Bacterial Endophytes: The Hidden Actor in Plant Immune Responses against Biotic Stress

Bacterial endophytes constitute an essential part of the plant microbiome and are described to promote plant health by different mechanisms. The close interaction with the host leads to important changes in the physiology of the plant. Although beneficial bacteria use the same entrance strategies as bacterial pathogens to colonize and enter the inner plant tissues, the host develops strategies to select and allow the entrance to specific genera of bacteria. In addition, endophytes may modify their own genome to adapt or avoid the defense machinery of the host. The present review gives an overview about bacterial endophytes inhabiting the phytosphere, their diversity, and the interaction with the host. Direct and indirect defenses promoted by the plant-endophyte symbiont exert an important role in controlling plant defenses against different stresses, and here, more specifically, is discussed the role against biotic stress. Defenses that should be considered are the emission of volatiles or antibiotic compounds, but also the induction of basal defenses and boosting plant immunity by priming defenses. The primed defenses may encompass pathogenesis-related protein genes (PR family), antioxidant enzymes, or changes in the secondary metabolism.

Automated summary

Bacterial endophytes play a crucial role in promoting plant health through different mechanisms and interacting closely with the plant to induce significant physiological changes. Despite using similar entrance strategies as bacterial pathogens, beneficial bacteria are selectively allowed entrance by the host, and may adapt or avoid host defense mechanisms through genomic modifications. The review provides an overview of endophytic bacteria inhabiting the phytosphere, their diversity, and their interactions with the host, emphasizing the importance of plant-endophyte symbionts in enhancing plant defenses against biotic stress.