[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Oomycetes cause devastating plant diseases of global importance, yet little is known about the molecular basis of their pathogenicity. Recently, the first oomycete effector genes with cultivar-specific avirulence (AVR) functions were identified. Evidence of diversifying selection in these genes and their cognate plant host resistance genes suggests a molecular "arms race" as plants and oomycetes attempt to achieve and evade detection, respectively. AVR proteins from Hyaloperonospora parasitica and Phytophthora infestans are detected in the plant host cytoplasm, consistent with the hypothesis that oomycetes, as is the case with bacteria and fungi, actively deliver effectors inside host cells. The RXLR amino acid motif, which is present in these AVR proteins and other secreted oomycete proteins, is similar to a host-cell-targeting signal in virulence proteins of malaria parasites (Plasmodium species), suggesting a conserved role in pathogenicity.
Trends in Microbiology 02/2006; 14(1):8-11. DOI:10.1016/j.tim.2005.11.007 · 9.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The perception of downy mildew avirulence (Arabidopsis thaliana Recognized [ATR]) gene products by matching Arabidopsis thaliana resistance (Recognition of Peronospora parasitica [RPP]) gene products triggers localized cell death (a hypersensitive response) in the host plant, and this inhibits pathogen development. The oomycete pathogen, therefore, is under selection pressure to alter the form of these gene products to prevent detection. That the pathogen maintains these genes indicates that they play a positive role in pathogen survival. Despite significant progress in cloning plant RPP genes and characterizing essential plant components of resistance signaling pathways, little progress has been made in identifying the oomycete molecules that trigger them. Concluding a map-based cloning effort, we have identified an avirulence gene, ATR1NdWsB, that is detected by RPP1 from the Arabidopsis accession Niederzenz in the cytoplasm of host plant cells. We report the cloning of six highly divergent alleles of ATR1NdWsB from eight downy mildew isolates and demonstrate that the ATR1NdWsB alleles are differentially recognized by RPP1 genes from two Arabidopsis accessions (Niederzenz and Wassilewskija). RPP1-Nd recognizes a single allele of ATR1NdWsB; RPP1-WsB also detects this allele plus three additional alleles with divergent sequences. The Emco5 isolate expresses an allele of ATR1NdWsB that is recognized by RPP1-WsB, but the isolate evades detection in planta. Although the Cala2 isolate is recognized by RPP1-WsA, the ATR1NdWsB allele from Cala2 is not, demonstrating that RPP1-WsA detects a novel ATR gene product. Cloning of ATR1NdWsB has highlighted the presence of a highly conserved novel amino acid motif in avirulence proteins from three different oomycetes. The presence of the motif in additional secreted proteins from plant pathogenic oomycetes and its similarity to a host-targeting signal from malaria parasites suggest a conserved role in pathogenicity.
The Plant Cell 07/2005; 17(6):1839-50. DOI:10.1105/tpc.105.031807 · 9.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The oomycete Phytophthora infestans causes late blight, the potato disease that precipitated the Irish famines in 1846 and 1847. It represents a reemerging threat to potato production and is one of >70 species that are arguably the most devastating pathogens of dicotyledonous plants. Nevertheless, little is known about the molecular bases of pathogenicity in these algae-like organisms or of avirulence molecules that are perceived by host defenses. Disease resistance alleles, products of which recognize corresponding avirulence molecules in the pathogen, have been introgressed into the cultivated potato from a wild species, Solanum demissum, and R1 and R3a have been identified. We used association genetics to identify Avr3a and show that it encodes a protein that is recognized in the host cytoplasm, where it triggers R3a-dependent cell death. Avr3a resides in a region of the P. infestans genome that is colinear with the locus containing avirulence gene ATR1(NdWsB) in Hyaloperonospora parasitica, an oomycete pathogen of Arabidopsis. Remarkably, distances between conserved genes in these avirulence loci were often similar, despite intervening genomic variation. We suggest that Avr3a has undergone gene duplication and that an allele evading recognition by R3a arose under positive selection.
