Phytohormone Dynamics Associated with Gall Insects, and their Potential Role in the Evolution of the Gall-Inducing Habit

Abstract

While plant galls can be induced by a variety of organisms, insects produce the most diverse and complex galls found in nature; yet, how these galls are formed is unknown. Phytohormones have long been hypothesized to play a key role in gall production, but their exact role, and how they influence galls, has been unclear. Research in the past decade has provided better insight into the role of plant hormones in gall growth and plant defenses. We review and synthesize recent literature on auxin, cytokinins, and abscisic, jasmonic, and salicylic acids to provide a broader understanding of how these phytohormones might effect gall production, help plants defend against galls, and/or allow insects to overcome host-plant defenses. After reviewing these topics, we consider the potential for phytohormones to have facilitated the evolution of insect galls. More specialized research is needed to provide a mechanistic understanding of how phytohormones operate in gall-insect-plant interactions, but current evidence strongly supports phytohormones as key factors determining the success and failure of insect galls.

Gall-forming root-knot nematodes hijack key plant cellular functions to induce multinucleate and hypertrophied feeding cells

“…Because plant hormones play a pivotal role in coordinating plant responses to parasitic biotrophs and insects (Erb et al, 2012), it seems likely that RKN manipulate the hormone balance in order to develop their highly specialized feeding structure. Such alterations in hormone signaling have also been observed for plantmanipulating insects and may be required for the successful development and nutrition of gall-inducing organisms and leaf-mining insects (Bartlett & Connor, 2014;Tooker & Helms, 2014;Zhang et al, this issue).…”

Leaf-mining by Phyllonorycter blancardella reprograms the host-leaf transcriptome to modulate phytohormones associated with nutrient mobilization and plant defense

“…Targeting (Chung et al, 2013;Giron et al, 2013; 103 Kästner et al, 2014;Favery et al, this issue). More specifically, phy-104 tohormones play a key role in gall-induction and may even have 105 facilitated the evolution of insect galls (Bartlett and Connor,106 2014; Tooker and Helms, 2014). They are also key players in the 107 interaction between leaf-miners and their host-plant with an intri-108 cate involvement of insect bacterial symbionts sometimes impli-109 cated (Connor and Taverner, 1997;Kaiser et al, 2010;Body et al, 110 2013; Giron and Glevarec, 2014;Gutzwiller et al, 2015).…”

How to be a fig wasp parasite on the fig–fig wasp mutualism

“…The molecular mechanism of galling is completely uninvestigated; secretions of the poison gland may be involved [25]. Gallers, particularly the large gall inducers, may inject cytokine-like compounds that manipulate plant tissue as in other systems [26].…”

“…The molecular mechanism of galling is completely uninvestigated; secretions of the poison gland may be involved [25]. Gallers, particularly the large gall inducers, may inject cytokine-like compounds that manipulate plant tissue as in other systems [26].Some NPFWs are kleptoparasites [27] incapable of inducing galls but ovipositing into existing galls; their larvae feed on galled tissue, starving the gall occupant. How such larvae out-compete existing gall inhabitants is unknown but is important to understand because developing fig wasps are confined to their own galls from which they must derive all nutrition [28 ].…”