The Physiological and Ecological Processes of Anthocyanins Accumulation and Red Gall Coloration

Many galls have red coloration, which is driven by the accumulation of anthocyanins. Several hypotheses have attempted to explain the physiological and evolutionary mechanisms underlying this visually and chemically remarkable and widespread phenomenon. Here, we revisit the main physiological explanations for this visually conspicuous trait and explore its potential biological functions. From the physiological point of view, anthocyanin accumulation is associated with photoprotection against high-light exposure and defense from damage from reactive oxygen species induced by biotic or abiotic stressors. Anthocyanin accumulation was suggested to be associated with sugar storage stimulated by cytokinin’s hijacking by the gall-inducing insects. In addition, the red gall phenomenon may in certain cases be either a by-product of the induced host plant’s metabolism, because the tissue sites where galls develop have already the predisposition to accumulate more pigment, or have a defensive signaling role. Indeed, the red gall phenomenon appears to be physiologically idiosyncratic and dependent on the stimulation of anthocyanin metabolism by different factors. Conspicuous gall coloration (especially red, orange, and yellow) is primarily controlled by the inducing insects (as in many gall traits) although the host plant tissue can impose both structural and chemical constraints. This conspicuous coloration may theoretically serve as a warning (aposematic) signal towards potential herbivores, predators, and parasitoids. Hypotheses-driven documentation of color expression in many species and controlled experiments could address the various alternative explanations for the remarkable red gall coloration. Nevertheless, different functions of red gall coloration may operate simultaneously.