Constitutive production of defense metabolites supplies a continual 'blanket of safety' against natural enemies at the energetic cost of lifetime investment by the plant while also increasing the likelihood of herbivore adaptation to specific defenses. Further, constitutive defense energy investments increase nutrient and pesticide requirements. On the other hand, inducible defenses are more dynamic and require short-term energetic investments in response to a perceived threat. In addition, because these defenses are only around for short bursts of time, there is a lower likelihood of comparable adaptation, allowing for potential pest and pathogen susceptibility maintenance. Finally, plant volatile organic compounds (VOCs) are low-carbon molecules that act directly on pests and pathogens while simultaneously inducing primed responses in neighboring plants. Therefore, breeding crops with an increased capacity for inducible defenses mediated by VOCs would increase the sustainability of agricultural systems by reducing nutrient and pesticide inputs. However, incorporation necessitates high-throughput methods to phenotype inducible plant responses in crops and wild relatives for introgression of resistance traits. For this study, we leverage the constitutive and inducible VOC and hyperspectral reflectance diversity of crop Helianthus annuus and 20 wild-relatives in the genus Helianthus to (1) assess the commonality of a signal of 'herbivore threat' across the genus and (2) describe induced qualitative and quantitative changes in VOC profiles during herbivory by a generalist lepidopteran, Vanessa cardui, and when exposed to herbivore-induced VOCs (HIPVs). Our findings suggest that herbivore- and HIPV-induced VOC responses across Helianthus are highly species-specific. However, a single classification model using hyperspectral reflectance applied across all species accurately distinguishes among controls, herbivore-, and HIPV-induced individuals with high specificity and sensitivity, corroborating patterns of delineation within each species. Results suggest that hyperspectral reflectance can detect induction across the genus, suggesting a potential conserved signal. Further, VOC diversity highlights a wide range of induced responses to a generalist herbivore across the genus.

Leveraging hyperspectral reflectance to assess volatile organic compound(VOC) mediated induced responses across the genus Helianthus

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ACS Fall 2022

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