Root Plaque Formation

Soluble Fe(ll) and Mn(II) are oxidized and precipitated on root surfaces, resulting in plaque formation. Although the actual site for Fe(II) and Mn(ll) oxidation in the root zone varies with plant species, the plaque formation typically follows oxygen release from roots. Details of plaque formation in the root zone of wetland plants are discussed in Chapter 7. In this section, consequences of root plaque formation will be discussed. Factors controlling iron plaque formation on roots have been reviewed in detail by Mendelssohn et al. (1995). Plaque on root surfaces can have both positive and negative effects on plants. These include 

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Geochemical analyses of the contaminated sediments in the root zone using sequential chemical extractions showed that greater than half of the arsenic is strongly adsorbed (Keon et al. 2000, 2001). A mixture of arsenic oxidation states and associations was observed and supported by bulk XANES and EXAFS data collected at the SSRL. Arsenic in the upper 40 cm of the wetland, which contains the peak corresponding to maximum deposition, appears to be controlled by iron phases, with a small contribution from sulfidic phases. The results suggest that iron oxide phases may be present in the otherwise reducing wetland sediments as a substrate onto which arsenic can adsorb, perhaps due to cattail root plaque formation. 

Wetland plants provide an oxygenated environment in the root zone, which can fnnction as an effective sink by oxidizing dissolved Fe(II) and Mn(ll). Several stndies have shown root plaque formation on snrface roots are due to the precipitation of Fe(lll) and Mn(IV) oxides (see Section 10.7 for additional details). 

The presence of iron oxyhydroxide coatings (i.e., Fe plaque, often dominated by ferrihydrite) on the surface of wetland plant roots is visual evidence that subsurface iron oxidation is occurring in otherwise anoxic wetland soils and sediments. Oxygen delivered via radial O2 loss may react with reduced iron in soil pore spaces to form oxidized iron that can be deposited on the plant roots as Fe plaque. Despite a long history of observing Fe plaque on wetland plant roots and understanding the basics of plaque formation [i.e., reaction of plant-transported O2 with Fe(II) in soils and sediments], it was largely assumed that plaque formation is predominately an abiotic (i.e., chemical) process because the kinetics of chemical oxidation can be extremely rapid (Mendelssohn et al., 1995). However, recent evidence has demonstrated that populations of lithotrophic FeOB are associated with Fe plaque and may play a role in plaque deposition. 

The rhizosphere is home to a diverse microbial community, including aerobic heterotrophs (Gilbert and Frenzel, 1998), methane oxidizers (Bosse and Frenzel, 1997 Calhoun and King, 1997), and ammonium and nitrite oxidizers (Bodelier et al., 1996 Arth et al., 1998). Microscopy has also shown that microbial cells are associated with Fe plaque (Trolldenier, 1988 St-Cyr et al., 1993), but visual examinations alone cannot determine if cells are responsible for plaque formation. Trolldenier (1988) demonstrated that rusty-colored colonies formed when root plaque was inoculated into an iron-containing medium, but further characterization of these colonies was not attempted. 

Oxidized root channels have been observed for few species, including rice (Oryza sativa), cattails (Typha sp.), reeds (Phragmites), Spartina sp., Carex sp., and Potomogeton sp. (see review of Mendelssohn et al, 1995). The iron-enriched plaques essentially consist of FeOOH minerals (see Section 7.8.1). Excessive ferric iron precipitation can block the uptake of nutrients. Gas exchange within the root can also be decreased by dense plaque formation. 

The dominant and most reported component of root plaques is various oxidized compounds of iron. Microscopic observations of root plaques show a highly heterogenous morphology composed mostly of an amorphous material dispersed throughout nodules (50-300 nm in diameter), needles (50-100 nm in length), and filaments with variable lengths. This iron plaque formation on roots results from diffusion of Fe + toward the root zone in response to concentration gradients at the interface (similar to those observed at the soil-floodwater interface). The oxidized rhizosphere functions as a sink for Fe + and other reduced substances. 

The oxidation and precipitation of reduced Fe(II) and Mn(II) in the root zone (a result of oxygen transport by wetland plants) results in iron and manganese plaque formation on the root surfaces. Iron plaque on root surfaces can protect plants from reduced phytotoxins such as sulfide, but it can also potentially create a barrier limiting nutrient diffusion into the root. 

Surfaces of these materials are critical to their successful use. For some implants, the surface should promote healing through growth of host tissue. Artificial bones or joints are examples of these. In other cases, materials to be used in a biological environment should repel growth. Examples are surfaces of food-handling equipment and the bottom surfaces of ships. Dental implants (artificial teeth) have both requirements the root must promote growth to anchor it into the gum and jaw, while the top must repel plaque formation by bacteria. 

Kammerer, I., R. Ringseis, and K. Eder. 2011a. Feeding a thermally oxidised fat inhibits atherosclerotic plaque formation in the aortic root of LDL receptor-deficient mice. JoumglafNjMrUioT 105 190-9. 

A lack of dental hygiene allows the plaque formation, causing gingivitis or inflammation caused by toxins produced by the bacteria. With time, the gum recedes, the fragile root dentine is exposed, and finally, the tooth will fall out. 

In the wetlands of Idaho, the formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots (Typha latifolia, cat tail and Phalaris arundinacea, reed canary grass) may provide a means of attenuation and external exclusion of metals and trace elements (Hansel et al, 2002). Iron oxides were predominantly ferrihydrite with lesser amounts of goethite and minor levels of siderite and lepidocrocite. Both spatial and temporal correlations between As and Fe on the root surfaces were observed and arsenic existed as arsenate-iron hydroxide complexes (82%). 

Taylor GJ, Crowder AA (1984) Formation and morphology of an iron plaque on the roots of Typha latifolia... 

Chlorhexidine initially was used as a general disinfectant because of its broad antibacterial action (9). It was later shown to inhibit dental caries and reduce the formation of dental plaque (10). In vitro inhibition studies have shown chlorhexidine to be effective against species found in infected root canals such as Enterococcus faecalis (11) and Streptococcus mutans (12), and because of this, it was introduced as an endodontic irrigant in the early 1960 s (10). Chlorhexidine is increasingly being incorporated into endodontic materials due to its ability to increase antimicrobial properties and improve prognosis. 

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