Tritrophic interactions

Dr. Sagar Pandit

One of our major goals is to decipher the role of glucosinolates in these tritrophic interactions. Glucosinolates, the major defense chemicals of cruciferous plants, are nitrogen- and sulfur-containing anionic compounds. They are actually pretoxins, which are converted to toxic isothiocyanates upon the action of myrosinase. Myrosinase is a thioglucosidase enzyme, which deglucosylates glucosinolates to release isothiocyanates. Myrosinase is stored in idioblasts and guard cells in some plants to avoid contact with the glucosinolates, which may be stored in specialized sulfur-rich cells close to the phloem; thus, under normal conditions plants avoid damaging themselves by isothiocyanate formation. However, glucosinolates and myrosinase come in contact with each other after herbivores chewing plant tissue and disrupt compartmentation. Although the glucosinolate-myrosinase system exerts negative effects, various herbivores have adapted to glucosinolate-containing hosts using sequestration, detoxification or toxicity-circumvention mechanisms. In lepidopteran herbivores, three types of glucosinolate- or isothiocyanate-coping mechanisms have been observed. 1) Larvae of the diamondback moth Plutella xylostella use a highly specialized mechanism by which they render glucosinolates harmless before they are hydrolyzed by the myrosinases. These larvae desulfate glucosinolates to less toxic desulfo-glucosinolates using a sulfatase enzyme thus thwarting isothiocyanate biosynthesis. 2) Larvae of the cabbage white butterfly Pieris rapae divert isothiocyanate biosynthesis to form less toxic nitriles using a nitrile-specifier protein. After formation in the midgut, nitriles are readily excreted in the frass. 3) Generalist herbivores of the family Noctuidae, H. armigera, H. zea, Heliothis virescens, Mamestra brassicae, Spodoptera exigua, S. littoralis and Trichoplusia ni use a similar mechanism as that of mammals. These larvae, which cannot circumvent isothiocyanate formation, conjugate isothiocyanates to the L-glutathione (GluCysGly) tripeptide to detoxify them and to accelerate their excretion.

Although the biochemistry of these mechanisms by which herbivores cope with glucosinolates is well studied, their ecological significance, especially their role in tritrophic interactions remains uninvestigated. We are using a modern reverse genetics approach, plant-mediated RNAi, to silence the genes responsible for these coping mechanisms. By feeding glucosinolates to these herbivores impaired in glucosinolate metabolism, we can learn more about the role of these mechanisms in herbivore performance, and the behavioral, biochemical and molecular responses of natural enemies.



Role of glucosinolates in the diamondback moth-parasitoid interactions

Sun Ruo

Certain hymenopteran parasitoids have been observed to differentiate between diamondback moth larvae feeding on the hostplants with different glucosinolate compositions. However, which individual glucosinolates are responsible for this differential response of parasitoids is not known. Similarly, whether the glucosinolate desulfation is beneficial or harmful to the larvae when they are under parasitoid attack is unknown. Ruo Sun proposes to answer these questions by studying the interaction between parasitoids and glucosinolate desulphatase-silenced diamondback moth larvae.