The involvement of glutathione S-tranferases in the interactions between Bemisia tabaci (Hemiptera: Aleyrodidae) and its Brassicaceae hosts

Generalist insects like Bemisia tabaci (Hemiptera: Aleyrodidae) are capable of feeding on a wide range of plant botanical families. As a result, they have to cope with a tremendous diversity of plant defense secondary compounds. In many cases, resistance to these compounds is achieved by induced activity of detoxification (metabolic) mechanisms that include several enzyme super-families. One of them, the glutathione S-tranferases (GSTs), is capable of detoxifying (among other substrates) glucosinolates, a group of chemical defense compounds produced mainly by plant species belonging to the Brassicaceae. In this paper, changes in expression level of three B. tabaci GST genes (BtGSTs) were monitored after the insects were allowed to switch between Brassicaceae and cotton hosts. Our analysis revealed a significant increase in the expression of one gene, BtGST2, after switching from cotton to a Brassicaceae host (1.97- and 2.08-fold for white mustard and cabbage, respectively). In addition, a significant decrease in BtGST2 expression (0.53-fold) was observed during switching from white mustard to cotton. A significant elevation in the expression of BtGST2, similar to the one observed when B. tabaci adult switched from feeding on cotton to feeding on white mustard, was observed when the nitrile 4-Hydroxybezyl cyanide was added to the artificial diet. 4-Hydroxybezyl cyanide is a toxic degradation product of Sinalbin, the major glucosinolate in white mustard. Taken together, these findings fit well our expectations from a detoxification system of generalist insect herbivores, because the ability to "turn on" the detoxification system only when required confers an adaptive plasticity that enables generalist insects to optimize their fitness according to the levels of toxins in their environment.