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Plants have developed elaborate defense strategies through the coevolutionary process (Jones & Dangl, 2006). In the classic gene-for-gene model of host–pathogen interactions, plant resistance (R) gene products recognize pathogen elicitors, encoded by avirulence (Avr) genes, and initiate signal transduction pathways (Flor, 1971; Keen, 1990). Induced resistance responses include the hypersensitive response (HR), a form of programmed plant cell death, cell-wall strengthening, and the expression of various defense-related genes (Dangl & Jones, 2001). Resistance gene-mediated resistance is a host-specific defense and can only be activated when both R gene and corresponding Avr gene are present (Staskawicz et al., 1995); the absence of either component results in disease, which is typically associated with damage and a reduction in yield of the host plant (Jarosz & Davelos, 1995).
Molecular genetic studies of plant R genes have revealed extensive variation in R gene loci (Caicedo et al., 1999; Mauricio et al., 2003; Rose et al., 2004; Bakker et al., 2006). In several cases, the resistance and susceptibility alleles have been maintained in natural populations for millions of years (Stahl et al., 1999; Tian et al., 2002). Mathematical models generally suggest that a trade-off between costs of resistance in pathogen-free environments and benefits of resistance under infection is required to explain the coexistence of resistant and susceptible individuals (Stahl et al., 1999; Bergelson et al., 2001). Theories without a cost–benefit balance instead require a spatial structure or a trade-off between alternative defensive strategies (Thrall & Burdon, 2002). It is generally agreed that the magnitudes of both costs and benefits of resistance vary among different resistance traits, genetic backgrounds and environmental conditions (Bergelson, 1994; Bergelson & Purrington, 1996; Kover & Schaal, 2002).
Costs of resistance have been successfully detected in various plant–pathogen systems (Baldwin, 1990; Bergelson, 1994; Mauricio, 1998; Tian et al., 2003), generally by utilizing genetically controlled lines to compare the performance of resistant and susceptible plants. Surprisingly, a less consistent picture emerges regarding the benefits of resistance. Whereas substantial empirical support for benefits of resistance exists for plant–fungus systems (Jarosz & Burdon, 1992; Melendez & Ackerman, 1993; Korves & Bergelson, 2004; Kniskern & Rausher, 2006), plant–bacterial systems reveal fitness effects of infection ranging from negative (reducing fitness) to positive (increasing fitness) as a consequence of variation in resistance and tolerance among genetic backgrounds (Goss & Bergelson, 2007). These studies focus on overall quantitative resistance traits of plants subject to natural infection, and little has been shown about the fitness effects resulting from R gene mediated resistance (but see Korves & Bergelson, 2004).
There are still several gaps in our understanding of the evolutionary ecology of host defense, especially to bacterial pathogens. First, as pointed out in Goss & Bergelson (2007), fitness effects of bacterial pathogens on plant hosts have generally been measured using isolates collected from other host species (Kover & Schaal, 2002; Korves & Bergelson, 2004; Kover et al., 2005; but see Goss & Bergelson, 2007). Experiments conducted with wild plants and their naturally occurring pathogens are preferential in having direct evolutionary relevance. Second, surveys of pathogens in naturally occurring plants reveal a wide range of bacterial titers in plant tissues (Dunning, 2008), and this variation appears to be important in shaping plant–bacteria interactions. For example, a study by Korves & Bergelson (2003) revealed different developmental responses of Arabidopsis thaliana hosts when infected by three initial densities of the bacterial pathogen, Pseudomonas syringae. It is thus instructive to test for fitness effects of infection at a range of titers. Finally, there remains the challenge of distinguishing between resistance and tolerance – two mechanisms that prevent plant fitness loss under infection (Mauricio et al., 1997). By definition, resistance traits reduce pathogen growth and spread whereas tolerance traits diminish the impact of infection on fitness (Simms & Triplett, 1994). It has been hypothesized that there should be a negative correlation between resistance and tolerance abilities such that susceptible plants are more tolerant (Fineblum & Rausher, 1995; Mauricio et al., 1997). In order to address whether resistance and tolerance are two exclusive defense strategies, at least regarding infection by one particular pathogen isolate, one can measure tolerance within a resistant accession and within a susceptible accession, and compare these estimates.
In this study, we sought to fill these gaps in our understanding through a detailed investigation of the interaction between A. thaliana and P. syringae, both of which are well-known model species. In particular, we used resistant (R) and susceptible (S) isolines, constructed in one naturally resistant line and one naturally susceptible line, to measure the benefit of resistance and to assess the relationship between tolerance and R gene resistance in A. thaliana. We examined the R gene, Rps5, which segregates for an insertion/deletion polymorphism in which susceptible plants have the entire locus deleted (Simonich & Innes, 1995; Tian et al., 2002). The Rps5 gene confers resistance to the P. syringae pv. tomato carrying avrPphB (Jenner et al., 1991) and to a P. syringae isolate recently found in naturally occurring A. thaliana leaves in Midwest USA (J. M. Kniskern, pers. comm.). Two sets of plant lines with controlled genetic backgrounds differing only in the presence or absence of a functional Rps5 gene were cross-infected with this wild P. syringae strain at three initial infection dosages. In conducting these inoculations, we were interested in whether there is a fitness advantage of R plants relative to S plants when infected with pathogens carrying an Avr gene, and how this fitness advantage varies over three initial infection dosages. Second, we compared tolerance abilities and asked whether there is a negative relationship between Rps5 resistance and tolerance.