Extracellular Trapping of Soil Contaminants by Root Border Cells: New Insights into Plant Defense

Abstract

Soil and water pollution by metals and other toxic chemicals is difficult to measure and control, and, as such, presents an ongoing global threat to sustainable agriculture and human health. Efforts to remove contaminants by plant-mediated pathways, or "phytoremediation", though widely studied, have failed to yield consistent, predictable removal of biological and chemical contaminants. Emerging research has revealed that one major limitation to using plants to clean up the environment is that plants are programmed to protect themselves: Like white blood cells in animals, border cells released from plant root tips carry out an extracellular trapping process to neutralize threats and prevent injury to the host. Variability in border cell trapping has been found to be correlated with variation in sensitivity of roots to aluminum, and removal of border cell results in increased Al uptake into the root tip. Studies now have implicated border cells in responses of diverse plant roots to a range of heavy metals, including arsenic, copper, cadmium, lead, mercury, iron, and zinc. A better understanding of border cell extracellular traps and their role in preventing toxin uptake may facilitate efforts to use plants as a nondestructive approach to neutralize environmental threats.

Drought-Tolerant Barley: II. Root Tip characteristics in Emerging Roots

“…Oberkochen, Germany) to ensure proper root tip and border cell formation. Root border cells, which dissociate from the root tip upon contact with water, were separated and collected from the root tip by micropipetting [16][17][18][19][20][21][22][23][24]. Border cell numbers were counted from 10 µL aliquot samples of the border cell-sdH 2 O suspension using an Olympus BX60F5 compound microscope.…”

Visualization of extracellular DNA released during border cell separation from the root cap

“…Furthermore, exDNA plays a critical role in the structural integrity of the complex extracellular trap structures surrounding border cells. Future studies to define the process and how it varies in response to pathogens, toxins, metals (Hawes et al, 2016b), and other dangers are needed. Rather than providing a convenient system to observe how infection occurs at the cellular level (e.g., Hawes and Pueppke, 1986, 1987), border cells instead may provide a tool to define how infection and injury are prevented.…”

“…Most plant species are programmed to synthesize and deliver populations of metabolically active cells from the root tip into the soil environment (Knudson, 1919; Hawes and Pueppke, 1986; Hawes et al, 1998; Hawes et al, 2016a, b). Gene expression patterns in these root “border” cells are distinct from progenitor cells in the root cap, and the cells actively export a complex matrix that includes >100 extracellular proteins (Brigham et al, 1995, 1998; Knox et al, 2007).…”

“…Subsequent studies revealed for the first time that exDNA also is a component of the border cell extracellular matrix (Wen et al, 2009a). Degrading the exDNA by adding DNase at the time of inoculation with a fungal pathogen results in 100% loss of root tip resistance to infection (Hawes et al, 2011, 2012, 2016b). DNase also reverses trapping of bacteria by border cells (Curlango‐Rivera et al, 2013; Tran et al, 2016).…”

Extracellular Traps: An Ancient Weapon of Multiple Kingdoms

“…The DNA staining was visible surrounding the border cells and surface of the peripheral root cap, with no staining inside the border cells. Interestingly, recent findings revealed that soil contaminants can be entrapped within extracellular DNA released by root border cells [164]. The authors proposed that a better understanding of border cell ETs would help develop a non-destructive approach to neutralise environmental threats [164].…”