Fitness costs of mutations affecting the systemic acquired resistance pathway in Arabidopsis thaliana

Abstract

This study investigated the fitness effects of four mutations (npr1, cpr1, cpr5, and cpr6) and two transgenic genotypes (NPR1-L and NPR1-H) affecting different points of the systemic acquired resistance (SAR) signaling pathway associated with pathogen defense in Arabidopsis thaliana. The npr1 mutation, which resulted in a failure to express SAR, had no effect on fitness under growth chamber conditions, but decreased fitness in the field. The expression of NPR1 positively correlated with the fitness in the field. Constitutive activation of SAR by cpr1, cpr5, and cpr6 generally decreased fitness in the field and under two nutrient levels in two growth chamber conditions. At low-nutrient levels, fitness differences between wild type and the constitutive mutants were unchanged or reduced (especially in cpr5). The reduced fitness of the constitutive mutants suggests that this pathway is costly, with the precise fitness consequences highly dependent on the environmental context.

Figure 1.—

Seed yield and rosette diameter in growth chamber and field experiments. Mean values (±SE) are shown for A–G. Letters above bars indicate significantly different values. (A) Seed yield in growth chamber experiment 1, high-nutrient treatment. (B) Seed yield in growth chamber experiment 1, low-nutrient treatment. (C) Rosette diameter in growth chamber experiment 1, high-nutrient treatment. (D) Rosette diameter in growth chamber experiment 1, low-nutrient treatment. (E) Seed yield in growth chamber experiment 2, high-nutrient treatment. (F) Seed yield in growth chamber experiment 2, low-nutrient treatment. (G) Rosette diameter in field experiment across both plantings. (H) Survival in field experiment for planting 1(solid bars) and planting 2 (open bars). Lowercase letters indicate significant differences within planting 1; uppercase letters indicate significant differences within planting 2.

Figure 2.—

Relative PR-1, PDF1.2, and COR78 expression by genotype in field experiment. Mean values (±SE) are shown of gene expression in the field experiment. Letters above bars indicate significantly different values. (A) PR-1 relative expression. (B) PDF1.2 relative expression. (C) COR78 relative expression.