Oral cues are not enough: induction of defensive proteins in Nicotiana tabacum upon feeding by caterpillars.

MAIN
CONCLUSION: The study challenges the general belief that plants are highly sensitive to oral cues of herbivores and reveals the role of the damage level on the magnitude of defense induction. Many leaf-feeding caterpillars share similar feeding behaviors involving repeated removal of previously wounded leaf tissue (semicircle feeding pattern). We hypothesized that this behavior is a strategy to attenuate plant-induced defenses by removing both the oral cues and tissues that detect it. Using tobacco (Nicotiana tabacum) and the tobacco hornworm (Manduca sexta), we found that tobacco increased defensive responses during herbivory compared to mechanical wounding at moderate damage levels (30%). However, tobacco did not differentiate between mechanical wounding and herbivory when the level of leaf tissue loss was either small (4%) or severe (100%, whole leaf removal). Higher amounts of oral cues did not induce higher defenses when damage was small. Severe damage led to the highest level of systemic defense proteins compared to other levels of leaf tissue loss with or without oral cues. In conclusion, we did not find clear evidence that semicircle feeding behavior compromises plant defense induction. In addition, the level of leaf tissue loss and oral cues interact to determine the level of induced defensive responses in tobacco. Although oral cues play an important role in inducing defensive proteins, the level of induction depends more on the level of leaf tissue loss in tobacco.