Proceedings of the National Academy of Sciences 06/2005; 102(21):7766-71. DOI:10.1073/pnas.0500113102 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: SUMMARY Peronospora parasitica is an obligate biotrophic oomycete that causes downy mildew in Arabidopsis thaliana and Brassica species. Our goal is to identify P. parasitica (At) genes that are involved in pathogenicity. We used suppression subtractive hybridization (SSH) to generate cDNA libraries enriched for in planta-expressed parasite genes and up-regulated host genes. A total of 1345 clones were sequenced representing cDNA fragments from 25 putative P. parasitica (At) genes (Ppat 1-25) and 618 Arabidopsis genes. Analyses of expression patterns showed that 15 Ppats were expressed only in planta. Eleven Ppats encoded peptides with homology (BlastP values < 1e-05) to proteins with roles in membrane or cell wall biosynthesis, amino acid metabolism, osmoregulation, cation transport, phosphorylation or protein secretion. The other 14 represent potentially novel oomycete genes with none having homologues in an extensive Phytophthora species EST database. A full-length sequence was obtained for four Ppats and each encoded small cysteine-rich proteins with amino-terminal signal peptide sequences. These results demonstrate the utility of SSH in obtaining novel in planta-expressed genes from P. parasitica (At) that complements other gene discovery approaches such as EST sequencing.
[Show abstract][Hide abstract] ABSTRACT: In Peronospora parasitica (At) (downy mildew), the genetic determinants of cultivar-specific recognition by Arabidopsis thaliana are the Arabidopsis thaliana-recognised (ATR) avirulence genes. We describe the identification of 10 amplified fragment length polymorphism (AFLP) markers that define a genetic mapping interval for the ATR1Nd avirulence allele, the presence of which is perceived by the RPP1Nd resistance gene. Furthermore, we have constructed a P. parasitica (At) bacterial artificial chromosome (BAC) library comprising over 630Mb of cloned DNA. We have isolated 16 overlapping clones from the BAC library that form a contig spanning the genetic interval. BAC sequence-derived markers and a total mapping population of 311 F(2) individuals were used to refine the ATR1Nd locus to a 1cM interval that is represented by four BAC clones and spans less than 250kb of DNA. This work demonstrates that map-based cloning techniques are feasible in this organism and provides the critical foundations for cloning ATR1Nd using such a strategy.
[Show abstract][Hide abstract] ABSTRACT: Amplified fragment length polymorphism (AFLP) fingerprints and internal transcribed spacer 1 (ITS1) sequences from 27 Peronospora parasitica isolates (collected from Arabidopsis thaliana or Brassica oleracea), 5 Albugo candida isolates (from the same hosts and from Capsella bursa-pastoris), and 1 Bremia lactucae isolate (from Lactuca sativa) were compared. The AFLP analysis divided the isolates into five groups that correlated with taxonomic species and, in most cases, with host origin. The only exception was a group consisting of A. candida isolates from both B. oleracea and C. bursa-pastoris. ITS1 sequence analysis divided the isolates into the same five groups, demonstrated the divergence between P. parasitica isolates from A. thaliana and B. oleracea, and, using previously published ITS1 sequences, clearly showed the relationship between A. candida isolates from different hosts.
[Show abstract][Hide abstract] ABSTRACT: Plants sense phosphate (Pi) deficiency and initiate signaling that controls adaptive responses necessary for Pi acquisition. Herein, evidence establishes that AtSIZ1 is a plant small ubiquitin-like modifier (SUMO) E3 ligase and is a focal controller of Pi starvation-dependent responses. T-DNA insertional mutated alleles of AtSIZ1 (At5g60410) cause Arabidopsis to exhibit exaggerated prototypical Pi starvation responses, including cessation of primary root growth, extensive lateral root and root hair development, increase in root/shoot mass ratio, and greater anthocyanin accumulation, even though intracellular Pi levels in siz1 plants were similar to wild type. AtSIZ1 has SUMO E3 ligase activity in vitro, and immunoblot analysis revealed that the protein sumoylation profile is impaired in siz1 plants. AtSIZ1-GFP was localized to nuclear foci. Steadystate transcript abundances of Pi starvation-responsive genes AtPT2, AtPS2, and AtPS3 were moderate but clearly greater in siz1 seedlings than in wild type, where Pi is sufficient. Pi starvation induced the expression of these genes to the same extent in siz1 and wild-type seedlings. However, two other Pi starvation-responsive genes, AtIPS1 and AtRNS1, are induced more slowly in siz1 seedlings by Pi limitation. PHR1, a MYB transcriptional activator of AtIPS1 and AtRNS1, is an AtSIZ1 sumoylation target. These results indicate that AtSIZ1 is a SUMO E3 ligase and that sumoylation is a control mechanism that acts both negatively and positively on different Pi deficiency responses