Conifer responses to a stylet‐feeding invasive herbivore and induction with methyl jasmonate: impact on the expression of induced defences and a native folivore

Abstract

1 Trees attacked by multiple herbivores need to defend themselves against dynamic biotic challenges; appropriate responses to one stressor can elicit hormonal responses that are antagonistic to another. Hemlock (Tsuga canadensis) infestation by hemlock woolly adelgid (HWA; Adelges tsugae) results in the accumulation of the defensive hormone salicylic acid. 2 We explored the potential for HWA infestation to interfere with anti-folivore-induced defence signalling and its implications for a native folivore (hemlock looper; Lambdina fiscellaria). Hemlocks were infested with HWA and/or sprayed with methyl jasmonate; foliar defences were analyzed and foliage quality for looper larvae was assessed. 3 Both treatments activated foliar defensive traits, including a HWA-mediated increase in peroxidase activity and an accumulation of cell wall-bound phenolics and lignin, as well as a methyl jasmonate-mediated increase in lipoxygenase activity. The two treatments had an additive effect on other defensive traits and both treatments negatively affected looper performance. 4 These results suggest that salicylic acid and jasmonic acid are not strictly antagonistic in conifers and that both have a role in anti-folivore defence signalling. The present study illustrates the need for a better understanding of hormone signalling, cross-talk and induced responses in conifers.

Phytohormones in galls on eucalypt trees and in the gall‐forming wasp Leptocybe invasa (Hymenoptera: Eulophidae)

“…JA is considered to be the primary regulator of plant defence against phytophagous insects, while SA is considered to be the dominant hormone regulating defence against pathogens (Glazebrook, 2005). JA‐Me, the methyl ester of JA, is known to play important roles in transmission of plant defence signals (Huang et al, 2015; Ye et al, 2019) and regulation of insect‐induced defence responses (Rigsby et al, 2019). In this study, we found that JA‐Me levels were higher in both the interior and exterior tissues of galls sampled from the resistant EA species compared to the more susceptible DH201‐2 at the early gall developmental stage only.…”

Facilitation between invasive herbivores: hemlock woolly adelgid increases gypsy moth preference for and performance on eastern hemlock

“…The enhanced performance of gypsy moth larvae reared on adelgid‐infested foliage may result from adelgid‐induced changes to hemlock defences. Adelgid infestation of hemlock increases foliar emissions of methyl SA (Pezet et al ., ; Pezet & Elkinton, ) and triggers SA accumulation in needles (Schaeffer et al ., ; Rigsby et al ., ), activating SA‐linked stress responses in hemlock. The accumulation of SA, and subsequent monomerisation of NPR1, has been shown to inhibit JA biosynthesis and JA‐responsive gene expression (Zarate et al ., ).…”

“…Recent work in the hemlock system suggests that adelgid‐induced phytochemical changes may influence interactions between hemlock and other herbivores (Schaeffer et al ., ; Wilson et al ., ; Rigsby et al ., ). Larvae of the native hemlock looper ( Lambdina fiscellaria ) had higher survival and enhanced larval development when reared on adelgid‐infested versus uninfested hemlock foliage (Wilson et al ., ).…”

Enhanced volatile emissions and anti-herbivore functions mediated by the synergism between jasmonic acid and salicylic acid pathways in tea plants

“…Currently, >80% of studies on JA–SA interactions have indicated that the interaction between the JA and SA pathways is reciprocally antagonistic [ 5 ]. Most of these studies focused on gene transcript levels and protein abundances in the upstream part of these pathways, with only a few focusing on the changes in downstream metabolites, which are particularly important for the ecological outcome of plant interactions with other organisms [ 5 , 6 ]. This may be because it is difficult to accurately detect and quantify the many diverse metabolites that are produced as a result of the interaction between the JA and SA pathways [ 6 , 7 ].…”