LJH685 Phytoremediation technology is defined as the use of
Phytoremediation technology is defined as the use of plants to remove contaminants from soils or to render them harmless, and is regarded as a cost-effective, environmental-friendly method for reclaiming soils contaminated by toxic metals (Lasat, 2002, Mahar et al., 2016, Marrugo-Negrete et al., 2016). Compositae plants have been shown to be excellent candidates for phytoremediation purposes due to their rapid growth, high biomass, strong breeding ability, adapted to growing in soils polluted with heavy metals, and low impact on the food chain and human health (Hernández and Pastor, 2008, Peng et al., 2006). We have already studied the content, subcellular distribution and chemical forms of heavy metals in three types of Compositae plants (Artemisia lavandulaefolia, Ageratum conyzoides L., Crassocephalum crepidioides) from one lead-zinc tailings area, and demonstrated that C. crepidioides demonstrated the basic characteristics of a Cd-hyperaccumulator, cell wall binding, vacuolar compartmentalization and distribution mainly in lower active chemical forms were supposed to be the main tolerance mechanisms to heavy metals (Zhu et al., 2017b, Zhu et al., 2018). Free amino acids have been shown to have functional roles in metal tolerance of plants. However, there is little information on the free amino LJH685 metabolism in Compositae plants under heavy metal stress. Therefore the present study was conducted to (1) confirm the identification of Cd hyperaccumulator for C. crepidioides by pot-culture upon exposed to various gradient of Cd stress, and (2) characterize the changes and differences in accumulation of free amino acids of two Compositae species with different Cd enrichment abilities -Ageratum conyzoides L. and Crassocephalum crepidioides, growing in pot experiment under Cd stress.
Materials and methods
Discussion Tolerance property, enrichment factor property and high biomass production are considered as the three key factors for using plants in phytoremediation strategies (Antonkiewicz and Para, 2016, Mahar et al., 2016). The tolerance characteristics show that there is no significant decrease in the shoot biomass compared with the control; enrichment factor property means that plants with high metal accumulation capacities also translocate metals effectively from root to the shoot. Several Cd-hyperaccumulator plants have low biomass production and growth rates which determine the extended use in soil remediation (Mahar et al., 2016). A. conyzoides and C. crepidioides, which are annual herb Compositae plants with fast growth and spread rates, occur widely throughout tropical and subtropical China. The results presented in this paper show that these two Compositae plants were identified as Cd accumulator plants, due to relatively high concentrations of Cd in the aerial parts and because their associated BCF were higher than 1, in addition to their associated BTF. However, the comparison of C. crepidioides and A. conyzoides considering biomass production and Cd accumulation, the former shows a higher growth tolerance under high Cd conditions. C. crepidioides can be basically used as a Cd-hyperaccumulator. Additionally, triple cropping would be possible within one year for C. crepidioides, which is a significant superiority over other previously reported Cd accumulators. Accordingly, C. crepidioides represents a promising candidate for phytoextraction of Cd from polluted soils. It's necessary to conduct field trials to evaluate the possible use of this plant on a large scale. Cadmium toxicity perturbed amino acid metabolism in plants, and the changes of amino acid levels can play a significant role in the mechanism of plant adaptation to Cd stress (Zemanová et al., 2014, Zoghlami et al., 2011). Studying the qualitative and quantitative changes in the content of individual amino acids can thus be informative. Amino acid homeostasis is essential for growth, development and defense of plants against environmental stress. The homeostasis is regulated by de novo biosynthesis, uptake/translocation, and protein synthesis/degradation (He et al., 2013). Chaffei et al. (2004) indicated that an increase in the proportion of high N:C amino acids is a protective strategy in plants for preserving roots as a nutritional safeguard organ to ensure future recovery. The results presented in this paper demonstrated an accumulation of large amounts of amino acids in Cd-tolerant C. crepidioides compared to A. conyzoides (Fig. 3), which was consistent with hypothesis explained above. The amino acid accumulation in high Cd accumulator C. crepidioides also suggested that the Cd-chelating molecules are highly active in binding Cd and forming a complex that can be translocated from the roots to the shoots.