Cadmium response of the hairy root culture of the endangered species Adenophora lobophylla

Physiological and molecular mechanisms of medicinal plants in response to cadmium stress: Current status and future perspective

Medicinal plants have a wide range of uses worldwide. However, the quality of medicinal plants is affected by severe cadmium pollution. Cadmium can reduce photosynthetic capacity, lead to plant growth retardation and oxidative stress, and affect secondary metabolism. Medicinal plants have complex mechanisms to cope with cadmium stress. On the one hand, an antioxidant system can effectively scavenge excess reactive oxygen species produced by cadmium stress. On the other hand, cadmium chelates are formed by chelating peptides and then sequestered through vacuolar compartmentalization. Cadmium has no specific transporter in plants and is generally transferred to plant tissues through competition for the transporters of divalent metal ions, such as zinc, iron, and manganese. In recent years, progress has been achieved in exploring the physiological mechanisms by which medicinal plants responding to cadmium stress. The exogenous regulation of cadmium accumulation in medicinal plants has been studied, and the aim is reducing the toxicity of cadmium. However, research into molecular mechanisms is still lagging. In this paper, we review the physiological and molecular mechanisms and regulatory networks of medicinal plants exposed to cadmium, providing a reference for the study on the responses of medicinal plants to cadmium stress.

Establishment of hairy root system of transgenic IRT1 brassica campestris L. and preliminary study of its effect on cadmium enrichment

Cadmium (Cd) is the main heavy metal pollutant in soil. The combination of genetic engineering technology and Rizobium rhizogenes mediated technology can effectively improve the enrichment efficiency of heavy metals in super accumulators and reduce soil heavy metal pollution. In this study, the transgenic hairy root system containing the IRT1 gene of Cd hyperaccumulator-Brassica campestris L. was successfully constructed by the R. rhizogenes mediated method (IRT1 gene come from Arabidopsis thaliana). The hairy roots of each subculture can grow stably within 6 weeks, and IRT1 gene will not be lost within 50 subcultures., which is detected using PCR method. The results of Cd enrichment experiments showed that after treatment with 100 µmol/L Cd for 14 days, the growth state of transgenic IRT1 hairy roots only showed slight browning. Also, the accumulation value of Cd reached 331.61 µg/g and the enrichment efficiency of transgenic IRT1 hairy roots was 13.8% higher than that of wild-type hairy roots. Western blotting results showed that the expression of IRT1 protein in transgenic hairy roots was significantly higher than that of wild-type hairy roots under Cd stress. The above results indicated that the overexpression of IRT1 gene can help B. campestris L. hairy roots to effectively cope with Cd stress and improve its ability to enrich Cd.

Rhizobium rhizogenes-mediated root proliferation in Cd/Zn hyperaccumulator Sedum alfredii and its effects on plant growth promotion, root exudates and metal uptake efficiency

In this study, Rhizobium rhizogenes-mediated root proliferation system in Sedum alfredii has been established. Twenty strains of R. rhizogenes were screened for root proliferation. A significant difference (P < 0.01) was observed in plant morphological characters under influence of different bacterial strains. The highest root fresh weight (3.236 g/plant) was observed with strain AS12556. Furthermore, significant difference (P < 0.05) was observed in the chemical composition of organic acids, Tartaric acid (TA), Succinic acid (SA), Malic acid (MA), Citric acid (CA) and Oxalic acid (OA), pH, Total Nitrogen (TN), Total Organic Carbon (TOC) and soluble sugars in root exudates with different R. rhizogenes mediated roots. Furthermore, a series of hydroponics experiments were conducted with varying concentrations of Cd (25, 50 and 75 µM) and Zn (100, 200 and 500 µM) to assess the phytoextraction efficiency of proliferated roots with Rhizobium. Several plants with proliferated roots showed enhanced growth and improved metal extraction efficiency. Five strains (LBA 9402, K599, AS12556, MSU440 and C58C1) were identified as potential strains for root proliferation in Sedum alfredii. R. rhizogenes strain AS12556 improved Cd/Zn phytoextraction by exogenous production of phytochemicals to promote root proliferation, improved shoot biomass, lowered oxidative damage and enhanced phytoextraction efficiency in S. alfredii. Therefore, it has been selected as a potential microbial partner of S. alfredii to develop extensive rooting system for better growth and enhanced phytoremediation potential. Results suggest that R. rhizogenes mediated root proliferation system can be used for optimizing metal extraction from contaminated soils.

Metal uptake and nanoparticle synthesis in hairy root cultures

: Hairy roots are a convenient experimental tool for investigating the interactions between plant cells and metal ions. Hairy roots of species capable of hyperaccumulating Cd and Ni have been applied to investigate heavy metal tolerance in plants; hairy roots of nonhyperaccumulator species have also been employed in metal uptake studies. Furnace treatment of hairy root biomass containing high concentrations of Ni has been used to generate Ni-rich bio-ore suitable for metal recovery in phytomining applications. Hairy roots also have potential for biological synthesis of quantum dot nanocrystals. As plant cells intrinsically provide the confined spaces needed to limit the size of nanocrystals, hairy roots cultured in bioreactors under controlled conditions are a promising vehicle for the manufacture of peptide-capped semiconductor quantum dots.

