enzyme in snail saliva induces herbivore-resistance in a marine alga

1. It is well understood that herbivory can cause plants to elevate production of defensive chemicals in their tissues. One of the key questions in understanding patterns of potential coevolutionary links between plant and herbivore is 'what switches these induced plant defences on?' Until cues are identified, understanding the evolutionary and ecological significance of defences in the context of the plant is difficult. 2. We induced host plant resistance in a marine macroalga (Ascophyllum nodosum) in the absence of herbivory by application of ox-amylase, known to exist in mollusc saliva. There was a demonstrable change in the behaviour of a subsequent herbivore (Littorina obtusata) consistent with herbivore induction, i.e. reduced consumption, more but smaller meals and greater movement. We also produced a concomitant increase in the level of phlorotannins, compounds associated with defence against herbivory. 3. Such changes in herbivore behaviour and plant chemistry provide evidence that brown algae and higher plants utilize similar salivate-based signals in the induction of defence against herbivory. Thus the possibility that such responses could be either highly conserved or a convergent strategy is discussed.