Actinobacteria and Their Role as Plant Probiotics.

Actinobacteria is one of the largest phyla within the domain Bacteria. This phylum comprises more than 400 genera heterogeneously distributed in up to 50 families, 20 orders and 6 classes, being composed with very diverse groups of microorganisms. Members included within this phylum were recovered from a wide range of aquatic and terrestrial environments and also from a huge number of higher organisms, including plants. Actinobacteria inhabiting soils and plants are well known as producers of bioactive molecules and as biocontrol agents, possessing antimicrobial activities mostly against pathogenic fungi and/or bacteria. Moreover, some of them have the capacity to exert beneficial effects on plant growth and development via different plant growth-promoting mechanisms, i.e., phytohormones biosynthesis, siderophore production, and phosphate solubilization, among others. The available genomic data revealed that members belonging to this phylum have a huge potential as Plant Probiotic Actinobacteria. A plethora of studies reported the isolation and identification of plant endophytic actinobacteria possessing those features and also their performance under controlled conditions. However, few studies show the effects of the inoculation of these actinobacteria on real field conditions. In this chapter, we will provide an overview of the available data on the Actinobacteria displaying plant growth-promoting features, particularly in the ones that already had applications in agriculture. Together with a correct taxonomic classification, we will present evidence that the Plant Probiotic Actinobacteria should be considered as a source of bacterial candidates that will be important for a future sustainable agriculture