While plants can phytoremediate soils that are contaminated with petroleum hydrocarbons, adding microbes to remediate contaminated sites with petroleum-degrading microorganisms and arbuscular mycorrhizal fungi (AMF) is not well understood. The phytoremediation of Arabian medium crude oil (ACO) was done with a Lolium multiflorum system inoculated with an AMF (Glomus intraradices) and a mixture of petroleum-degrading microorganisms--the bacterium, Sphingomonas paucimobilis (Sp) and the filamentous fungus, Cunninghamella echinulata (Ce, SpCe)--or with a combination of microorganisms (AMF + SpCe). Based on an earlier study on screening plants for phytoremediation of ACO, L. multiflorum (Italian ryegrass) was selected for its tolerance and rapid growth response (Alarcón, 2006). The plants were exposed to ACO-contaminated soil (6000 mg kg(-1)) for 80 d under greenhouse conditions. A modified Long Ashton Nutrient Solution (LANS) was supplied to all treatments at 30 microg P mL(-1), except for a second, higher P, control treatment at 44 microg P mL(-1). Inoculation with AMF, SpCe, or AMF + SpCe resulted in significantly increased leaf area as well as leaf and pseudostem dry mass as compared to controls at 30 microg P mL(-1). Populations of bacteria grown on a nitrogen-free medium and filamentous fungi increased with AMF + SpCe and SpCe treatments. The average total colonization and arbuscule formation of AMF-inoculated plants in ACO-contaminated soil were 25% and 8%, respectively. No adverse effects were caused by SpCe on AMF colonization. Most importantly, ACO degradation was significantly enhanced by the addition of petroleum-degrading microorganisms and higher fertility controls, as compared to plants at 30 microg P mL(-1). The highest ACO degradation (59%) was observed with AMF + SpCe. The phytoremediation of ACO was also enhanced by single inoculation of AMF or SpCe. The effect of AMF and petroleum-degrading microorganisms on plant growth and ACOdegradation was not attributable to differences in proline, total phenolics, nitrate reductase levels, or variation in plant-gas exchange.