Heat shock proteins are a group of proteins expressed under heat and other abiotic stresses (reviewed in Lindquist and Craig, 1988). They are classified into five different families, HSP100, HSP90, HSP70, HSP60, and sHSP (12 to 42 kDa), based on their molecular masses. sHSPs are unusually abundant and diverse in plants (19 in Arabidopsis thaliana, Scharf et al., 2001; 23 in Oryza sativa: Waters et al., 2008; 36 in Populous trichocarpa: Waters et al., 2008), compared with other organisms (four in Drosophila melanogaster, 10 in Homo sapiens, 16 in Caenorhabditis elegans; reviewed in Haslbeck et al., 2005), suggesting that sHSPs may confer enhanced protection to abiotic stresses in sessile plants. sHSPs are divided into 11 subfamilies based on their cellular localization and sequence homology (Waters, 2013). There are six subfamilies in the cytoplasm/nucleus (CI to CVI), two in the mitochondria (MTI and MTII), and one each in the endoplasmic reticulum, peroxisome, and chloroplast. A number of studies have reported that sHSPs function as molecular chaperones by preventing protein denaturation and/or restoring folding of partially denatured proteins under abiotic stresses (reviewed in Sun et al., 2002).
Our previous studies have reported that DcHsp17.7, a cytosolic (CI) sHSP in carrot (Daucus carota L.), is expressed under heat (Ahn et al., 2004), cold (Song and Ahn, 2010), salinity (Song and Ahn, 2011) as well as osmotic and oxidation (Ahn and Song, 2012) stresses. Transgenic carrot (Malik et al., 1999) and potato (Solanum tuberosum L.; Ahn and Zimmerman, 2006) plants overexpressing DcHsp17.7 showed increased thermotolerance. When heterologously expressed in E. coli, DcHsp17.7 increased survival rates in a transgenic cell line, compared with vector control, under various abiotic stresses (Ahn and Song, 2012; Kim and Ahn, 2009; Song and Ahn, 2010, 2011), functioning as a molecular chaperone. Our results suggested that DcHsp17.7 could confer tolerances to not only heat, but also multiple abiotic stresses.
In this study, we examined the expression and function of DcHsp17.7 under heavy metal (lead ion) and metalloid (arsenate) stresses. Heavy metal and metalloid contamination poses a serious health risk to all living organisms, including humans and plants. Lead ion enters the food chain through living organisms such as microorganisms, insects, and plants taking up lead from various sources including residues from mining of metalliferous ores, burning of leaded gasoline, and municipal sewage (Gisbert et al., 2003). Within the cell, lead ions can replace essential ions and thus disrupt redox balance (Wang et al., 2011). In plants, lead ion inhibits a number of metabolic processes, including mineral acquisition and phytosynthesis (Brunet et al., 2009). Arsenate compounds have been widely used in agriculture such as pesticides for insects, fungi, and rodents and wood preservatives (Murcott, 2012). The level of arsenic is rapidly increasing in groundwater and soils (Goswami et al., 2010). Inorganic forms of arsenic, i.e., arsenate and arsenite, are more toxic than the organic forms (Chakrabarty et al., 2009). However, the mechanism(s) of its uptake and translocation is largely unknown in plants.
The production of stress-tolerant proteins is one of the mechanisms that plants and other living organisms have developed to withstand heavy metal contamination. Phytochelatin and methallothionine are the most well-known proteins that bind to and detoxify heavy metals in living organisms (Cobbett and Goldsbrough, 2002). HSPs have also been reported to accumulate under heavy metal conditions, such as lead (HSP70 and HSP60, Solanum lycopersicum, Wang et al., 2008; Spinacia oleraces, Wang et al., 2011) and arsenic (HSP70 in Oryza sativa, Goswami et al., 2010; HSP21 in Crambe abyssinica, Paulose et al., 2010; HSP23 in Nicotiana tabacum, Lee et al., 2012). These results suggest that HSPs may be involved in heavy metal stress tolerance in plants.
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