Phytoremediation—A Novel and Promising Approach for Environmental Clean-up

Phytoremediation is an eco friendly approach for remediation of contaminated soil and water using plants. Phytoremediation is comprised of two components, one by the root colonizing microbes and the other by plants themselves, which degrade the toxic compounds to further non-toxic metabolites. Various compounds, viz. organic compounds, xenobiotics, pesticides and heavy metals, are among the contaminants that can be effectively remediated by plants. Plant cell cultures, hairy roots and algae have been studied for their ability to degrade a number of contaminants. They exhibit various enzymatic activities for degradation of xenobiotics, viz. dehalogenation, denitrification leading to breakdown of complex compounds to simple and non-toxic products. Plants and algae also have the ability to hyper accumulate various heavy metals by the action of phytochelatins and metallothioneins forming complexes with heavy metals and translocate them into vacuoles. Molecular cloning and expression of heavy metal accumulator genes and xenobiotic degrading enzyme coding genes resulted in enhanced remediation rates, which will be helpful in making the process for large-scale application to remediate vast areas of contaminated soils. A few companies worldwide are also working on this aspect of bioremediation, mainly by transgenic plants to replace expensive physical or chemical remediation techniques. Selection and testing multiple hyperaccumulator plants, protein engineering ofphytochelatin and membrane transporter genes and their expression would enhance the rate of phytoremediation, making this process a successful one for bioremediation of environmental contamination. Recent years have seen major investments in the R&D, which have also resulted in competition of filing patents by several companies for economic gains. The details of science & technology related to phytoremediation have been discussed with a focus on future trends and prospects of global relevance.

An ATP-binding cassette transporter related to yeast vacuolar ScYCF1 is important for Cd sequestration in Chlamydomonas reinhardtii

We generated a Cd-sensitive insertional mutant, Cds18, in Chlamydomonas reinhardtii and elucidated the deletion of a 10 kb fragment containing the promoter and a portion of the coding region for CrMRP2 gene that silenced the transcription of CrMRP2 in mutant Cds18. The association between CrMRP2 and Cd sensitivity was confirmed by complementing mutant Cds18 with a cloned genomic DNA fragment containing the promoter and complete coding sequence for CrMRP2. The genomic region and the full-length cDNA for CrMRP2 were cloned and sequenced. Computer searches detected the significant resemblance of CrMRP2 with HsMRP1, AtMRP3 and ScYCF1, in Homo sapiens, Arabidopsis thaliana and Saccharomyces cerevisiae, respectively. All are members of the multidrug resistance-associated protein (MRP)/cystic fibrosis transmembrane conductance regulator (CFTR) subfamily of ATP-binding cassette (ABC) transporters. When the cDNA of CrMRP2 was cloned into the yeast expression vector pEGKT and transformed into the yeast mutant strain DTY168 lacking ScYCF1, it restored the function of ScYCF1, a yeast vacuolar glutathione (GSH)-conjugate ABC transporter. A putative vacuolar-targeting motif (T/I/K)LP(L/K/I) was detected in the N-terminal part of CrMRP2. In wild-type C. reinhardtii, CrMRP2 transcription was significantly up-regulated upon Cd treatment. Comparing with mutant Cds18, the wild-type algal cells accumulated and sequestered more Cd in the stable high molecular weight (HMW) phytochelatin (PC)-Cd complex; the labile low molecular weight (LMW) PC-Cd complex was detected in mutant Cds18 at an earlier stage of Cd treatment. This study demonstrated the expression of CrMRP2 in C. reinhardtii and implicated its function in the formation/accumulation of stable HMW PC-Cd complex.

Ni-induced oxidative stress in roots of the Ni hyperaccumulator, Alyssum bertolonii

•  The aim of this study was to determine whether superior antioxidative defences contribute to Ni tolerance in roots of the hyperaccumulator species, Alyssum bertolonii . Antioxidative responses were compared in hairy roots of A. bertolonii and the nonhyperaccumulator, Nicotiana tabacum . •  Growth, Ni uptake, antioxidative enzyme activities, lipid peroxidation and concentrations of H ₂ O₂ and surface -SH groups were measured in hairy root cultures exposed to 25 ppm (426 µm) Ni. •  Growth of A. bertolonii roots was not affected by Ni, whereas Ni prevented N. tabacum root growth. Endogenous activities of superoxide dismutase and catalase were 2.4 and > 500 times greater, respectively, in A. bertolonii roots than in N. tabacum . H ₂ O₂ levels rose significantly with Ni treatment in both species, by factors of 3.6 and 8.6, respectively. •  Compared with N. tabacum , oxidative damage may be minimised in A. bertolonii roots by high endogenous activities of catalase and, to a lesser extent, superoxide dismutase. As accumulation of H ₂ O₂ was not detrimental to A. bertolonii, enhanced mechanisms for tolerating active oxygen species may also be present.