A useful weed put to work: genetic analysis of disease resistance in Arabidopsis thaliana

Genetic characterization of RRS1, a recessive locus in Arabidopsis thaliana that confers resistance to the bacterial soilborne pathogen Ralstonia solanacearum

The soilborne, vascular pathogen Ralstonia solanacearum, the causative agent of bacterial wilt, was shown to infect a range of Arabidopsis thaliana accessions. The pathogen was capable of infecting the Col-5 accession in an hrp-dependent manner, following root inoculation. Elevated bacterial population levels were found in leaves of Col-5, 4 to 5 days after root inoculation by the GMI1000 strain. Bacteria were found predominantly in the xylem vessels and spread systematically throughout the plant. The Nd-1 accession of A. thaliana was resistant to the GMI1000 strain of R. solanacearum. Bacterial concentrations detected in leaves of Nd-1, inoculated with an hrp+ strain of R. solanacearum, were only slightly higher than those detected in the susceptible accession, Col-5, following inoculation with a strain whose hrp gene cluster was deleted. Leaf inoculation of the GMI1000 strain on the resistant accession Nd-1 induced the formation of lesions in the older leaves of the rosette whereas the same strain of R. solanacearum provoked complete wilting of Col-5. Resistance to strain GMI1000 of R. solanacearum segregated as a simply inherited recessive trait in a genetic cross between Col-5 and Nd-1. F9 recombinant inbred lines generated between these two accessions were used to map a locus, RRS1, that was the major determinant of resistance between restriction fragment length polymorphism markers mi83 and mi61 on chromosome V. This region of the A. thaliana genome is known to contain many other pathogen recognition capabilities.

Entry of Xanthomonas campestris pv. campestris into hydathodes of Arabidopsis thaliana leaves: a system for studying early infection events in bacterial pathogenesis

Xanthomonas campestris pv. campestris (Xcc) is a vascular pathogen of cruciferous plants that normally gains entry to plants via hydathodes. In order to study the basis of the preference for this protal of entry we have developed an Arabidopsis thaliana model with attached or detached leaves partially immersed in a bacterial suspension. Entry of bacteria into leaves, assessed by resistance to surface sterilization, could be detected after 1 h. Dissection of leaves and histochemical staining for beta-glucuronidase produced by the bacteria indicated that they were located in hydathodes. In contrast, similar experiments with the leaf-spotting pathogen X. campestris pv. armoraciae gave patterns of localized staining dispersed over the leaf area, indicative of entry through stomata. A survey of 41 A. thaliana accessions showed that they fell into three classes distinguishable by total numbers of Xcc that entered under standard conditions and by preference for hydathode colonization. Previously isolated Xcc mutants affected in pathogenicity were tested for hydathode colonization: an hrp mutant behaved indistinguishably from the wild type, and rpf regulatory mutants gave 10-fold reduced colonization, whereas with rfaX mutants with altered lipopolysaccharide, few if any viable bacteria were recoverable from hydathodes. This fact, together with the rapid induction of superoxide dismutase in the bacteria located in hydathodes, suggests that an early defense reaction is mounted in the hydathode.

Isolation of new Arabidopsis mutants with enhanced disease susceptibility to Pseudomonas syringae by direct screening

To identify plant defense components that are important in restricting the growth of virulent pathogens, we screened for Arabidopsis mutants in the accession Columbia (carrying the transgene BGL2-GUS) that display enhanced disease susceptibility to the virulent bacterial pathogen Pseudomonas syringae pv. maculicola (Psm) ES4326. Among six (out of a total of 11 isolated) enhanced disease susceptibility (eds) mutants that were studied in detail, we identified one allele of the previously described npr1/nim1/sai1 mutation, which is affected in mounting a systemic acquired resistance response, one allele of the previously identified EDS5 gene, and four EDS genes that have not been previously described. The six eds mutants studied in detail (npr1-4, eds5-2, eds10-1, eds11-1, eds12-1, and eds13-1) displayed different patterns of enhanced susceptibility to a variety of phytopathogenic bacteria and to the obligate biotrophic fungal pathogen Erysiphe orontii, suggesting that particular EDS genes have pathogen-specific roles in conferring resistance. All six eds mutants retained the ability to mount a hypersensitive response and to restrict the growth of the avirulent strain Psm ES4326/avrRpt2. With the exception of npr1-4, the mutants were able to initiate a systemic acquired resistance (SAR) response, although enhanced growth of Psm ES4326 was still detectable in leaves of SAR-induced plants. The data presented here indicate that eds genes define a variety of components involved in limiting pathogen growth, that many additional EDS genes remain to be discovered, and that direct screens for mutants with altered susceptibility to pathogens are helpful in the dissection of complex pathogen response pathways in plants.

