The Extended Plant Immune System

When disease resistance was still viewed as a vague physiological response rather than a genetically defined process, Flor laid the foundation for our modern understanding of plant immunity with his gene-for-gene concept. Initially developed for simple host–pathogen interactions, this idea evolved into a more complex framework in which plants engage in continuous dialogue with a diverse microbiome. Within this community, beneficial, commensal, and pathogenic microbes interact both directly and indirectly through the plant host, extending the boundaries of plant immunity beyond the individual organism. This broader perspective envisions an “extended plant immune system” that integrates the plant’s microbial partners into a coordinated, community-level defense. Like early views of disease resistance, this concept was first described in broad physiological or ecological terms. As the field has matured with the advent of next-generation sequencing, it has become clear that the microbiome-mediated extension of the plant immune system is also grounded in genetically determined molecular processes. These range from host-driven recruitment of protective microbiota, to microbial traits that suppress pathogens, and to plant mechanisms that enable beneficial microbes to trigger induced systemic resistance. This review is a symphony composed of historical progression of research from molecular recognition to community-level defense, distilling the principles that connect classical plant immunity with emerging plant-microbiome concepts and framing microbiome-mediated disease protection as an extension of the plant’s innate immune system. This integrated perspective not only reframes our understanding of plant immunity but also offers a conceptual foundation for harnessing the extended immune system in sustainable crop protection.