Plants have evolved a variety of mechanisms to cope with the negative effects of herbivory, including plant resistance, structural and chemical traits that reduce tissue damage and plant tolerance, the ability to compensate for tissue loss. It has been argued that resistance and tolerance represent two alternative and redundant strategies and therefore should be negatively correlated. However, plant resistance and tolerance appear to be controlled via the same molecular pathway, the Oxidative Pentose-Phosphate Pathway. Here, we take the first step in showing that the two strategies are positively rather than negatively correlated. Specifically, using HPLC, we measured glucosinolate levels following the removal of apical dominance (simulating natural herbivory) in two ecotypes of Arabidopsis thaliana: an undercompensator, Landsberg erecta, and an overcompensator, Columbia-4 and recombinant inbred lines generated from a cross between Columbia and Landsberg erecta to assess the relationship between tolerance and resistance.
We report that glucosinolate concentration in Columbia plants were significantly higher than unclipped controls following the removal of apical dominance, and that glucosinolate concentration is positively associated with attributes of fitness (fruit and seed production). In contrast, clipped Landsberg erecta showed a significant decrease in glucosinolate concentration and a decrease in seed and fruit production compared to unclipped controls. There is also a general relationship between tolerance and resistance. RILs from a cross between Columbia and Landsberg erecta demonstrate that lines that overcompensate have higher induction of glucosinolates than the lines that undercompensate following the removal of apical dominance. Such a positive correlation (R2=0.853, P=0.025) between resistance and fitness compensation serves as support for the idea that tolerance and resistance are molecularly interdependent. Given that both traits (fitness compensation and resistance) are polygenic, one would expect the relationship to fall apart when crossed if the traits are unrelated, however, the relationship is maintained. This positive association between tolerance and resistance is the first step in showing that both defense strategies are mediated by the same genetic pathway. We are currently conducting manipulative gene experiments to assess cause and effect.