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

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.