Root niches for colonization by diazotrophic bacteria. Rhizospheric bacteria (orange cells) colonize the rhizosphere soil area without invading internal plant tissues. Associative bacteria (blue cells) are in close interaction with the plant surface and sometimes can be found within plant tissues. Endophytic bacteria (dark red cells) colonize any region within the epidermis of the plant root, and they can reside in apoplastic intercellular spaces and the xylem vessel apoplast. In general, the endophytes invade the internal plant tissues through sites of injury in the epidermis, root tips, and root cracks formed at the sites of lateral root emergence. Some endophytic bacteria can spread to distant plant organs (stem, leaves, seeds, and fruits).