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Abstract

Plants are inhabited by a large diversity of microbes, most of which are taken up from the environment, and some which are vertically transmitted from generation to generation. Plant–microbe symbioses include mutualistic interactions where microbe-derived nutrients are exchanged for plant-derived photosynthates, and antagonistic interactions where plant tissues are exploited for the growth and reproduction of pathogens. Many years of research of mutualistic and antagonistic plant symbioses have provided insight into the complex interactions of plants with their symbionts. Manipulation of the plant immune system is crucial in the establishment of both mutualism and parasitism. Antagonistic and mutualistic microbial symbionts produce an arsenal of effector molecules that interact with the plant immune system to allow plant colonization. Accumulating evidence suggests that the plant immune system and effector molecules also are instrumental in determining the composition of the plant microbiota. In this chapter, we consider plant–microbe symbiosis in the context of the plant immune system. We discuss the crucial role of effector molecules in plant–microbe interactions and also possible microbe–microbe interactions.