Salicylic acid (SA) plays a key role in plant disease resistance and hypersensitive cell death but is also implicated in hardening responses to abiotic stressors. Cadmium (Cd) exposure increased the free SA contents of barley (Hordeum vulgare) roots by a factor of about 2. Cultivation of dry barley caryopses presoaked in SA-containing solution for only 6 h or single transient addition of SA at a 0.5 mm concentration to the hydroponics solution partially protected the seedlings from Cd toxicity during the following growth period. Both SA treatments had little effect on growth in the absence of Cd, but increased root and shoot length and fresh and dry weight and inhibited lipid peroxidation in roots, as indicated by malondialdehyde contents, in the presence of Cd. To test whether this protection was due to up-regulation of antioxidant enzymes, activities and transcript levels of the H 2 O 2 -metabolizing enzymes such as catalase and ascorbate peroxidase were measured in control and SA-treated seedlings in the presence or absence of 25 m Cd. Cd stress increased the activity of these enzymes by variable extent. SA treatments strongly or completely suppressed the Cd-induced up-regulation of the antioxidant enzyme activities. Slices from leaves treated with SA for 24 h also showed an increased level of tolerance toward high Cd concentrations as indicated by chlorophyll a fluorescence parameters. The results support the conclusion that SA alleviates Cd toxicity not at the level of antioxidant defense but by affecting other mechanisms of Cd detoxification.Cd is a highly toxic and persistent environmental poison for plants and animals (di Toppi and Gabbrielli, 1999). Cd interferes with many cellular functions mainly by complex formation with side groups of organic compounds such as proteins resulting in inhibition of essential activities. Although the mechanisms of cytoplasmic toxicity are identical in all organisms, different plant species and varieties show a wide range of plasticity in Cd tolerance, reaching from the high degree of sensitivity of most plants on the one hand to the hyperaccumulating phenotype of some tolerant higher plants on the other hand (McGrath et al., 2001). On an expanded concentration scale, even sensitive species vary considerably in their response to Cd. For example pea (Pisum sativum) is considerably more sensitive to Cd than barley (Hordeum vulgare cv Gerbel), which still grows well at concentrations above 10 m under nutrient rich conditions. Cd induces genetic and biochemical changes in plant metabolism that are related to general and Cd-specific stress responses (Blinda et al., 1997). Cd tolerance is correlated with intracellular compartmentalization and hence specific transport processes that allow the toxic effects of low Cd levels to decrease at least (Brune et al., 1995; Gonzalez et al., 1999). The activation of the cellular antioxidant metabolism belongs to the general stress responses induced by heavy metals (Dietz et al., 1999). Although an active antioxidative metabolism does not...

The SGECd(t) mutant represents a novel and unique model to study adaptation of plants to toxic heavy metal concentrations.

The inter-relation between nitrogen availability and cadmium toxicity was studied in roots of barley seedlings with emphasis on the analysis of expression of 10 selected genes relevant for growth in the presence of toxic Cd concentrations. The response to Cd exposure differed quantitatively or qualitatively for the 10 genes in dependence of the N supply. Transcripts of glutathione synthase, glutathione reductase, glutathione peroxidase and dehydroascorbate reductase were measured as parameters involved in antioxidant defence, metallothionein, phosphoenolpyruvate carboxylase and phytochelatin synthase (PCS) were analysed as genes related to heavy metal binding, and vacuolar ATPase subunits VHA-E and VHA-c and a NRAMPtransporter as genes being implicated in Cd transport. Reprogramming of the Cd response was most obvious for PCS and NRAMP whose transcript levels were unaltered and down-regulated, respectively, in the presence of Cd at adequate N, but strongly up-regulated upon Cd exposure under conditions of nitrogen deficiency. Different responses to Cd at varying N supply were also seen for the antioxidant genes. The results on gene expression are discussed in context with the changes in biochemical parameters, and underline the importance of evaluating the general growth conditions of a plant when discussing its specific response to a stressor such as Cd.The sequence of the nramp cDNA was filed at the EMBL/GenBank/DDBJ Databases under the accession number AJ514946.

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