Plant responses to insect herbivory are known to be shaped by prior insect egg deposition. However, little knowledge is available in this respect for perennial K-selected plants. Project B1 aims to elucidate the impact of egg deposition by the multivoltine elm leaf beetle Xanthogaleruca luteola on the anti-herbivore defence of Ulmus minor.
In CRC-973-phase I, we showed that anti-herbivore defence of egg-deposited elms is more effective than of egg-free elms. We recorded increased mortality of larvae feeding on previously egg-deposited elm and fewer adult females resulting from surviving larvae. As fewer female beetles will limit the reproductive capacity of the herbivore´s population, elms likely benefit in the long run from taking the eggs as warning signals. Chemical analyses of numerous defence-related parameters of feeding-damaged leaves with and without prior egg deposition showed that especially phenolic compounds play a central role for the egg-mediated effects on the elm´s anti-herbivore defence. Currently we study transcriptomic and phytohormonal changes mediated by egg deposition in feeding-damaged and undamaged clonal elm trees propagated from a tissue culture maintained in our lab.
In CRC-973-phase II, our major questions are: (1) How is the egg-mediated increased anti-herbivore defence regulated on a spatial and temporal scale? These studies aim to elucidate how far the information about the priming signal (the egg deposition) is spread within a tree, how fast chemical and molecular changes occur in egg-deposited leaves damaged by feeding larvae and for how long they are maintained. (2) Do the known egg-induced changes in leaf odour act as priming stimulus, similarly to the well-known priming activity of feeding-induced leaf odour? (3) How is the priming effect of elm leaf beetle eggs affected by the eggs of co-occurring predators (here: coccinellids)? (4) How does feeding by predatory larval or adult coccinellids upon the elm leaf beetle eggs affect the elm´s priming of anti-herbivore defence? When coccinellids devour elm leaf beetle eggs, they usually do not feed 100% of the eggs, but leave the egg basis with a rest of the egg shell attached to the leaf surface. Do these left-overs of elm leaf beetle eggs destroyed by predators still affect the plant´s anti-herbivore defence against elm leaf beetle larvae? Or does the plant “relax” when the elm leaf beetle eggs are destroyed and does no longer keep a “warned” (primed) state?
We intend to study these questions on the mechanisms of priming of the elm´s anti-herbivore defence and on its ecological conditions by insect performance studies and analyses of transcript levels and metabolites of differently treated elm leaves.