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Cadmium (Cd) toxicity has been widely studied in different plant species; however, the mechanism involved in its toxicity as well as the cell response against the metal have not been well established. In this work, using pea (Pisum sativum) plants, we studied the effect of Cd on antioxidants, reactive oxygen species (ROS), and nitric oxide (NO) metabolism of leaves using different cellular, molecular, and biochemical approaches. The growth of pea plants with 50 mM CdCl2 affected differentially the expression of superoxide dismutase (SOD) isozymes at both transcriptional and posttranscriptional levels, giving rise to a SOD activity reduction. The copper/zinc-SOD down-regulation was apparently due to the calcium (Ca) deficiency induced by the heavy metal. In these circumstances, the overproduction of the ROS hydrogen peroxide and superoxide could be observed in vivo by confocal laser microscopy, mainly associated with vascular tissue, epidermis, and mesophyll cells, and the production of superoxide radicals was prevented by exogenous Ca. On the other hand, the NO synthase-dependent NO production was strongly depressed by Cd, and treatment with Ca prevented this effect. Under these conditions, the pathogen-related proteins PrP4A and chitinase and the heat shock protein 71.2, were up-regulated, probably to protect cells against damages induced by Cd. The regulation of these proteins could be mediated by jasmonic acid and ethylene, whose contents increased by Cd treatment. A model is proposed for the cellular response to long-term Cd exposure consisting of cross talk between Ca, ROS, and NO.
Cadmium (Cd) is a toxic element whose presence in the environment is mainly due to industrial processes and phosphate fertilizers and then is transferred to the food chain (Pinto et al., 2004). Cd is rapidly taken up by plant roots and can be loaded into the xylem for its transport into leaves. Most plants are sensitive to low Cd concentrations, which inhibit plant growth as a consequence of alterations in the photosynthesis rate and the uptake and distribution of macronutrients and micronutrients (Lozano-Rodriguez et al., 1997; Sandalio et al., 2001; Benavides et al., 2005). It is known that the content of polyvalent cations can be affected by the presence of Cd through competition for binding sites of proteins or transporters (Gussarson et al., 1996). Thus, Cd produced a decrease of calcium (Ca)...