The effect of glomalin on root stabilization of lead in clover plant inoculated with Rhizophagus irregularis fungus

Root stabilization of toxic metals by mycorrhizal plants is one of the protective mechanisms of symbiotic arbuscular mycorrhizal (AM) fungi in response to metal stress. Role of the glomalin as a specific glycoprotein of spore and hyphal cell wall of AM fungi can be remarkable in sequestration of toxic metals and reduction of stress effects on host plant. Considering this hypothesize, the study was conducted to investigate the role of glomalin produced by Rhizophagus irregularis fungus symbiosis of clover plant in root stabilization of Pb.

A pot culture experiment was performed as completely randomized block design by two factors including AM fungus (inoculated with R. irregularis and non-inoculated) and four levels of Pb+2 (0, 150, 300 and 450 µM) with five replications. For glomalin extraction, root samples were autoclaved at 121˚C with 50 mM sodium citrate buffer for 1 hr in three cycles. Glomalin concentration in the extracted samples were determined by ELISA method using monoclonal antibody 32B11. After precipitation of the glomalin and its digestion in concentrated nitric acid, Pb-sequestrated by the glomalin were measured. Shoot and root dry weights, root colonization percentage, shoot and root P and Pb contents, and plant uptake, extraction and translocation efficiency of Pb were assessed.

Shoot and root dry weights of mycorrhizal (M) and non-mycorrhizal (NM) plants were decreased by increasing of Pb levels. At the levels of 150, 300 and 450 μm Pb, shoot and root dry weights were decreased by 11.2%, 12.9%, 18.3% and 7.5%, 18.1%, 36.7% compared to the control (0 μm Pb), respectively. Shoot and root dry weights of M plants were increased by 24.9% and 43.2% compared to the NM ones. P contents of shoot and root were affected by AM fungus, so that the shoot and root P contents of M plants were increased by 32.2% and 45.8 % compared to the NM ones. At different levels of Pb, shoot and root Pb contents in M plants significantly were higher than NM ones. Maximum contents of Pb of shoot and root were recorded at level of 450 µM Pb in M plants which were increased by 46.5% and 80.7% compared to the NM ones at the same level. At the levels of 150, 300 and 450 µM Pb, the uptake efficiency of Pb in M plants was increased by 8%, 14.5% and 80.7% compared to the NM ones at the same levels. Based on ANOVA results, Pb-extraction and translocation efficiency were affected by Pb treatments. Pb-extraction efficiency of plants was increased as Pb concentration increased, so that the content of Pb-extraction efficiency at 450 µM of Pb was increased by 69.3% and 27.8% compared to the 150 and 300 µM of Pb, respectively. Plant Pb-translocation efficiency from root to shoot was decreased as Pb concentration increased. The percentage of root colonization was increased as the Pb concentration increased up to 300 µM Pb and then was slightly decreased as the level of Pb rose from 300 to 450 µM, but there was no significant difference between the levels of 150, 300 and 450 µM Pb. Glomalin production in root and Pb sequestrated by glomalin was significantly increased as Pb concentration increased.

Root colonization of clover plant by R. irreqularis led to improved growth and phosphorus nutrition of M plants compared to the NM ones, under Pb stress condition. Pb uptake was greater in roots than in shoots, therefore the clover plants played a more effective role in root stabilisation of Pb.