Trichomes in plant-herbivore interactions : the first line of defense of the wild tobacco Nicotiana attenuata against attacking insects
Trichomes represent a first meal for neonate larvae of the tobacco hornworm Manduca sexta and we explored the functions of their exudates and contents in terms of plant defense. We analyzed the composition of O-acyl sugars in Nicotiana attenuata and demonstrated that their mixture is even more complex than previously described. Fractionation of crude plant extracts and subsequent HPLC-ToFMS analysis revealed the presence of at least 15 different compounds. In addition, MS2 experiments showed that O-acyl sugars can be classified into three different structural classes and that different O-acyl sugar classes vary in their substitution pattern of the sucrose core molecule. We could demonstrate that O-acyl sugars containing trichome exudates are consumed by the larvae of Manduca sexta and thereby cause a release of branched chain aliphatic acids from either the larval frass or body. Furthermore we could show that these volatile acids in M. sexta’s body odor are utilized by the omnivorous ant Pogonomyrmex rugosus to locate its prey and forage on M. sexta. In addition to these findings we could demonstrate that not only omnivorous ants but most likely lizards also use M. sexta’s body and frass odor to locate their prey. Further studies of plant trichomes led to the identification of a trichome metabolite novel to the Solanaceae. Comparative NMR- and MS studies revealed the presence of phaseoloidin - a homogentisic acid glucoside - in the trichomes of the wild tobacco N. attenuata. We could show that besides O-acyl sugars, phaseoloidin and nicotine are present in the different trichome types of N. attenuata and that their abundance varies between different trichome parts. In addition we could demonstrate that phaseoloidin negatively affects the growth of specialist Manduca sexta and generalist Spodoptera littoralis larvae. These results underline the important defensive function of trichomes and their exudates in nature.