Carnivore Attractant or Plant Elicitor? Multifunctional Roles of Methyl Salicylate Lures in Tomato Defense

Abstract

Synthetic plant volatile lures attract natural enemies, but may have non-target effects due to the multifunctional nature of volatile signals. For example, methyl salicylate (MeSA) is used to attract predators, yet also serves as a signaling hormone involved in plant pathogen defense. We investigated the consequences of deploying MeSA lures to attract predators for tomato (Solanum lycopersicum) defense against herbivores. To understand the spatial distribution of the lure's effect, we exposed tomatoes in the field to MeSA along a linear distance gradient and induced defenses by simulating feeding by hornworm caterpillars in a fully crossed factorial design (+/- MeSA, +/- herbivory). Subsequently, we analyzed activity of several defensive proteins (protease inhibitors, polyphenol oxidase, peroxidase), development of hornworm larvae (Manduca sexta), growth of fungal pathogens (Cladosporium and Alternaria), and attractiveness to herbivores and predators. Overall, MeSA-exposed plants were more resistant to both insects and pathogens. Secondary pathogen infection was reduced by 25% in MeSA exposed plants, possibly due to elevated polyphenol oxidase activity. Interestingly, we found that lures affected plant pathogen defenses equivalently across all distances (up to 4 m away) indicating that horizontal diffusion of a synthetic volatile may be greater than previously assumed. While thrips avoided colonizing hornworm- damaged tomato plants, this induced resistance was not observed upon pre-exposure to MeSA, suggesting that MeSA suppresses the repellant effect induced by herbivory. Thus, using MeSA lures in biological control may inadvertently protect crops from pathogens, but has mixed effects on plant resistance to insect herbivores.

Volatile Organic Compounds as Insect Repellents and Plant Elicitors: an Integrated Pest Management (IPM) Strategy for Glasshouse Whitefly (Trialeurodes vaporariorum)

“…2b ) after 20 dpi were most likely due to the comparatively large size of the plants, resulting in the limonene dispensers having less effect on whitefly behaviour as the compound diffused to lower concentrations throughout the glasshouse. Although previous studies have shown MeSA to have effects against insects up to four metres away from dispensers (although at high concentrations; Rowen et al ( 2017 )) and some pheromone-based dispensers have effects at eight metres (Braasch and Kaplan 2012 ), the nature of volatile repellence is understood to be dose dependant with higher concentrations having more of an effect (Ben Issa et al 2016 ). Maintaining high concentrations of limonene throughout the canopy may prove key in repelling whiteflies in a commercial glasshouse environment, where tomato plants can reach up to 12 ft. in height.…”

“…We propose that spraying of exogenous MeSA has induced this signalling pathway, similar to how methyl jasmonate (MeJA) has been shown to influence the JA signalling pathway (Wu et al 2008 ). There is currently a disparity amongst published literature with some groups showing MeSA to induce JA related defences (Rowen et al 2017 ) and other groups showing induction of SA related defences (Park 2008 ). Whilst we can associate MeSA with induction of SA related defences, which may correlate with increased resistance to whiteflies, the way in which MeSA influences plant signalling pathways remains unclear.…”

“…All five treatments for this experiment (C, L, MeSA, Tv and MeTv) were implemented as described above. PPO and POD activities were measured using modified methods from Thaler et al ( 1996 ) and Rowen et al ( 2017 ). For all treatments, plant tissue apical to the dicotyledonous leaves was removed and flash frozen in liquid nitrogen.…”

“…However, an alternative hypothesis is that MeSA would make tomatoes more vulnerable to whitefly attack by inducing SA induction and reducing expression of JA-induced defences, as shown for Bemisia tabaci (Su et al 2015 ; Zhang et al 2013 ). There is also evidence of possible repellence of other Hemipterans by MeSA in the field (Braasch et al 2012 ; Rowen et al 2017 ).…”

Àcars en cítrics: defensa de les plantes i interacció del microbioma

“…urticae most probably obtains this volatile from its host plants (Oku et al 2015). Because SA has been widely recognized as a key factor for predator recruitment by infested plants (i.e., indirect defense) (Mallinger et al 2011;Rodríguez-Saona et al 2011;Kaplan 2012;Rowen et al 2017;Salamanca et al 2017), the question of why a plant volatile exploited by natural enemies as a kairomone is not immobilized/degraded by its potential prey, deserves further investigations.…”