A Functional View of Plant Microbiomes: Endosymbiotic Systems That Enhance Plant Growth and Survival

Over the past several decades, it has become clear that numerous nonpathogenic or weakly pathogenic microbes inhabit plants both internally and externally. The challenge for plant biologists who study endophytism lies not only in the discovery of endophytes in plants but also in articulating the precise mechanisms whereby these endophytes function to support the growth and survival of their plant hosts. In this chapter, we discuss the phenomenon of microbial endophytism from a functional perspective. We propose that endophytic microbes in plants comprise a critical part of the plant’s functional systems. We propose three broad categories of endosymbiotic systems, including (1) Defensive Endosymbiotic Systems, (2) Stress Tolerance Endosymbiotic Systems, and (3) Nutritional Endosymbiotic Systems. We will also consider potential interactions between endosymbiotic organisms of plants and relativity of function of endosymbionts. A particular endophyte may serve multiple functions in the ecology of its host plant, and predominant functions of an endophyte may change depending on the ecological circumstances affecting its host. Only now are we beginning to realize how important endophytic microbes are to plants. Much research is needed to elucidate the mechanisms of action and the roles that endophytes play in modulating host plant ecology and enhancing plant growth and survival.