Plasticity of secreted saliva in two polyphagous Lepidoptera insects and its effect on plant defense responses

Polyphagous herbivores insects need to adapt their physiology when feeding on different host plants. The oral secretions of phytophagous insects play an important role in mediating plant defense responses. Plasticity in salivary secretions may have direct effect on the insects’ ability to deal with plant defenses. The objective of this project was to determine whether or not the composition of secreted saliva in herbivorous insects changes when they feed on different host plants and how it affects plant defense responses. We used the polyphagous insects Spodoptera frugiperda and Helicoverpa zea as a model system. Ablated (impaired to salivate) and intact 5^th instar caterpillars were allowed to feed on tomato and maize plants for 24 hours. The expression of the plant defense genes Maize proteinase inhibitor (MPI) and tomato proteinase inhibitor 2 (pin 2) were tested using quantitative real time PCR. Protein profiles of secreted saliva were obtained using SDS polyacrylamida gels. In addition, the glucose oxydase (GOX) activity was tested in labial glands after caterpillar feeding on different host plants.  Our results show that secreted saliva of S. frugiperda induces plant defense response genes in maize but suppresses the expression of Pin 2 in tomato. Differences in the protein profile and activity of GOX were also obtained. We conclude that host plants influence the saliva composition in herbivorous insects; further, changes in saliva have a direct effect on the plant response to caterpillar feeding.