Accumulation of phenanthrene by roots of intact wheat (Triticum acstivnmL.) seedlings: passive or active uptake?

Abstract

BackgroundPolycyclic aromatic hydrocarbons (PAHs) are of particular concern due to their hydrophobic, recalcitrant, persistent, potentially carcinogenic, mutagenic and toxic properties, and their ubiquitous occurrence in the environment. Most of the PAHs in the environment are present in surface soil. Plants grown in PAH-contaminated soils or water can become contaminated with PAHs because of their uptake. Therefore, they may threaten human and animal health. However, the mechanism for PAHs uptake by crop roots is little understood. It is important to understand exactly how PAHs are transported into the plant root system and into the human food chain, since it is beneficial in governing crop contamination by PAHs, remedying soils or waters polluted by PAHs with plants, and modeling potential uptake for risk assessment.ResultsThe possibility that plant roots may take up phenanthrene (PHE), a representative of PAHs, via active process was investigated using intact wheat (Triticum acstivnm L.) seedlings in a series of hydroponic experiments. The time course for PHE uptake into wheat roots grown in Hoagland solution containing 5.62 μM PHE for 36 h could be separated into two periods: a fast uptake process during the initial 2 h and a slow uptake component thereafter. Concentration-dependent PHE uptake was characterized by a smooth, saturable curve with an apparent Km of 23.7 μM and a Vmax of 208 nmol g-1 fresh weight h-1, suggesting a carrier-mediated uptake system. Competition between PHE and naphthalene for their uptake by the roots further supported the carrier-mediated uptake system. Low temperature and 2,4-dinitrophenol (DNP) could inhibit PHE uptake equally, indicating that metabolism plays a role in PHE uptake. The inhibitions by low temperature and DNP were strengthened with increasing concentration of PHE in external solution within PHE water solubility (7.3 μM). The contribution of active uptake to total absorption was almost 40% within PHE water solubility. PHE uptake by wheat roots caused an increase in external solution pH, implying that wheat roots take up PHE via a PHE/nH+ symport system.ConclusionIt is concluded that an active, carrier-mediated and energy-consuming influx process is involved in the uptake of PHE by plant roots.

The halophytic model plant Thellungiella salsuginea exhibited increased tolerance to phenanthrene-induced stress in comparison with the glycophitic one Arabidopsis thaliana: Application for phytoremediation

“…In this context, it was established that a fast accumulation phase of phenanthrene occurs during the first hours (the first two hours in the case of wheat) of treatment, followed by a slower one. Its uptake by plant roots is certainly realized through a passive mechanism; however an involvement of active mechanisms is also likely (Zhan et al, 2010). This suggests that Thellungiella salsuginea and Arabidopsis thaliana accumulated high quantities of the pollutant during the first hours of treatment, which can explain the noticeable reduction in their fresh weights.…”

Mechanistic studies of perfluorooctane sulfonate, perfluorooctanoic acid uptake by maize (Zea mays L. cv. TY2)

“…2 and Table 2). Organic compounds could be taken up via passive process, which involves simple and facilitated diffusion, and/or active process, depending on the properties and the concentrations of the organic compounds and the biological species (Zhan et al 2010). The absorption of non-ionized organic compounds by roots of higher plants is generally regarded as a passive, diffusive, partitioning and nonmetabolic process (Li et al 2010;Chiou et al 2001).…”

“…Michaelis-Menten is a useful model to examine transporter-mediated uptake processes. When uptake kinetics followed a Michaelis-Menten pattern, it could indicate uptake via a carrier-mediated process, by analogy to the relationship between an enzyme and its substrate (Zhan et al 2010). Yang et al (2010) revealed that uptake of perfluorooctanoate (PFO) by OAT4-expressing CHO cells is a carrier-mediated process, which was demonstrated by a kinetics that can be described by Michaelis-Menten equation.…”

“…To further distinguish active transport from facilitated diffusion, metabolic inhibitors were employed to affect active uptake by disabling cellular energy production. 2,4-dinitrophenol (2,4-DNP) is considered primarily as metabolic inhibitor (Kong et al 2007;Zhan et al 2010;Wang et al 2011). It is regarded as an uncoupler of oxidative phosphorylation, and of proton-coupled fluxes at the plasma membrane and endomembrane via the dissipation of trans-membrane electrochemical gradients of protons (Kong et al 2007).…”

“…Data from the relatively short uptake period (1 h) were used to investigate the uptake while minimizing the possible efflux of PFOS and PFOA across the plasma membrane back into the external solution. The uptake kinetics of PFOS and PFOA was described with a Michaelis-Menten equation (Zhan et al 2010):…”