Extracellular DNA-Based Trapping of Heavy Metals by Root Border Cells

Abstract

Plant root interactions with biotic and abiotic environmental stressors affect numerous aspects of human and environmental health. Plant root border cells comprise a dynamic barrier that mediates harmful effects to the plant by protecting vulnerable root tips from injury and infection and thereby facilitating plant growth and development. Recent studies have revealed that, as in animals, extracellular DNA (exDNA) trapping plays a central role in defense, but the mechanism remains poorly understood. Upon immersion in water, border cells and the associated exDNA mucilage structures rapidly disperse and are primed for function in defense of the root tip. The goal of this research was to utilize lead (Pb) as a model to examine the hypothesis that border cells function to protect plant health by utilizing exDNA-based defense mechanisms against biotic and abiotic threats. The experimental approaches were (1) to detect and study the role of exDNA in the response of border cells to pathogenic microbes; (2) to analyze the interactions of lead (Pb) with border cells; and (3) to explore the role of exDNA in Pb trapping by using various nucleases and a Pb-specific stain. The results revealed that exDNA is present in a matrix surrounding root border cells. The central role of the matrix in defense against pathogens and toxic metals was revealed by the discovery that its degradation by nuclease eliminates resistance of the root tip to infection and injury. Border cell exDNA traps immobilized phytopathogenic bacteria Ralstonia solanacearum, which were then dispersed upon treatment with a nuclease. This indicates that the trapping mechanism is exDNA-based and an important defense function of border cells. To test its role in defense against abiotic stressors, we investigated the interactions of border cells and Pb. The ability of border cells to remove large amounts of Pb from solution was demonstrated by ICP-MS analysis. Up to 20% of the Pb in a 1 mM solution was removed by the border cells of one corn root during one hour incubation. This relatively large trapping capacity could be of use in phytoremediation efforts. To further examine the interactions of Pb with border cells, a rapid and sensitive Pb staining protocol was developed. Rhodizonic acid is a chromogenic stain which, under certain chemical conditions, changes color from pale yellow to dark red. This stain was used to characterize Pb-border cell interactions, as well as the effect of various nucleases on Pb trapping. The addition of nucleases resulted in reduced Pb trapping and staining, much like previous studies demonstrating reduced microbial trapping after nuclease treatment. These combined results indicate that exDNA is a key component of border cell defense from both biotic and abiotic threats. Nuclease degradation of border cell exDNA results in reduced microbial trapping, reduced Pb trapping, increased damage to root tips, and cessation of root growth. These new insights reveal nuclease-exDNA interactions as a critical target for improving crop health and production using nontoxic management of soilborne pathogens and pollutants.