Plant Volatiles Modulate Immune Responses of Spodoptera litura
Plants emit a specific blend of volatiles in response to herbivory and these volatiles, which often attract predators and parasitoids function as an indirect plant defense. The impact of plant volatiles in shaping herbivore defenses is unclear. Here, we report that specific plant volatiles induce immune responses in the polyphagous herbivore, Spodoptera litura. We characterized the hemocyte profile and established their functional significance with respect to ontogeny and exposure to specific plant volatiles. Fifth instar larvae showed the highest number and hemocytes diversity. We characterized seven different types of hemocytes, of which granulocytes performed phagocytosis, oenocytoids showed melanization activity, and plasmatocytes along with granulocytes and oenocytoids were found to be involved in encapsulation. Among the six volatiles tested, exposure to (E)-β-ocimene caused the highest increase in total hemocytes number (THC) followed by linalool and (Z)-3-hexenyl acetate exposure. Although THC did not differ between these three volatile treatments, circulating hemocytes diversity varied significantly. (E)-β-ocimene exposure showed higher number of plasmatocytes and phenol oxidase activity. The interaction of the parasitic wasp Bracon brevicornis with (E)-β-ocimene exposed larvae was poor in terms of delayed paralysis and lower egg deposition. In choice assays, the wasp showed clear preference towards control larvae indicating (E)-β-ocimene treatment renders the host unattractive. Hemocyte profiles post-parasitoid exposure and (E)-β-ocimene treatment were similar indicating cue-based priming. When challenged with Bacillus thuringiensis, linalool exposure resulted in the highest survival as compared to other volatiles. Our results show that specific HIPVs can modulate cellular immunity of S. litura, revealing a new role for HIPVs in tri-trophic interactions.
KeywordsPlant volatiles Herbivory Ocimene Linalool Insect immunity Tritrophic interaction
The authors acknowledge Dr. Tina Mukherjee lab (inStem, Bengaluru) and Dr. Deepa Agashe (NCBS, Bengaluru) for their help. We thank Prof. Paul Ode for his valuable comments and help in editing the manuscript. The authors thank National Bureau of Agriculturally Important Insect Resources, Bengaluru for providing insects. NCBS central imaging and flow cytometry facility is gratefully acknowledged.
VR conceived and supervised the study and experimental design. EG designed and performed the experiments and analyzed the data. EG and VR wrote the manuscript.
This work was supported by Department of Science and Technology (Early Career Award, Ramanujan Fellowship), Max Planck Society (DST-Max Planck Partner group program) and Department of Biotechnology.
Compliance with Ethical Standards
Conflict of Interest
The authors declare no competing interests.
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