54 fungal and Oomycete pathogens, of which 176 are from animal pathogens, 227 from plant pathogens and 3 from pathogens with a fungal host. PHI-base is the first on-line resource devoted to the identification and presentation of information on fungal and Oomycete pathogenicity genes and their host interactions. As such, PHI-base is a valuable resource for the discovery of candidate targets in medically and agronomically important fungal and Oomycete pathogens for intervention with synthetic chemistries and natural products. Each entry in PHI-base is curated by domain experts and supported by strong experimental evidence (gene/transcript disruption experiments) as well as literature references in which the experiments are described. Each gene in PHI-base is presented with its nucleotide and deduced amino acid sequence as well as a detailed description of the predicted protein's function during the host infection process. To facilitate data interoperability, we have annotated genes using controlled vocabularies (Gene Ontology terms, Enzyme Commission Numbers and so on), and provide links to other external data sources (e.g. NCBI taxonomy and EMBL). We welcome new data for inclusion in PHI-base, which is freely accessed at www4.rothamsted.bbsrc.ac.uk/phibase/.

Members of the Arabidopsis HRT/RPP8 family of resistance genes confer resistance to both viral and oomycete pathogens

Turnip crinkle virus (TCV) inoculation onto TCV-resistant Arabidopsis leads to a hypersensitive response (HR) controlled by the dominant gene HRT. HRT is a member of the class of resistance (R) genes that contain a leucine zipper, a nucleotide binding site, and leucine-rich repeats. The chromosomal position of HRT and its homology to resistance gene RPP8 and two RPP8 homologs indicate that unequal crossing over and gene conversion may have contributed to HRT evolution. RPP8 confers resistance to an oomycete pathogen, Peronospora parasitica. Despite very strong similarities within the HRT/RPP8 family, HRT and RPP8 are specific for the respective pathogens they detect. Hence, the HRT/RPP8 family provides molecular evidence that sequence changes between closely related members of multigene families can generate novel specificities for radically different pathogens. Transgenic plants expressing HRT developed an HR but generally remained susceptible to TCV because of a second gene, RRT, that regulates resistance to TCV. However, several transgenic plants that overexpressed HRT produced micro-HRs or no HR when inoculated with TCV and were resistant to infection. Expression of the TCV coat protein gene in seedlings containing HRT resulted in massive necrosis and death, indicating that the avirulence factor detected by the HRT-encoded protein is the TCV coat protein.

The evolutionary phylogeny of the oomycete "fungi"

Molecular sequencing has helped resolve the phylogenetic relationships amongst the diverse groups of algal, fungal-like and protist organisms that constitute the Chromalveolate "superkingdom" clade. It is thought that the whole clade evolved from a photosynthetic ancestor and that there have been at least three independent plastid losses during their evolutionary history. The fungal-like oomycetes and hyphochytrids, together with the marine flagellates Pirsonia and Developayella, form part of the clade defined by Cavalier-Smith and Chao (2006) as the phylum "Pseudofungi", which is a sister to the photosynthetic chromistan algae (phylum Ochrophyta). Within the oomycetes, a number of predominantly marine holocarpic genera appear to diverge before the main "saprolegnian" and "peronosporalean" lines, into which all oomycetes had been traditionally placed. It is now clear that oomycetes have their evolutionary roots in the sea. The earliest diverging oomycete genera so far documented, Eurychasma and Haptoglossa, are both obligate parasites that show a high degree of complexity and sophistication in their host parasite interactions and infection structures. Key morphological and cytological features of the oomycetes will be reviewed in the context of our revised understanding of their likely phylogeny. Recent genomic studies have revealed a number of intriguing similarities in host-pathogen interactions between the oomycetes with their distant apicocomplexan cousins. Therefore, the earlier view that oomycetes evolved from the largely saprotrophic "saprolegnian line" is not supported and current evidence shows these organisms evolved from simple holocarpic marine parasites. Both the hyphal-like pattern of growth and the acquisition of oogamous sexual reproduction probably developed largely after the migration of these organisms from the sea to land.