Qualitative Assessment of Cadmium Stress Using Genome Template Stability in Hordeum vulgare

Abstract

Cadmium, a non essential heavy metal, does not play any metabolic role and is considered as toxic element even at very low concentration. It is mainly generated through anthropogenic activities, such as mining deposits, aerial fallout from smelters and from industrial, agricultural, energy and municipal sources (McGrath et al., 2001; McBride, 2003; Entezari et al., 2006). Cadmium (Cd) contamination has become a severe global issue and is considered a dangerous environmental pollutant not only for its neurotoxic, mutagenic and carcinogenic effects, but also for the high water solubility and thereby easier entry into human body via food (Koizumi and Yamada, 2003). Phytotoxicity of heavy metals such as Cd, Cu and Zn is long known and well documented (Prasad, 2004; Panda and Choudhury, 2005). Metal ions may directly interfere with the metabolic activities by altering the conformation of proteins, for example enzymes, transporters or regular proteins, owing to their strong affinities as ligands to sulfhydryl and carboxylic groups (Sharma et al., 2004).
Genotoxicity of Cd is directly related to its effect on structure and function of DNA, which may be determined using a number of laboratory methods (Angelis et al., 2000; Unyayar et al., 2006). However, there have been few direct experimental demonstrations of the wider relationships between DNA effects and their subsequent consequences at higher levels of biological organization. To address this issue, it is necessary to develop reliable and reproducible genotoxicity assays that can then be used in conjunction with traditional assays for detecting any impairment of population parameters (e.g. growth, reproduction and viability of offspring).
Recently, advances in molecular biology have led to the development of new tools for detection of genetic alteration in response to toxic chemicals tolerance at the level of DNA sequence and structure. DNA based techniques (RFLP, QTL, RAPD, AFLP, SSR and VNTR) are used to evaluate the variation at the DNA sequence level (Cenkci et al., 2009). Random amplified polymorphic DNA (RAPD), developed by Williams et al. (1990) and Welsh and McClelland (1990), is a PCR-based technique that amplifies random DNA fragments of genomic DNA with single short primers of arbitrary nucleotide sequence under low annealing conditions. This technique is used extensively for species classification, genetic mapping and phylogeny etc. in addition, their use in surveying genomic DNA for evidence of various types of DNA damage and mutational events (e.g., rearrangements, point mutation, small insert or deletions of DNA changes) in cells of bacteria, plants and animals (Atienzar et al., 2000; Cenkci et al., 2010; Cambier et al., 2010). It is suggested that alterations in RAPD profiles due to genotoxic exposure can be regarded as changes in genomic template stability (GTS, a qualitative measure of genotoxic effects) (Atienzar et al., 1999).
It is known that, heavy metal hyperaccumulation potential of some plant species is very high (Ozturk et al., 2010). This feature makes hyperaccumulators highly suitable for phytoremediation, for clean-up of soil and water. On the other hand, an excess of toxic heavy metal ions induces several cellular stress responses and damage to different cellular components such as membranes, proteins and DNA (Patra et al., 1998; Waisberg et al., 2003; Jimi et al., 2004). Therefore, several plant species, such as Allium cepa, Hordeum vulgare, Arabiodopsis thaliana, Vicia faba and Zea mays. etc., have been used as good bioindicators of genetic toxicity of environmental pollutants in recent years. There are many studies on heavy metal uptake and accumulation by crop plants (Chlopecka, 1996; Otabbong et al., 1997; Ali et al., 2004; Al-Qurainy, 2009). On the other hand, tolerant food crops may be dangerous as carriers of toxic metals in the food chain leading to food toxicity.
Barley (Hordeum vulgare) has been identified as a plant for efficient uptake and accumulation of Cd with a phytoremediation potential equal to that of mustard plants (Ebbs and Kochain, 1998; Sridhar et al., 2007). To be efficient metal accumulator, plants require efficient detoxification and tolerance mechanisms at both the cellular and plant level. Hence it is important to study how the different plant and cellular characteristics are affected when Cd are accumulated in barley plants.
The aims of this study were to investigate the DNA changes induced by Cd using RAPD technique, and to compare changes in RAPD profiles with growth parameters such as root and shoot growth of barley seedlings in order to improve the knowledge about the capacity of Graminae species as phytoextractors. Hordeum vulgare was chosen as test species because of its worldwide economic importance and extensive cultivation.