The Roles And Mechanisms Of Cytokinin In The Arsenic Hyperaccumulation By Hyperaccumulators

By means of field survey and laboratory experiments, the roles and mechanisms of cytokinin(CTK) in the arsenic(As) hyperaccumulation of plants were studied in this paper. Firsty, a field survey was conducted to evaluate the soil metal pollution and the contents of CTK in the leaves of5plants at six sampling sites in a metal-contaminated area located at Shadian County, Gejiu City, Yunnan Province. Secondarily, one arsenic hyperaccumulator (Pteris cretica var. nervosa) and one non-hyperaccumulator (Pteris ensiformis) were selected to evaluate the effects of As(V) stress (0-200mg·kg-1) on the plant biomass, height, CTK, chlorophyll, soluble proteins and As contents in plant fronds by a pot experiment. By using step regression statistical method, the relationship among the contents of CTK. As, chlorophyll and soluble protein contents in the fronds of the two plants were also analyzed. In addition, the variations of CTK, As speciation, chlorophyll and soluble protein contents in the two plants exposed to100mg·kg-1As(V) with culture time extension were examined. Finally, the effects of exogenously applied CTK (0-1.0mg·L-1) on the biomass, As uptake and speciation, antioxidative enzyme activities and malondialdehyde (MDA) contents in the fronds of the two plants exposed to2.0mg·L-1As (Ⅲ), As (V) or DMA were determined. The endogenous CTK in the present study was trans-Zeatin and the exogenous one was kinetin. The two names were simplified as CTK in the full paper. The main results showed that:1. The concentrations of Pb, Zn, Cd and As in soils of the study area were far beyond the national environmental quality standard for soils (GB15618-1995, Grade Ⅲ) except Cu. The6sampling sites were classified into4pollution levels:light, low, middle and severe according to the metal concentrations. With the increasing levels of soil heavy metal pollution, the contents of CTK could maintain or increase in the fronds of Pteris vittata (an As hyperaccumulator) and Bidens pilosa (a Pb hyperaccumulator). However, the contents of CTK in the fronds of the other3plants decreased. Therefore, the higher contents of CTK in the hyperaccumulators under heavy metal stress played an important role in the normal growth of plants.2. In the pot experiment, P. cretica var. nervosa could grow well but the growth of P. ensiformis was inhibited under the stress of different concentrations of As(V). The contents of CTK increased in the fronds of P. cretica var. nervosa but decreased in P. ensiformis under middle or high As(V) stress. Therefore, the higher CTK level in P. cretica var. nervosa contributed to its normal growth under As stress. The main forms of As in the fronds were presented as As(III) and As(V) in P. cretica var. nervosa and P. ensiformis, respectively. Results from step regression statistical method showed that a positive correlation was found between CTK and the As(V), chlorophyll contents in the fronds of two plants.3. The main forms of As were still presented as As(III) in the fronds of P. cretica var. nervosa but as As(V) in P. ensiformis when the two plants were exposed to2mg·L-1inorganic As solution. However, in the DMA treatment, the form of frond As was presented as DMA in the two plants. With the addition of CTK, the concentrations of original As forms in the fronds of the two plants increased, especially in P. cretica var. nervosa when supplied with0.5mg·L-1CTK. This increase was much higher in P. cretica var. nervosa than in P. ensiformis. With the addition of CTK, the activities of three antioxidative enzymes, superoxide dismutase (SOD), catalase(CAT) and peroxidase(POD), could maintain or significantly increase in the fronds of P. cretica var. nervosa, but MDA contents decreased significantly. The MDA contents were significantly higher in P. ensiformis than those in P. cretica var. nervosa, although a decrease was observed in the former. On the whole, the antioxidative ability and As species uptake were significantly higher in P. cretica var. nervosa compared to P ensiformis. Therefore, a suitable concentration of exogenously applied CTK could increase the activities of antioxidative enzymes of As hyperaccumulators, which contribute to alleviating the damage of cell membrane-lipid peroxidation caused by As stress.