Most plant species produce thousands of metabolically active root border cells, comprising
up to 90% of daily fixed carbon, and destined to be shed from root tips into the soil environment. The
function of this seemingly wasteful process has remained unclear until parallels with newly defined
functions of neutrophils were discovered. In 2004, neutrophil extracellular traps (NETs) were first
reported to be a central component of the mammalian immune system (Science 303:1532). In response to
microbial challenge, neutrophils export a complex that immobilizes the invading pathogen. Extracellular
DNA (exDNA) is a key component of NETs, and treatment with DNase eliminates the capacity of cells to
control microbial invasion. Extracellular DNases (exDNases) in pathogens including Streptococcus
Group A are required for pathogenesis; mutant strains lacking exDNase lose virulence. Emerging
evidence that this is an ancient underpinning of immune systems includes the observation that DNAbased
extracellular traps also operate in plants: root border cells protect root tips from infection as they
penetrate the soil environment, using mechanisms that operate in parallel with NETs. Altered expression
of plant pathogen exDNases, as in animals, results in altered virulence. Understanding mechanisms
controlling extracellular trapping and microbial escape from trapping may yield new applications in
agriculture and medicine. How plants like tomato and Arabidopsis defend their root tips with very few
viable border cells will be of special interest.
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