The Roles And Mechanisms Of3-Indole Acetic Acid In Arsenic Hyperaccumulation By Plants

To study the roles and mechanisms of3-indoleacetic acid (IAA) in arsenic (As) hyperaccumulation by plants, firstly, a field survey was conducted at6sites in a mine reclamation farmland located at Datun town, Gejiu City, Yunnan Province. The IAA contents in the leaves of12plants were determined. Secondarilly, one arsenic hyperaccumulator (Pteris cretica var. nervosa) and one non-hyperaccumulator (Pteris ensiformis) were selected from Pteris genus to assess the effects of arsenic stress (0,50,100and200mg As/kg) on the plant biomass, height, frond indole-3-acetic acid (AA) Icontents, arsenic accumulation, the activities of IAA oxidase and antioxidant enzymes (superoxide dismutase SOD, peroxidase POD and catalase CAT), as well as the content of malondialdehyde (MDA, a per-oxidation product of cell membrane lipid) using a pot experiment. The relationship among the contents of IAA, arsenic concentration, activities of3antioxidant enzymes and MDA contents in the fronds of two plants were also analyzed by step regression statistical method. In addition, the variation of IAA, IAAO, SOD, POD, CAT and MDA in two plants exposed to100mg/kg with culture time was examined. Finally, the effects of exogenously applied IAA (0-50mg/L) on the biomass, total As uptake, arsenic, As speciation in plant different parts (root and leaves), antioxidant enzyme activities and MDA contents in P. cretica var. nervosa and P. ensiformis were evaluated at2mg/L As treatment with three As forms (As (Ⅲ), As (Ⅴ) and dimethyl arsenic DMA) by a hydroponic experiment. The results showed that:1. The farmlands polluted by heavy metals had variable concentrations of Pb, Zn, Cu, Cd and As. The average concentrations of these5heavy metals in study area were far beyond the allowable threshod values of environmental quality standard for soils in China (GB15618-1995). The metal pollution of soil was evaluated by using a cluster statistic method and the6polluted sties were classified into4pollution degrees (light, low, middle and severe pollution), but most of the12plants grew well. With the increasing levels of heavy metal pollution, IAA contents could remain at a relatively high level or increased significantly in the leaves of As hyperaccumulators Pteris vittata and Pb hyperaccumulator Bidens pilosa. This might play a significant role on promoting or maintaining plants’normal growth and development. In the same heavy metal pollution level, the IAA contents in P. vittata and B. pilosa were significantly higher than those in non-hyperaccumulators (Erigeron speciosus or Saccharum sinensis). Therefore, the high biomass and metal-accumulative ability of hyperaccumulators under heavy metal pollution might ascribe to the high IAA contents in hyperaccumulators.2. No significant difference was observed in the biomass and height of P. cretica var. nervosa under arsenic stress compared to the control (0mg/kg), but a significant decrease was noted in P. ensiformis under medium or high arsenic stress. The concentrations of arsenic and IAA increased but the activity of IAAO decreased in the fronds of two plants exposed to medium or high arsenic stress, but this change was more significant in P. cretica var. nervosa. The activities of3antioxidant enzymes could maintain or increase in the fronds of P. cretica var. nervosa exposed to arsenic stress, but a significant decrease of POD was observed in P. ensiformis, in which the activities of SOD and CAT could maintain. Therefore, much stronger antioxidant ability was shown in P. cretica var. nervosa exposed to arsenic stress. The results from step regression analysis showed a significantly positive correlation between the IAA contents and arsenic concentrations in the fronds of two plants, and a significantly negative correlation was also observed between the IAA contents and CAT activities in the fronds of P. cretica var. nervosa. The result from time-course effect research showed that the highest IAA contents, the lowest IAAO and CAT activity were found in the13th days in the culture process of P. cretica var. nervosa exposed to100mg/kg, but similar result was not observed for P. ensiformis. Therefore, a higher IAA contents, a lower IAAO and CAT activities may have contributed to arsenic hyperaccumulation in P. cretica var. nervosa.3. When the two plants were exogenously treated with a suitable concentration of IAA, their biomass and contents of As speciations significantly increased under the stress of different As forms. In the As(Ⅲ) or As (V) treatment without IAA, the main form of As was presented as As(V) in the roots and stems of two plants and in the leaves of P. ensiformis, and as As(Ⅲ) in the leaves of P. cretica var. nervosa, respectively. The As forms in the different parts of two plants was not change with IAA addition, but the contents of original As forms were increased, especially in the30mg/L IAA treatment. In the DMA treatment without IAA, the main form of As was presented as DMA in the roots, stems and leaves in the two plants. The As forms in the different parts of two plants was still not change with IAA addition, but the contents of DMA were increased, especially for the stems and leaves of two plants and for the roots in P. ensiformis in the30mg/L IAA treatment. The best enhancement uptake of DMA was observed in the roots of P. cretica var. nervosa at10mg/L IAA treatment.4. Under the stress of3As forms (2mg/L), compared to the control (0mg/L IAA), the activities of3antioxidative enzymes in the leaves of P. cretica var. nervosa could maintain or significant increase. However, the POD activities could only maintain even decrease in P. ensiformis, althouth the activities of SOD and CAT was higher compared to the control. Whenever under the stress of As forms, the MDA contents significantly decreased in the leaves of P. cretica var. nervosa with IAA addition. Howecer, the MDA contents in the leaves of P. ensiformis were much higher than those in P. cretica var. nervosa, althouth this contents decreased or unchanged in the former. Therefore, a suitable concentration of IAA could alleviate the damage caused by lipid peroxidation of plant cell membrane.