Three Closely Related Spodoptera Species Similarly Affect Gene Expression and Phytohormone Levels but Differentially Induce Volatile Emissions in Maize

Plants can perceive specific elicitors in the oral secretions (OS) of herbivorous insects and respond by increasing their defences. Whether plants can discriminate among similar herbivorous insect species and differentially modulate their defence responses against them is largely unknown. Here, we investigated the responses of the maize transcriptome, phytohormones, and volatile emissions to the OS of three closely related Spodoptera caterpillars: the fall armyworm S. frugiperda, the beet armyworm S. exigua, and the cotton leafworm S. littoralis. Maize plants strongly increased their phytohormone levels and volatile emissions when treated with each of the OS, which was reflected in the transcription levels of genes involved in phytohormone signalling, and primary and secondary metabolism. Compared to the OS of S. exigua and S. littoralis, the secretion of the maize specialist S. frugiperda, elicited greater changes in the maize transcriptome but triggered considerably lower volatile emissions. Besides revealing the generality and specificity of maize responses to different lepidopteran caterpillars, the dataset provides a molecular resource for studies that aim to identify and characterise herbivore-specific elicitors and effectors and their receptors. This information can then be used to elucidate and possibly disrupt the mechanisms that allow well-adapted herbivorous insects to manipulate maize defences.