Contribution of glomalin to Pb sequestration by arbuscular mycorrhizal fungus in a sand culture system with clover plant

Abstract

Glomalin (a glycoprotein) is produced by arbuscular mycorrhizal fungi and has the ability to sequester toxic heavy metals. We hypothesized that an increase in the concentration of Pb would lead to increased glomalin production and consequently higher Pb-sequestration. In a two compartment pot culture experiment, clover plants (Trifolium repens L.) were inoculated with arbuscular mycorrhizal fungus, Rhizophagus irregularis, in sterile washed sand and then treated with Pb concentrations of 0, 150, 300 and 450 μM as Pb(NO3)2. The root compartment (RC) was isolated from the hyphal compartment (HC) by nylon mesh (37 μm). The glomalin was extracted from both RC and HC and assayed by using Bradford method and monoclonal antibody MAb32B11. The amount of sequestrated Pb was determined after wet digestion of glomalin. The results showed that the amount of Bradford reactive and immunoreactive glomalin is not correlated with hyphal biomass at different Pb levels. There was a positive and significant relationship between the percentage of root colonization and both immunoreactive and Bradford reactive glomalin. The content of Bradford and immunoreactive glomalin in both RC and HC was significantly increased by rising Pb levels, compared to the unleaded control. The maximum immunoreactive glomalin in HC (10.04 μg/mg hyphae) and in RC (7.79 μg/mg root) was recorded at 450 μM of Pb, which was markedly different from other levels of Pb. Total Pb sequestrated by HC glomalin enhanced as the level of Pb increased and it was 665.72 μg Pb/mg HC glomalin at 450 μM of Pb, while the total Pb sequestrated by RC glomalin was enhanced as the levels of Pb increased up to 300 μM (228.91 μg Pb/mg RC glomalin). Also, there were positive and significant correlations between both immunoreactive and Bradford reactive glomalin in HC with the amount of Pb sequestration. Consequently, this study provides evidence on the role of glomalin in Pb sequestration which should be considered for biostabilization of toxic elements in polluted sites.