Endophytic actinobacteria: Nitrogen fixation, phytohormone production, and antibiosis

Abstract

Rhizospheric and endophytic microbial population plays a key role in sustaining plant production under either nutrient-limited or sufficient conditions. Endophytic microorganisms that are internalized in plants offer a competitive advantage over rhizospheric microorganisms in the growing environments. Among various endophytic microorganisms, the Gram-positive actinobacteria have gained considerable interests due to their secondary metabolites production. Besides Frankia, a large number of cultivable actinobacteria such as Streptomyces spp., Nocardiopsis, Actinoplanes spp., Micromonospora, Microbispora, and Streptosporangium show endophytic lifestyles in diverse plant species. Recent advances in molecular tools show the existence of “yet to be uncultured but viable organisms” within this group. The colonization of endophytic actinobacteria depends on plant species, soil types, and varied environmental factors. Their metabolic capabilities even make them an important source of plant hormones, antibiotics, and other bioactive molecules that are used in agriculture and pharmaceutical industries. In the legume plants, the nodule inhabiting actinobacteria such as Micromonospora, Streptomyces sp., Nocardia alba, Nonomuraea rubra, and Actinomadura glauciflava have probiotic effects with Rhizobium. In the present chapter, the potential role of the plant-associated actinobacteria in sustainable agriculture and their endophytic lifestyles has been reviewed.