Analyzing plant defense chemistry at the genus-level: patterns and processes in the tropical legume Inga

Background/Question/Methods Genus-level analysis of plant defense chemistry can give important insights into the nature of plant-herbivore interactions at both ecological and evolutionary time scales, thus providing direct evidence of chemotype evolution under herbivore selection. We are pursuing just such a large-scale analysis of chemical defenses in the Neotropical legume genus Inga, a speciose taxon which has undergone explosive radiation since the Miocene. To date, we have explicitly characterized chemical defenses in 40 species of Inga. Using a generalist lepidopteran as a bioassay organism, we identified active metabolite classes, isolated abundant metabolites from each class and solved their structures by NMR. With the structural information thus gained, we made inferences about the composition of the metabolite class as a whole using HPLC-mass spectrometry.

Results/Conclusions

We have found that Inga chemical defenses are largely based on two chemically complex metabolite classes, phenolics and saponins. Phenolics include flavan-3-ols (condensed tannins), flavone glycosides, phenolic acids, and the primary amino acid tyrosine (and derivatives). High structural variability is generated in this group through modifications to stereochemistry, oxidation pattern, glycosylation and polymerization. Inga saponins are composed of one of several triterpene cores which are moderately to highly substituted with an array of saccharide, terpene and phenolic acid moieties. Among the 40 species analyzed, there was very little overlap in chemistry: more than 50% of chemotypes were represented by a single species. True structural novelty was almost never encountered: those previously undescribed structures we did encounter were recombinations of common metabolites. Two major conclusions can be drawn from this analysis. 1) Evolutionary innovation in Inga chemical defenses occurs almost entirely at the level of regulation, i.e. pathway switching, modulation of the timing and location of expression of structural genes and modulation of end-product accumulation levels. 2) Switches in defense chemotypes appear to happen rapidly. Given Inga’s rapid radiation, this can be inferred from both the rarity of intermediate defense chemotypes and the high percentage of species with narrowly defined chemical defenses.