Climate and ecology interactively influence foliar concentrations of salicylic acid in Glycine max and Arabidopsis thaliana

Background/Question/Methods

           To understand how climate influences a plant's responses to its biotic environment, we conducted two studies to test for interacting influences between factors of climate and ecology on endogenous pools of the phytohormone salicylic acid. In a growth chamber experiment, we cultivated Arabidopsis thaliana under variable CO₂ (400 ppb and >550 ppb) and light levels (60-90 μE m⁻² s⁻¹ and 120-150 μE m⁻² s⁻¹) before applying herbivory by fifth-instar Trichoplusia ni caterpillars. In a field experiment at the SoyFACE test site, we sampled leaflets from soybean, Glycine max, grown under variable conditions of CO₂ (400 ppb and 550 ppb) and temperature (ambient and +3.5°C) at the vegetative, flowering and fruiting stages of development. Concentrations of salicylic acid in leaf tissue collected from each experimental treatment were measured via LC/MS.

Results/Conclusions

           Insect herbivory significantly induced salicylic acid concentrations in A. thaliana grown under elevated CO₂.This effect was absent in plants grown under ambient CO₂ levels. Constitutive salicylic acid levels significantly rose by a factor of four during the flowering stage of soybean, relative to the vegetative and flowering stages. This effect exclusively occurred under ambient conditions of CO₂ and temperature.

            Our two experiments broadly demonstrate that climate modulates the concentrations of an ecologically relevant defense hormone, salicylic acid. Based on what is known about salicylic acid as an agonist of jasmonic acid, the results of the first study suggest that plants lose their capacity to activate defenses against herbivores with chewing mouthparts under future conditions of CO₂. Considering that soybean in the Midwest typically experience herbivory from Japanese beetles (Popillia japonica) followed by soybean aphids (Aphis glycines) later in the growth season, the observed increase of constitutive salicylic acid during the fruiting stages of soybean suggests that climate could alter crop tolerance to herbivory from different feeding guilds. The ecological implications of these results under field conditions require substantiation from further experiments.