Responses of feral Brassica to above- versus below-ground herbivores: From ecology to transcriptomics and back

Background/Question/Methods

When Brassica plants are attacked by herbivores they produce glucosinulate compounds that function in anti-herbivore defenses and they also emit volatiles that attract enemies of the herbivores, such as parasitoid wasps. The defense response is local as well as systemic, so that distant plant tissues also show increased defenses. Observations that leaf glucosinulate and volatile production is qualitatively different when plants are attacked above-ground (leaves) or below-ground (roots) suggests high specificity of the systemic response depending on which tissue is attacked.  We explored the transcriptome profile of this above-belowground interaction, using microarrays to evaluate gene expression in both root and shoot tissue after attack (mimicked by jasmonic acid application) either below-ground or above-ground. From a functional perspective, our interest was in identifying genes and pathways that are involved in the jasmonic acid-induced defense responses. From a broader ecological perspective, we aimed to gain insight in the specificity of induced effects in the attacked (“local”) vs. non-attacked (“non-local”) compartment (e.g. effects of induction belowground on roots vs. shoots). Specifically, if the local and nonlocal transcriptome responses are very different then this could reflect adaptive modulation of the response towards tissue-specific herbivore defenses. Alternatively, if local and nonlocal transcriptome responses are very similar then the nonlocal response may be merely a correlated side effect with limited evolutionary significance.

Results/Conclusions

We use this case study in a non-model species to highlight several aspects of the data analysis and interpretation. This includes statistical analysis to detect differentially expressed genes, going from differentially expressed genes to functional insight, validating key results (including checks of consistency with metabolic changes measured in the plants), and linking the transcriptome data back to questions of ecological and evolutionary interest. In this study we found significant differences in primary and secondary metabolism, hormone metabolism, cell wall formation, signalling, and developmental pathways depending on whether the original defense induction was local or systemic, which could indicate that there is indeed a modulation of the response towards tissue-specific herbivore defences.