Disease resistance gene homologs correlate with disease resistance loci of Arabidopsis thaliana

The disease resistance genes RPS2 of Arabidopsis and N of tobacco, among other recently cloned resistance genes, share several conserved sequences. Degenerate oligonucleotide primers, based on conserved sequences in the nucleotide binding site (NBS) and a weak hydrophobic domain of RPS2 and N, were used to amplify homologous sequences from Arabidopsis thaliana. Amplification products were obtained that were similar in sequence to the disease resistance genes RPS2, RPM1, N and L6. The Arabidopsis CIC-YAC library was used to identify the position of the disease resistance homologs on the Arabidopsis genome. Their map positions could be correlated with the disease resistance loci RPS5, RAC1, RPP9, CAR1, RPP7, RPW2, RPP1, RPP10, RPP14, RPP5, RPP4, RPS2, RPW6, HRT, RPS4, RPP8, RPP21, RPP22, RPP23, RPP24 and TTR1. This method was therefore not only successful in the identification of sequences located within gene clusters that are involved in disease resistance, but could also contribute to the cloning of disease resistance genes from Arabidopsis.

Isolation of a mutant of Arabidopsis thaliana carrying two simultaneous mutations affecting tobacco mosaic virus multiplication within a single cell

Tobacco mosaic virus strain Cg (TMV-Cg) infects A. thaliana systemically. In order to identify host factors involved in the multiplication of TMV-Cg, we isolated mutant of A. thaliana from an M2 population mutagenized by fast neutron irradiation, in which the accumulation of the coat protein in upper systemic leaves was reduced to low levels. The phenotype of the mutant, YS241, was controlled primarily by a single nuclear recessive mutation named tom2-1, which was distinct from tom1, a separate mutation which also affects TMV-Cg multiplication. The tom2-1 mutation affected the accumulation of TMV-related RNAs in protoplasts in a tobamovirus-specific manner, suggesting that the wild-type TOM2 gene product is necessary for efficient amplification of TMV-related RNAs within a single cell, through specific interaction with virus-coded factors. Furthermore, we found that YS241 contained a single dominant modifier named ttm1, which increased the efficiency of multiplication of TMV-Cg and a tomato strain of TMV in a tom2-1 genetic background, both in plants and in protoplasts. We propose that the ttm1 element might be a translocated form of the TOM2 gene.

Identification of R-gene homologous DNA fragments genetically linked to disease resistance loci in Arabidopsis thaliana

Disease resistance in plants is a desirable economic trait. A number of disease resistance genes from various plant species have been cloned so far. The gene products of some of these can be distinguished by the presence of an N-terminal nucleotide binding site and a C-terminal stretch of leucine-rich repeats. Although these gene products are structurally related, the DNA sequences are poorly conserved. Only parts of the nucleotide binding site share enough DNA identity to design primers for polymerase chain reaction amplification of related DNA sequences. Such primers were used to amplify different resistance-gene-like (RGL) DNA fragments from Arabidopsis thaliana accessions Landsberg erecta and Columbia. Almost all cloned DNA fragments were genetically closely linked with known disease resistance loci. Most RGL fragments were found in a clustered or dispersed multi-copy sequence organization, supporting the supposed correlation of RGL sequences and disease resistance loci.

Isolation of an Arabidopsis thaliana mutant in which accumulation of cucumber mosaic virus coat protein is delayed

