Genetic variation for disease resistance and tolerance among Arabidopsis thaliana accessions

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.

病原体可作为植物演化过程中的关键选择因子，因其能够显著降低植物的适合度与生长表现。然而，病原体选择对植物演化的调控作用，取决于抗性性状的遗传变异幅度，以及该性状与宿主适合度之间的遗传协变关系。尽管学界通常假设抗性性状会与植物适合度呈正相关，但这一假设尚未在植物-病原体互作体系中得到严格验证。诸多植物物种对植食性动物具有耐受性，这打破了抗性与适合度之间的固有关联；而对病原体的耐受性，则会削弱针对宿主抗性的选择压力，并改变病原体对植物演化的影响效应。本研究针对拟南芥（Arabidopsis thaliana）对抗丁香假单胞菌（Pseudomonas syringae）的三项抗性组分——病原菌增殖量、病害症状严重程度以及宿主适合度——进行了测定，并探究了三者间的相关关系，以明确抗性与耐受性的相对重要性。我们在19个受丁香假单胞菌侵染的拟南芥生态型中，观测到病害严重程度、细菌种群规模以及侵染对宿主适合度的影响均存在广泛的数量性状变异。病害症状的严重程度与细菌种群规模呈显著正相关。尽管受侵染植株的平均适合度低于未受侵染植株，但我们并未发现不同拟南芥生态型的细菌增殖量或病害症状，与其侵染后的相对适合度之间存在相关性。上述结果表明，本研究涉及的拟南芥生态型对丁香假单胞菌的耐受性存在差异，这会削弱抗性性状所承受的选择压力；同时，病害症状与细菌增殖量并不能作为宿主适合度的可靠预测指标。