Functional Diversity of Endophytic Bacteria

Abstract

Bacterial endophytes colonize the inner tissues of plants but do not cause detrimental effects on them. Moreover, some endophytic bacteria are useful to overcome certain environmental stresses by the plants. Thus, it has been proposed that endophytic plant growth-promoting bacteria (PGPB) may act increasing the acquisition of nutritional resources, preventing the proliferation of pathogenic microorganisms, or altering plant hormone levels. However, bacterial endophytes are a subpopulation of the bacterial soil community combined with seed borne bacteria that prosper during the plant growth. Therefore, endophytic bacteria encompass a much more diverse community than the PGPB, but their diversity has been scarcely investigated. In this chapter, the affiliation and diversity of endophytic bacteria is presented with particular emphasis in two functional groups, diazotrophs and methanotrophs. Though with different intensities, diazotrophs and methanotrophs are two of the functional groups of endophytes more studied due to their biotechnological and ecological implications. Albeit no methanotrophs have been isolated as endophytes, their presence has been evidenced by enrichments and molecular detection, and its diversity seems to be restricted to a few genera. Most of the isolated endophytic diazotrophs are affiliated to the Proteobacteria. However, a wider diversity has been observed according to the nifH gene, which encodes for the nitrogenase reductase subunit, sequences retrieved from plant tissues, though a restricted fraction of these genes seems to be actually expressed. The study of these two groups illustrates the complexity of the endophytic bacterial communities and evidences the need of further studies to understand the interactions among bacterial endophytes and plants.