Abstract
The symbiosis between leguminous plants and soil bacteria of the genus Rhizobium is of considerable agronominal importance. Recently it has been found, that polycyclic aromatic hydrocarbons (PAHs; e.g. anthracene, phenanthrene, fluoranthene), occurring as ubiquitous environmental contaminants can inhibit nodulation of Medicago sativa. Fluoranthene is one of the dominant PAHs found in urban particulate matter, sewage sludge or beside motorways. Several organisms have been shown to be able to metabolize and mineralize fluoranthene but the uptake of fluoranthene is limited due to low solubility of fluoranthene in water and strong adsorption to humic substances in soil. Rhizobium meliloti cannot degrade fluoranthene. Toxic effects of fluoranthene on bacterial growth have never been observed. In contrast to their rhizobial symbiotic partners, alfalfa plants grown on a solidified fluoranthene-containing medium, exhibited symptoms of toxicity. They showed a dose-responsive decrease in shoot length and, if inoculated with R. meliloti, inhibition of nodule formation. Growth retardation is accompanied by a decrease in anthocyanin pigmentation of shoots, and an atypical accumulation of anthocyanins in roots. Plant flavonoids are known to play a central role in the signal exchange of the Legume-Rhizobium symbiosis. Phenylpropane derived compounds and flavonoids have been implicated in nodule development. Since fluoranthene impairs nodulationmore » and induces the production of anthocyanins, it is possible that these events are causally linked via phenylpropanoid metabolism. These experiments attempt to overcome the inhibitory effects of fluoranthene by exogeneous application of the flavonoid luteolin. This paper demonstrates that luteolin antagonizes the fluoranthene mediated inhibition of nodule formation and prevents the accumulation of anthocyanins in roots. 29 refs., 4 figs., 1 tab.« less
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