| Endophytic fungi are fungal microorganisms that colonize healthy plant tissue intercellularly and/or intracellularly without causing any apparent symptoms of disease. It has known that endophytic fungi are potential bio-adsorbents and play important roles in host plant growth, heavy metal tolerance and phytoremediation.Thus, the objectives of this study were (1) to analyse the species composition of endophytic root-associated fungi (RAF) isolated from Salix variegate and Myricaria laxiflora growing in the water-level-fluctuation zone of Three Gorges Reservoir Region, China, (2) to assess the heavy metal tolerance or IAA production potential of these fungi in vitro, and (3) to understand the possible impact of the plant associated fungi on metal tolerance, and their roles in the uptake and the potential transference of heavy metals in the plant.A total of 53 RAF strains were isolated from S. variegate collected from bank of Jialing river and identified to 14 morphotaxa, in which Chromosporium, Fusarium and Gonatobotrys were dominant genera. Among them,27 isolates were selected to measure their resistance to 0.02 mg-ml-1 Cd2+and 81.5% of the isolates showed resistance to Cd2+(TI>50%),40.7% of the isolates were growth stimulated (Tolerance index>100%). Of these active isolates, four dark septate endophyte (DSE) isolates (Paraphaeosphaeria sp. SR46, Pyrenochaeta sp. SR35, Rhizopycnis vagum SR37 and R. vagum SR44) were further tested for minimum inhibitory concentrations (MICs) against Cd and SR46 was found to be the most tolerant isolate with MIC of 0.39 mg·ml-1. Additionally, the maximum uptake values of these DSEs ranged from 3.01 to 7.89 mg·g-1, but there was no significant correlation between metal uptake with fungal biomass and metal tolerance. Subsequently, a pot experiment was conducted for investigating the impact of SR46 on corn seedlings in Cd-enriched soil. The results obtained suggested that SR46 reduced the Cd bioaccumulation of plant under low (100 mg-kg-1) Cd stress and enhanced the Cd translocation from root zone to aerial parts under high (200 mg·kg-1) Cd stress. Besides, it promoted plant growth without Cd stress. These findings indicated S. variegate harbors an RAF flora showing a high genetic diversity as well as a high level of metal resistance to Cd that has potential values in cadmium cycling and restoration of plant, soil and water system.A total of 37 RAF strains were isolated from M. laxiflora collected from beach of Yangtze Valley and identified to 11 taxa, in which Paraconiothyrium cyclothyrioides, Pleosporales sp. and Sordariales sp. were dominant taxa. Besides,5 taxa including 29 strains were dematiaceous fungi.21 isolates of 11 taxa were selected to detect their heavy metal resistance to 0.04 mg-ml-1 Cd2+ or Hg2+ and 47.6% of the isolates showed resistance to Cd2+(TI>50%),71.4% of the isolates showed resistance to Hg2+, and 42.8% of the isolates showed resistance to both Cd2+and Hg2+. Among of 21 isolates, Coprinopsis sp. MR32-1 produced the highest level of IAA (72.29μg·ml-1). A root elongation assay on filter paper culture experiment suggested that culture filtrates of MR32-1 showed different levels of root length promotion under heavy metal stress. In addition, an inoculation experiment was performed to monitor the effects of the MR32-1 on the plant growth and metal translocation and uptake by the corn host. The results obtained sugguested that Coprinopsis sp. MR32-1 enhanced the metal translocation, metal uptake and physiology of corn seedlings, supporting its potential for phytoremediation or phytoextraction by host plant in metal contaminated soil. Our findings revealed that M. laxiflora harbored an endophytic RAF flora showing genetic, metal tolerance and IAA production diversity that has potential values in phytoremediation.These findings demonstrated that the two flooding-resistant plants harbor RAF flora showing great diversity and different level of heavy metal reistance and/or IAA production. The endophytic fungal symbionts associated with plants play pivotal roles in promoting plant growth, enhancing host tolerance and assisting phytoremediation. |
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