Microbe-induced galls and plant defence: metabolite crosstalk in a co-evolutionary battle

Plants and pathogens engage in complex biochemical communication, mediated by proteins, specialized metabolites, phytohormones and phytohormone-mimicking compounds. These interactions drive a dynamic 'co-evolutionary arms race', as plants and microbes compete to gain an advantage, ultimately transforming the infection site into a distinct micro-ecosystem. Microbe-induced galls are abnormal plant organogenesis induced by specific pathogens, creating a niche where the pathogen manipulates the host plant machinery to ensure its own survival. Such galls adversely affect agricultural and horticultural productivity by stunting plant growth and causing deformities. However, the rapid cell proliferation in tumourigenesis, along with the mechanisms driving robust shoot and root re-differentiation, present exciting opportunities for innovative biotechnological applications. Therefore, elucidation of these mechanisms is crucial for advancing basic research with significant potential for agricultural applications. This review focuses on galls induced by Agrobacterium tumefaciens and Rhodococcus fascians-two phytopathogens that utilize phytohormones as tumour-inducing molecules-to highlight the mechanisms underlying plant-pathogen interactions within this specialized microenvironment. It also explores the evolutionary adaptations and strategies of these pathogens. Gaining insight into these biological processes is key to understanding the mechanisms driving biological diversity and evolution, with implications extending beyond plant pathology into the broader field of molecular plant physiology. This article is part of the theme issue 'Life in natural microcosms'.