Co-variation of chemical and mechanical defenses in lima bean (Phaseolus lunatus L.)

Background/Question/Methods

Plants often express various chemical and mechanical defenses simultaneously. While in some plant species these defenses act together forming defense syndromes, in other species defensive traits show tradeoffs. Lima bean (Phaseolus lunatus) is a well-established experimental plant system in chemical ecology for which different types of chemical defenses have been thoroughly analyzed. These defenses include cyanogenesis (wound-induced release of hydrogen cyanide from preformed cyanogenic glucosides), the synthesis of herbivore-induced volatile organic compounds (HIVOCs; indirect defense against herbivores) and the activity ofpolyphenol oxidases (PPOs; enzymes which are mainly involved in resistance to pathogens). In lima bean, cyanogenesis shows a distinct tradeoff with both, HIVOCs and PPOs making this system particularly interesting for studying the function of traits or combination of traits under different biotic and abiotic conditions. Interestingly, in this model plant system mechanical defenses as well as the co-variation of mechanical and chemical defenses have not been investigated so far. In this study, we microscopically analyzed the occurrence of hook-shaped trichomes on leaves serving as mechanical defense against arthropod herbivores among 19 different lima bean genotypes. We mapped this trait together with data on chemical defenses (cyanogenesis, VOCs and PPO activity) on an AFLP-based tree.

Results/Conclusions

Hook-shaped, defense associated trichomes varied among different lima bean genotypes. Five out of 19 genotypes developed trichomes, while 14 did not. Trichomes were only found in one monophyletic group of the AFLP-based tree indicating a single evolutionary origin within the species. Four genotypes (CV8078, CV1315, CV2357, CV2116) possessing trichomes were high cyanogenic, whereas one (CV2441) was low cyanogenic, but nested within a high cyanogenic clade. This finding highlights that high cyanogenesis—in contrast to the presence of trichomes—is not specific to a single clade. As cyanogenesis shows a distinct quantitative tradeoff with the emission of herbivore-induced volatile organic compounds (HIVOCs), four lima bean genotypes with trichomes show low emission of HIVOCs, while one has the ability to emit large amounts of HIVOCs. Cyanogenesis, which is an efficient anti-herbivore defense, has an inhibitory effect on polyphenol oxidase activity in plants, a defense against fungal pathogens. Thus, high cyanogenic plants are more susceptible to colonization by fungi. By analyzing the presence of trichomes, we added another level of complexity in investigating plant defenses. Taken together, our findings indicated that even within a single species, plants invest in different types of defense, but that there are restrictions in the combinations of traits.