Cucumber mosaic virus (CMV) is known to systemically infect Arabidopsis thaliana ecotype Columbia plants. In order to identify the host factors involved in the multiplication of CMV, we isolated an A. thaliana mutant in which the accumulation of the coat protein (CP) of CMV in upper uninoculated leaves was delayed. Genetic analyses suggested that the phenotype of delayed accumulation of CMV CP in the mutant plants was caused by a single, nuclear and recessive mutation designated cum1-1, which was located on chromosome IV. The cum1-1 mutation did not affect the multiplication of tobacco mosaic virus, turnip crinkle virus or turnip yellow mosaic virus, which belong to different taxonomic groups from CMV. Accumulation of CMV CP in the inoculated leaves of cum1-1 plants was also delayed either when CMV virion or CMV virion RNA was inoculated. On the other hand, when cum1-1 and the wild-type Col-0 protoplasts were inoculated with CMV virion RNA by electroporation, the accumulations of CMV-related RNAs and the coat protein were similar. These results suggest that the cum1-1 mutation did not affect the uncoating of CMV virion and subsequent replication in an initially infected cell but affected the spreading of CMV within an infected leaf, possibly the cell-to-cell movement of CMV in a virus-specific manner.

Cauliflower Mosaic Virus Isolate NY8153 Breaks Resistance in Arabidopsis Ecotype En-2

ABSTRACT Arabidopsis thaliana ecotype En-2 was previously shown to be resistant to cauliflower mosaic caulimovirus (CaMV) isolate CM4-184. In this study, En-2 plants were screened with eight other isolates of CaMV to identify viruses capable of overcoming resistance and to determine if the mechanism of resistance was the same for each virus. En-2 resistance to most CaMV isolates was mediated by the same mechanism, i.e., preventing virus long-distance movement. One CaMV isolate, NY8153, was found that produced a severe systemic infection on En-2 plants. In addition, the CM1841 isolate was able to spread systemically through En-2 plants, to a limited extent, without producing visible symptoms. These data indicate that the resistance shown by En-2 plants is not an all-or-none phenomenon. En-2 plants were susceptible to turnip mosaic potyvirus, suggesting that resistance is specific to CaMV.

PLANT DISEASE RESISTANCE GENES

In "gene-for-gene" interactions between plants and their pathogens, incompatibility (no disease) requires a dominant or semidominant resistance (R) gene in the plant, and a corresponding avirulence (Avr) gene in the pathogen. Many plant/pathogen interactions are of this type. R genes are presumed to (a) enable plants to detect Avr-gene-specified pathogen molecules, (b) initiate signal transduction to activate defenses, and (c) have the capacity to evolve new R gene specificities rapidly. Isolation of R genes has revealed four main classes of R gene sequences whose products appear to activate a similar range of defense mechanisms. Discovery of the structure of R genes and R gene loci provides insight into R gene function and evolution, and should lead to novel strategies for disease control.

Signaling in plant-microbe interactions

Analysis of viral and bacterial pathogenesis has revealed common themes in the ways in which plants and animals respond to pathogenic agents. Pathogenic bacteria use macromolecule delivery systems (types III and IV) to deliver microbial avirulence proteins and transfer DNA-protein complexes directly into plant cells. The molecular events that constitute critical steps of plant-pathogen interactions seem to involve ligand-receptor mechanisms for pathogen recognition and the induction of signal transduction pathways in the plant that lead to defense responses. Unraveling the molecular basis of disease resistance pathways has laid a foundation for the rational design of crop protection strategies.

Genetic engineering for fungal and bacterial diseases

Significant new advances at the molecular level in the field of plant-pathogen interactions form the basis for novel transgenic approaches to crop protection. The cloning of disease resistance genes and the dissection of the signal transduction components of the hypersensitive response and systemic acquired resistance pathways have greatly increased the diversity of options available for transgenic disease resistance. These new approaches will supplement our rapidly increasing repertoire of antimicrobial peptides, defense-related proteins and antimicrobial compounds. The combinatorial deployment of these strategies will be exploited for engineering effective and durable resistance to pathogens in the field. The integration of transgenic approaches with classical resistance breeding offers a potentially chemical-free and environmentally friendly solution for controlling pathogens.

Positional cloning of a gene for nematode resistance in sugar beet

The Hs1(pro-1) locus confers resistance to the beet cyst nematode (Heterodera schachtii Schmidt), a major pest in the cultivation of sugar beet (Beta vulgaris L.). The Hs1(pro-1) gene was cloned with the use of genome-specific satellite markers and chromosomal break-point analysis. Expression of the corresponding complementary DNA in a susceptible sugar beet conferred resistance to infection with the beet cyst nematode. The native Hs1(pro-1) gene, expressed in roots, encodes a 282-amino acid protein with imperfect leucine-rich repeats and a putative membrane-spanning segment, features similar to those of disease resistance genes previously cloned from higher plants.