Roles of root border cells in plant defense and regulation of rhizosphere microbial populations by extracellular DNA 'trapping'

Background: As roots penetrate soil, specialized cells called 'border cells' separate from root caps and contribute a large proportion of exudates forming the rhizosphere. Their function has been unclear. Recent findings suggest that border cells act in a manner similar to that of white blood cells functioning in defense. Histone-linked extracellular DNA (exDNA) and proteins operate as 'neutrophil extracellular traps' to attract and immobilize animal pathogens. DNase treatment reverses trapping and impairs defense, and mutation of pathogen DNase results in loss of virulence. Scope: Histones are among a group of proteins secreted from living border cells. This observation led to the discovery that exDNA also functions in defense of root caps. Experiments revealed that exDNA is synthesized and exported into the surrounding mucilage which attracts, traps and immobilizes pathogens in a host-microbe specific manner. When this plant exDNA is degraded, the normal resistance of the root cap to infection is abolished. Conclusions: Research to define how exDNA may operate in plant immunity is needed. In the meantime, the specificity and stability of exDNA and its association with distinct microbial species may provide an important new tool to monitor when, where, and how soil microbial populations become established as rhizosphere communities.