Hydroponic experiments were conducted using cadmium (Cd)-sensitive (cv Guiyan 1) and Cd-tolerant (Yunyan 2) tobacco cultivars to evaluate cultivar differences in response to Cd toxicity in the presence of selenium (Se). The results showed that addition of 3 µM Se in 50 µM Cd solution markedly reduced Cd accumulation in plants, alleviated Cd-induced growth inhibition, and increased nitrogen and chlorophyll contents as well as photosynthetic performance (i.e., net photosynthetic rate, stomatal conductance, and transpiration rate). External Se dramatically depressed Cd-induced O2 (•-) , H2 O2 , and malondialdehyde accumulation, especially in the sensitive cultivar. Selenium significantly elevated Cd-depressed activities of superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, glutathione-peroxidase, and glutathione reductase in the both cultivars after 7-d treatments. Meanwhile, Se counteracted Cd-induced alterations in certain nutrient elements; for example, it significantly increased Zn and Ca concentrations and reduced Mg concentration in both cultivars. Furthermore, Se significantly elevated Cd-depressed H(+) -K(+) -adenosine triphosphatase (ATPase), Na(+) -K(+) -ATPase, and Ca(2+) -Mg(2+) -ATPase activities. The beneficial effect of Se under Cd stress may be related mainly to the increased ATPase activity and reduced Cd uptake and reactive oxygen species accumulation, thus reducing the negative consequences of oxidative stress caused by Cd toxicity.

Tobacco easily accumulates certain heavy metals in leaves and thus poses a potential threat to human health. To systematically dissect Cr-responsive microRNAs (miRNAs) and their targets at the global level, 4 small RNA libraries were constructed from the roots of Cr-treated (Cr) and Cr-free (control) for 2 contrasting tobacco genotypes,Yunyan2 (Cr-sensitive) and Guiyan1 (Cr-tolerant). Using high-throughput-sequencing-technology, the authors identified 53 conserved and 29 novel miRNA families. Comparative genomic analysis of 41 conserved Cr-responsive miRNA families revealed that 11 miRNA families showed up-regulation in Guiyan1 but unaltered in Yunyan2, and 17 miRNA families were up-regulated only in Yunyan2 under Cr stress. Only 1 family, miR6149, was down-regulated in Yunyan2 but remained unchanged in Guiyan1. Of the 29 novel miRNA families, 14 expressed differently in the 2 genotypes under Cr stress. Based on a high-throughput degradome sequencing homology search, potential targets were predicted for the 41 conserved and 14 novel Cr-responsive miRNA families. Clusters of Orthologous Groups functional category analysis revealed that some of these predicted target transcripts of miRNAs are responsive to biotic and abiotic stresses. Furthermore, the expression patterns of many Cr-responsive miRNAs were validated by stem-loop real-time transcription polymerase chain reaction. The results of the present study provide valuable information and a framework for understanding the function of miRNAs in Cr tolerance.

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The effect of sodium chloride (NaCl) on cadmium (Cd) uptake, translocation, and oxidative stress was investigated using 2 tobacco cultivars differing in Cd tolerance. The growth inhibition of the tobacco plants exposed to Cd toxicity was in part alleviated by moderate addition of NaCl in the culture solution. Cadmium concentration of shoots and roots in the 2 cultivars increased with increasing Cd levels in the solution and decreased with the addition of NaCl. The addition of NaCl could alleviate the oxidative stress caused by Cd toxicity, as reflected by reduced production of malondialdehyde and recovered or enhanced activities of antioxidative enzymes catalase and glutathione peroxidase. The results also showed that the enhancement of antioxidative enzyme activity by NaCl for the tobacco plants exposed to Cd stress is related to induced Ca signaling.

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