Insect-induced foliar galls: a cross-talk among phytohormones for tissue growth and endogenous defense

Insect-induced gall tissue has a unique ability to influence its hosts' phenotypic expression. When plants are stressed by insects, phytochemical manipulations in galling tissue strengthen the plant's resilience to subsequent herbivore attacks, and as a result, the damaged plant tissue regenerates and rejuvenates. Gall tissue development and differentiation are initiated by changes in and modulation of a variety of phytohormones in the affected galling sites. Such hormonal changes ultimately boost the plant's ability to respond to herbivore invasions. In this study, the insect-induced gall tissues of three model plants—mature and immature galls and non-gall tissue—were evaluated for five phytohormone gradients. Phytohormone gradients are continually altered and compared from non-differentiated (non-gall) tissue to moderately (immature gall) and highly (mature gall) developed tissue. The results indicate that phytohormones serve a dual role in stimulating the plant's endogenous defense and promoting tissue growth, pointing to a complex chemogenesis process in galling tissue associated with developing neoplasm and plant defensive responses. Tissue abnormalities in galls are thought to have resulted from the herbivore's interactions with the plant on which it infests. Insects' ovipositing fluids or oral discharge may have also contributed to the accumulation of phytohormones in the stressed and wounded tissue. Foliar galls are thus a sign of manifestation of insects' adaptation since the herbivore and their hosts are likely to co-evolve in the context of chemical adaptation.