Eavesdropping plants prepare to be attacked: seeds exposed to herbivore kairomones become seedlings that are less palatable to herbivores

Background/Question/Methods

Incidental chemical cues (i.e. kairomones) are widely used in animals and have important consequences.  For example, predator kairomones alter prey behavior, growth, and survival, presumably because predator kairomones indicate an increased likelihood of prey attack.  Despite a large body of knowledge regarding the cues that plants use to induce defense when under actual attack by herbivores, the role of herbivore kairomones (i.e. chemical cues produced by herbivore movement or metabolism but that are not produced by the act of an attack) has remained almost entirely unexamined in plant defense. We exposed seeds and early seedlings of Arabidopsis thaliana and Brassica nigra to mucus from the herbivorous snail (Helix aspersa).  After exposure, seedlings were planted in individual pots and differences in seedling palatability were evaluated using herbivory assays.

Results/Conclusions

We show that exposing seeds of A. thaliana to herbivore kairomones (i.e. mucus from herbivorous snails) during germination and for the first 6 days of development leads to seedlings that are less palatable to herbivores when 25-27 days of age.  We also show that exposure of kairomone duration is important for maintaining induced defense: B. nigra plants that were consistently exposed to snail mucus were less palatable to snails compared to plants that were only exposed to mucus as seeds.  Our work provides one of the first examples that plants are capable of utilizing herbivore kairomones and further illustrates that plants utilize information very early in development to initiate defense.  The response of plants to incidental herbivore cues provides a basis to understand the variation often observed in levels of defense and induction.  Moreover,  the finding that plants utilize herbivore kairomones to induce defense opens up interesting new lines of inquiry regarding the specificity of kairomone use, molecular pathways involved in kairomone recognition, the degree to which kairomones are used relative to other kinds of information, and the degree to which use of herbivore kairomones might be widespread among terrestrial plants.