Gall-inducing Insects and Plants: The Induction Conundrum

Galls induced by insects and mites (insects, hereafter) have been a subject of interest to insect ecologists because of the unusual habit of gall induction and for their tightly connected relationships. These specialist insects and mites have been explored to explain the nature of interactions between them and the plants by entomologists, ecologists and plant physiologists over the last two centuries. However, the questions why only certain insect taxa induce galls on specific species of plants and how galls are induced remain challenging. Whereas several efforts made across the world implicate plant-growth regulators (PGRs) in answering the question on how galls are induced, this article emphasizes the establishment of a metaplasied cell at the location where the tip of the chitinous mandible or ovipositor first hits in the plant. In the light of the differentiation of a metaplasied cell, the earliest plant response, it is but critical to evaluate the physiology of that cell and the ‘new’ physiological events triggered around it, heralding gall initiation. PGRs certainly play a role in gall growth, but only during later stages. This article does not answer the question on how galls are induced. However, it brings to light the gaps that need to be addressed in future in the backdrop of the efforts made over the years. Since we need to deal with the physiological changes that occur in a metaplasied cell and a few adjacent cells, the use of sophisticated optical equipment and pertinent software to achieve a structured and articulate explanation impresses as the way to go.