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Pathogens can be an important selective agent in plant evolution because they can severely reduce plant fitness and growth. However, the role of pathogen selection on plant evolution depends on the extent of genetic variation for resistance traits and their covariance with host fitness. Although it is usually assumed that resistance traits will covary with plant fitness, this assumption has not been tested rigorously in plant-pathogen interactions. Many plant species are tolerant to herbivores, decoupling the relationship between resistance and fitness. Tolerance to pathogens can reduce selection for resistance and alter the effect of pathogens on plant evolution. In this study, we measured three components of Arabidopsis thaliana resistance (pathogen growth, disease symptoms, and host fitness) to the bacteria Pseudomonas syringae and investigated their covariation to determine the relative importance of resistance and tolerance. We observed extensive quantitative variation in the severity of disease symptoms, the bacterial population size, and the effect of infection on host fitness among 19 accessions of A. thaliana infected with P. syringae. The severity of disease symptoms was strongly and positively correlated with bacterial population size. Although the average fitness of infected plants was smaller than noninfected plants, we found no correlation between the bacterial growth or symptoms expressed by different accessions of A. thaliana and their relative fitness after infection. These results indicate that the accessions studied vary in tolerance to P. syringae, reducing the strength of selection on resistance traits, and that symptoms and bacterial growth are not good predictors of host fitness.

The Arabidopsis genes EDS1 and NDR1 were shown previously by mutational analysis to encode essential components of race-specific disease resistance. Here, we examined the relative requirements for EDS1 and NDR1 by a broad spectrum of Resistance (R) genes present in three Arabidopsis accessions (Columbia, Landsberg-erecta, and Wassilewskija). We show that there is a strong requirement for EDS1 by a subset of R loci (RPP2, RPP4, RPP5, RPP21, and RPS4), conferring resistance to the biotrophic oomycete Peronospora parasitica, and to Pseudomonas bacteria expressing the avirulence gene avrRps4. The requirement for NDR1 by these EDS1-dependent R loci is either weak or not measurable. Conversely, three NDR1-dependent R loci, RPS2, RPM1, and RPS5, operate independently of EDS1. Another RPP locus, RPP8, exhibits no strong exclusive requirement for EDS1 or NDR1 in isolate-specific resistance to P. parasitica, although resistance is compromised weakly by eds1. Similarly, resistance conditioned by two EDS1-dependent RPP genes, RPP4 and RPP5, is impaired partially by ndr1, implicating a degree of pathway cross-talk. Our results provide compelling evidence for the preferential utilization of either signaling component by particular R genes and thus define at least two disease resistance pathways. The data also suggest that strong dependence on EDS1 or NDR1 is governed by R protein structural type rather than pathogen class.

The Arabidopsis thaliana disease resistance genes RPS2 and RPM1 belong to a class of plant disease resistance genes that encode proteins that contain an N-terminal tripartite nucleotide binding site (NBS) and a C-terminal tandem array of leucine-rich repeats. RPS2 and RPM1 confer resistance to strains of the bacterial phytopathogen Pseudomonas syringae carrying the avirulence genes avrRpt2 and avrB, respectively. In these gene-for-gene relationships, it has been proposed that pathogen avirulence genes generate specific ligands that are recognized by cognate receptors encoded by the corresponding plant resistance genes. To test this hypothesis, it is crucial to know the site of the potential molecular recognition. Mutational analysis of RPS2 protein and in vitro translation/translocation studies indicated that RPS2 protein is localized in the plant cytoplasm. To determine whether avirulence gene products themselves are the ligands for resistance proteins, we expressed the avrRpt2 and avrB genes directly in plant cells using a novel quantitative transient expression assay, and found that expression of avrRpt2 and avrB elicited a resistance response in plants carrying the corresponding resistance genes. This observation indicates that no bacterial factors other than the avirulence gene products are required for the specific resistance response as long as the avirulence gene products are correctly localized. We propose that molecular recognition of P. syringae in RPS2- and RPM1-specified resistance occurs inside of plant cells.

Sunflower rust, incited by Puccinia helianthi Schw., is a significant disease problem in sunflower (Helianthus annuus L.) production areas of North America, Argentina, and Australia and has been identified on sunflower grown in all regions of the world. Five races of rust presently have been identified in the USA. Recently, a new rust isolate gave a previously unreported virulence reaction pattern and was designated North American Race 6. A genetic study was conducted to determine the inheritance of resistance to Race 6 found in MC29 (AUS), a germplasm line introduced from Australia. [F.sub.1], [F.sub.2], and testcross progeny were produced from two crosses following hand pollinations: HA89/MC29 (AUS) and MC29 (AUS)/MC69. A multiple race inoculation technique was utilized under greenhouse conditions to evaluate the resistance or susceptibility of each plant to North American Races 1, 3, and 6. Chi-square tests for goodness of fit and heterogeneity were utilized to test each [F.sub.2 and testcross family. A single, dominant gene in MC29 (AUS) was found to confer resistance to North American Races 1, 3, and 6. The gene was independent and nonallelic to the genes [R.sup.1] and [R.sup.2], and was designated [R.sup.10]. Identification of resistance to Race 6 has increased the genetic diversity of resistance to sunflower rust in North America.

