Fitness benefits of systemic acquired resistance during Hyaloperonospora parasitica infection in Arabidopsis thaliana

We investigated the fitness benefits of systemic acquired resistance (SAR) in Arabidopsis thaliana using a mutational and transformational genetic approach. Genetic lines were designed to differ in the genes determining resistance signaling in a common genetic background. Two mutant lines (cpr1 and epr5) constitutively activate SAR at different points in SAR signaling, and one mutant line (npr1) has impaired SAR. The transgenic line (NPR1-H) has enhanced resistance when SAR is activated, but SAR is still inducible similarly to wild type. The fitness benefits were also investigated under two nutrient levels to test theories that preventing pathogen damage and realized resistance benefits may be affected by nutrient availability. Under low-nutrient conditions and treatment with the pathogenic oomycete, Hyaloperonospora parasilica, wild type had a higher fitness than the mutant that could not activate SAR, demonstrating that normal inducible SAR is beneficial in these conditions; this result, however, was not found under high-nutrient conditions. The mutants with constitutive SAR all failed to show a fitness benefit in comparison to wild type under a H. parasitica pathogen treatment, suggesting that SAR is induced to prevent an excessive fitness cost.