Biology

Dr. Sagar Pandit
Max Planck Institute for Chemical Ecology, Germany

Plants produce plethora of chemical defenses to thwart the herbivore attack. In response, herbivores have evolved counter-adaptation mechanisms which facilitate detoxification, sequestration or excretion of the plant’s toxins. Some herbivores, especially the phytotoxin sequestering ones, even co-opt them as their own defenses against natural enemies. On the other hand, some natural enemies have evolved the detection of such chemicals to locate their herbivore prey. Overall, the vast literature from the classical ecological studies suggests that plant defense and herbivore counter-defense mechanisms significantly affect the tri- or even multi-trophic interactions. Furthermore, it has been surmised that plants with their specialized metabolites control their ecosystem, wherein the metabolite serves as an infochemical and the ecosystem as an infochemical network. The molecular and biochemical mechanisms underlying the plant defense and their ‘real world’ significance has been thoroughly investigated. However, these aspects of herbivore counter-adaptations remain poorly studied. The literature clearly indicates that the success of plant defense studies can be largely attributed to the use of reverse genetics. In case of insects, especially the lepidopteran insects, their recalcitrance to gene silencing has created a bottleneck. RNAi effects are transient in lepidopteran insects because these insects apparently lack RNA dependent RNA polymerases (RdRPs). We overrode the RdRP function by using plant mediated RNAi (PMRi), in which dsRNA is continuously supplied to the insect through their host plant. Plants are transformed to produce dsRNA of targeted insect genes. The ingested dsRNA penetrates midgut cells and causes post-transcriptional silencing in the insect. We used this system to unravel the counter-defense mechanisms of Manduca sexta against its host Nicotiana attenuata’s defense compounds like nicotine, chlorogenic acid and diterpene glycosides. We selected candidate M. sexta genes that were upregulated in response to the ingestion of these xenobiotics; we silenced these genes using PMRi and studied the ‘loss of function’ phenotypes in laboratory. Further, we applied PMRi for the in situ analysis of M. sexta’s ecological interactions by introducing PMRi plants to the native habitat (Great Basin Desert, Utah, USA). Responses of native predators to the candidate gene silenced larvae helped to find the cryptic mechanisms underlying the tritrophic interactions. The remarkable success of the trophic dsRNA delivery approach led us to a new hypothesis for the future research: Plants’ endogenous small RNAs effect cross-kingdom regulation of herbivore genes. If such cross-kingdom gene regulation is discovered, small RNAs will comprise a brand new category of plant infochemicals separate from proteins and secondary metabolites, opening up a novel branch of plant-herbivore interaction research.