Biochemical markers linked to pest resistance genes would be useful to plant breeders. This study was conducted to determine if biochemical markers could be found closely linked to genes conferring leaf rust resistance in wheat (Triticum aestivum L.). Near-isogenic lines have been developed in 'Thatcher' for a number of genes, Lr1 to Lr31, conferring resistance to leaf rust, caused by Puccinia recondita Rob. ex Desm. Fifteen of these Thatcher near-isogenic lines were evaluated for the isozyme endopeptidase. Two of the near-isogenic lines (Lr1 and Lr19) had endopeptidase phenotypes that differed from the Thatcher backcross parent. The endopeptidase phenotype observed in the near-isogenic line possessing Lr19 was completely different from any other phenotype normally observed in wheat. The leaf rust resistant Thatcher near-isogenic line possessing Lr19 was crossed with the leaf rust susceptible cultivar 'GA 100' (PI538257). The 59 F(sub 2) progeny were grown in the greenhouse and evaluated for leaf rust resistance and endopeptidase phenotype. All F(sub 2) progeny exhibiting the Lr19 endopeptidase phenotype were resistant to leaf rust. Maximum likelihood estimation, using the F(sub 2) phenotypic data, indicated a very tight linkage with a recombination value of 0.01 + or - 0.009 map units for the Lr19 and Ep-D1d loci. In addition, no recombinant F(sub 2) genotypes were found among the 30 F(sub 2) plants for which F(sub 3) progeny were obtained. Thus, the gene Lr19, which confers resistance to leaf rust, and the gene Ep-D1d which encodes endopeptidase phenotype EP-D1d, are linked. This linkage will enable rapid detection of seedlings possessing Lr19 in wheat improvement programs.

Genomic positions of phenotypically defined disease resistance genes (R genes) and R gene homologues were analyzed in three solanaceous crop genera, Lycopersicon (tomato), Solanum (potato), and Capsicum (pepper). R genes occurred at corresponding positions in two or more genomes more frequently than expected by chance; however, in only two cases, both involving Phytophthora spp., did genes at corresponding positions have specificity for closely related pathogen taxa. In contrast, resistances to Globodera spp., potato virus Y, tobacco mosaic virus, and tomato spotted will virus were mapped in two or more genera and did not occur in corresponding positions. Without exception, pepper homologues of the cloned R genes Sw-5, N, Pto, Prf, and 12 were found in syntenous positions in other solanaceous genomes and in some cases also mapped to additional positions near phenotypically defined solanaceous R genes. This detailed analysis and synthesis of all available data for solanaceous R genes suggests a working hypothesis regarding the evolution of R genes. Specifically, while the taxonomic specificity of host R genes may be evolving rapidly, general functions of R alleles (e.g., initiation of resistance response) may be conserved at homologous loci in related plant genera.

Tripsacum dactyloides L., has long been recognized as a potential source of resistance to corn rootworm, Diabrotica spp. However, sterility of intergeneric hybrids obtained from crossing Zea mays L. spp. mays and Tripsacum has limited use of Tripsacum germplasm in corn improvement. Intergeneric hybrids derived from crossing T. dactyloides with Zea diploperennis Iltis, Doebley and Guzman, however, are fully fertile, cross-fertile with maize, and thus a genetic bridge for imparting rootworm resistance and other beneficial traits from Tripsacum to corn. Results of small-scale bioassays of Tripsacum-Z. diploperennis plants and of corn lines crossed with Tripsacum-Z. diploperennis indicate there is a gene for corn rootworm resistance that is inherited in accordance with Mendelian laws. Larger-scale bioassays under controlled conditions are being conducted to see if this observation holds. Two to four week old plants grown in pots are infested with first instar larvae, and allowed to continue growing for two weeks till the third instar stage is reached. Plants are harvested, surviving larvae collected, and roots of individual plants scored for root damage. Plants are then dried and weighed to collect comparative biomass data on treated and control plants. Combined data are evaluated to calculate how many plants are resistance and how many susceptible. Chi square analysis will be employed to test the hypothesis of Mendelian segregation for a gene that confers resistance to Diabrotica.