Transport Mechanism Of Polycyclic Aromatic Hydrocarbons (Phenanthrene) Across Wheat Root Interface

Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants (POPs) widely existing in the environment. They are a unique class of pollutants constituted by hundreds of individual substances. These compounds contain two or more fused aromatic rings made up of carbon and hydrogen atoms. The behaviors and fates of PAHs in the environment are of increasing concern, due to their carcinogenic, mutagenic and tumorigenic properties of certain of them. Over90%of PAHs in the environment reside in surface soil. Plants grown in PAH-contaminated soils or water can become contaminated with PAHs due to their absorption. Therefore, they may pose human and animal health hazards. 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 helpful to produce PAH-free crop products by means of genetic engineering, to remove PAHs from PAH-polluted soils or water through phytoremediation, and to model potential uptake for risk assessment. Consequently, this study was carried out to explore the transport mechanism of PHE across the wheat root interface.The possibility that plant roots may take up 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 containing5.62μM PHE for36h could be separated into two periods:a fast uptake process during the initial2h and a slow uptake component thereafter. Concentration-dependent PHE uptake was characterized by a smooth, saturable curve with an apparent Km of23.7μM and a Vmax of208nmol g-1fresh 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 and2,4-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 (7.3μM). The contribution of active uptake to total absorption was almost40%within PHE water solubility. PHE uptake by wheat roots caused an increase in external solution pH, representing that PHE was absorbed by wheat roots via a PHE/nH+symport system.Relationship between channels and PHE uptake by wheat seedlings under hydroponic conditions was also studied. It was found that PHE into wheat roots was via water channel. When water channel inhibitors of different kinds and concentrations were added to Hoagland solution, they could both inhibit PHE uptake obviously (p<0.05); the inhibitory rate was31.05%-37.93%. However, inhibiting anion channel, Ca2+channel and K+channel didn’t influence PHE uptake at all; their inhibitory rates were3.93%-8.34%,-0.58%-7.78%and-7.12%-6.50%respectively. Consequently, we could.conclude that PHE into wheat roots was via water channel instead of anion channel, Ca+channel and K+channel.Effects of PHE on the activities of ATPase, H+-ATPase in plasma membrane and nitrate reductase (NR) in wheat roots were researched under hydroponic conditions, polyphenol oxidase activity and catalase activity in soil were also investigated to further discuss the mechanism of PHE uptake by wheat seedlings. The results showed that PHE could activate ATPase, H+-ATPase in plasma membrane and NR in wheat roots, and their activities became higher and higher with an increase in PHE concentration. The activities of ATPase and H+-ATPase in plasma membrane were highest when the pH of reaction solution was at8.0, and NR activity at7.5. ATPase and NR could activate their activities each other, and there was a positive correlation between them (rmax=0.996**). Furthermore, a good or significant positive correlation also existed among ATPase, H+-ATPase and PHE, and their maximum relative.coefficients rmax were0.977**and1.000respectively. Polyphenol oxidase in soil reached the maximum value at the14th day with an increase rate in the range of14.72%-46.52%; but catalase got to the minimum activity value at the7th day with an inhibition rate in the range of36.13%-94.79%; at the same time, there was a good negative correlation between polyphinol oxidase and catalase (rmax=-0.599**).Hydroponic experiment with wheat was conducted to study the influence of metabolism inhibitors in PHE uptake by wheat seedlings. Results suggested that DNP、NaN3、KCN and aresenate could inhibit PHE uptake pronouncely, and the degree of inhibitory effect strengthed with the concentration of inhibitors. Furthermore, different inhibitors showed distinct inhibitory rate. For example, when DNP, NaN3, KCN and aresenate were at higher concentrations, their inhibitory rates were59.55%,51.99%,21.27and33.17%, respectively; on the basis of the inhibitory rates, the four metabolism inhibitorscould be arranged in the following order:DNP>NaN3>Aresenate>KCN. However, Succinate, Malate and Glucose could not cause inhibitory effect at all.