Exogenous Glycinebetaine Reduces Cadmium Uptake and Mitigates Cadmium Toxicity in Two Tobacco Genotypes Differing in Cadmium Tolerance.

Greenhouse hydroponic experiments were conducted using Cd-sensitive (cv. Guiyan1) and Cd-tolerant (cv. Yunyan2) tobacco cultivars to study the ameliorative effects of exogenous glycinebetaine (GB) upon 5 µM Cd stress. The foliar spray of GB markedly reduced Cd concentrations in plants and alleviated Cd-induced soil plant analysis development (SPAD) value, plant height and root length inhibition, with the mitigation effect being more obvious in Yunyan2. External GB markedly reduced Cd-induced malondialdehyde (MDA) accumulation, induced stomatal closure, ameliorated Cd-induced damages on leaf/root ultrastructure, and increased the chlorophyll content and fluorescence parameters of Fo, Fm, and Fv/Fm in both cultivars and Pn in Yunyan2. Exogenous GB counteracted Cd-induced alterations of certain antioxidant enzymes and nutrients uptake, e.g., the depressed Cd-induced increase of superoxide dismutase (SOD) and peroxidase (POD) activities, but significantly elevated the depressed catalase (CAT) and ascorbate peroxidase (APX) activities. The results indicate that alleviated Cd toxicity by GB application is related to the reduced Cd uptake and MDA accumulation, balanced nutrients and antioxidant enzyme activities, improved PSII, and integrated ultrastructure in tobacco plants.

Identification and comparative analysis of the microRNA transcriptome in roots of two contrasting tobacco genotypes in response to cadmium stress.

Tobacco (Nicotiana tabacum L.) is more acclimated to cadmium (Cd) uptake and preferentially enriches Cd in leaves than other crops. MicroRNAs (miRNAs) play crucial roles in regulating expression of various stress response genes in plants. However, genome-wide expression of miRNAs and their target genes in response to Cd stress in tobacco are still unknown. Here, miRNA high-throughput sequencing technology was performed using two contrasting tobacco genotypes Guiyan 1 and Yunyan 2 of Cd-sensitive and tolerance. Comprehensive analysis of miRNA expression profiles in control and Cd treated plants identified 72 known (27 families) and 14 novel differentially expressed miRNAs in the two genotypes. Among them, 28 known (14 families) and 5 novel miRNAs were considered as Cd tolerance associated miRNAs, which mainly involved in cell growth, ion homeostasis, stress defense, antioxidant and hormone signaling. Finally, a hypothetical model of Cd tolerance mechanism in Yunyan 2 was presented. Our findings suggest that some miRNAs and their target genes and pathways may play critical roles in Cd tolerance.

Genotype-dependent effect of exogenous 24-epibrassinolide on chromium-induced changes in ultrastructure and physicochemical traits in tobacco seedlings.

Greenhouse hydroponic experiments were carried out using three different heavy metal accumulation tobacco genotypes to evaluate how different genotypes responded to chromium (Cr) toxicity in the presence of 24-epibrassinolide (EBR; a biologically active brassinosteroid). The results showed that Cr stress caused a marked reduction in plant biomass, chlorophyll content, chlorophyll fluorescence, and photosynthesis parameters but induced malondialdehyde accumulation and ultrastructure damage, with 2010-38 (L) less affected. Foliar application of 24-epibrassinolide (0.1 µM) on Cr-stressed plants greatly alleviated Cr-induced inhibition of growth and photosynthesis, oxidative stress and ultrastructure damage, decreased Cr accumulation in different parts of leaves and roots, with the exception of the upper and lower of leaves of genotype L, and maintained ion homeostasis. Regarding genotypes, L was more tolerant than M and H, as it absorbed less Cr and also performed better in all of the studied parameters. These findings suggest a potential role for 24-epibrassinolide in Cr stress alleviation and the utilization of elite genetic resources in future breeding programs to develop low Cr accumulation genotypes. These results advocate a positive role for 24-epibrassinolide in reducing pollutant residues from health point of view.

Genome-wide identification of chromium stress-responsive micro RNAs and their target genes in tobacco (Nicotiana tabacum) roots.

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.

Comparative proteomic analysis of two tobacco (Nicotiana tabacum) genotypes differing in Cd tolerance.

Tobacco can easily accumulate cadmium (Cd) in leaves and thus poses a potential threat to human health. Cd-stress-hydroponic-experiments were performed, and the proteomic and transcriptional features of two contrasting tobacco genotypes Yun-yan2 (Cd-tolerant) and Guiyan1 (Cd-sensitive) were compared. We identified 18 Cd-tolerance-associated proteins in leaves, using 2-dimensional gel electrophoresis coupled with mass spectrometry, whose expression were significantly induced in Yunyan2 leaves but down-regulated/unchanged in Guiyan1, or unchanged in Yunyan2 but down-regulated in Guiyan1 under 50 µM Cd stress. They are including epoxide hydrolase, enoyl-acyl-carrier-protein reductase, NPALDP1, chlorophyll a-b binding protein 25, heat shock protein 70 and 14-3-3 proteins. They categorized as 8 groups of their functions: metabolism, photosynthesis, stress response, signal transduction, protein synthesis, protein processing, transport and cell structure. Furthermore, the expression patterns of three Cd-responsive proteins were validated by quantitative real-time PCR. Our findings provide an insight into proteomic basis for Cd-detoxification in tobacco which offers molecular resource for Cd-tolerance.

Genotypic differences in physiological characteristics in the tolerance to drought and salinity combined stress between Tibetan wild and cultivated barley.

Greenhouse pot experiments were conducted to investigate genotypic differences in response to individual and combined stresses of drought and salinity between Tibetan wild barley genotypes (XZ5, drought-tolerant; XZ16, salinity/aluminum tolerant) and cv. CM72 (salinity-tolerant). Either drought (D) or salinity (S) alone and in combination (D + S) stresses significantly decreased plant growth, chlorophyll content, net photosynthetic rate (Pn), maximal photochemical efficiency of PSII (Fv/Fm), water potential and osmotic potential, with the largest suppression under combined stress, and two wild genotypes showing more tolerance than CM72. Water use efficiency (WUE) increased significantly in XZ5 and XZ16 after D + S, but no significant change in CM72. XZ5 and XZ16 showed 30.9% and 12.1% higher K(+) level and 30.5% and 24.1% lower Na(+)/K(+) ratio in plants, compared with CM72, with increased metal nutrients as Ca, Fe and Mn under D + S. The peak accumulation in proline and glycine-beatine was recorded in combined stress with larger accumulation in two wild genotypes. Moreover, larger increases in the level of ASA and GSH, and the activities of Ca(2+)Mg(2+)-ATPase, and superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), guaiacol peroxidase (POD) and glutathione reductase (GR) under D + S vs control were observed in XZ5 and XZ16 than CM72, with less accumulation of H(2)O(2) and malondialdehyde. These results suggest that high tolerance to D + S stress of XZ5 and XZ16 is closely related to lower Na(+)/K(+) ratio and enhanced Ca(2+)Mg(2+)-ATPase, proline, glycine-beatine and WUE, and improved capacity of antioxidative performance to scavenge reactive oxygen species and thus suppressed level of lipid peroxidation.