Root border cells and secretions as critical elements in plant host defense

Border cells and border-like cells are released from the root tip as individual cells and small aggregates, or as a group of attached cells. These are viable components of the root system that play a key role in controlling root interaction with living microbes of the rhizosphere. As their separation from root tip proceeds, the cells synthesize and secrete a hydrated mucilage that contains polysaccharides, secondary metabolites, antimicrobial proteins and extracellular DNA (exDNA). This exDNA-based matrix seems to function in root defense in a way similar to that of recently characterized neutrophil extracellular traps (NETs) in mammalian cells. This review discusses the role of the cells and secreted compounds in the protection of root tip against microbial infections.