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As one of the most toxic heavy metals in the environment, cadmium (Cd) poses a severe threat to plant growth. We previously reported that overexpression of the Tamarix hispida V-ATPase c subunit (ThVHAc1) improved the Cd tolerance of Saccharomyces cerevisiae. In the current study, we further explored the Cd tolerance conferred by ThVHAc1 in Arabidopsis and T. hispida. ThVHAc1 transgenic Arabidopsis had higher seed germination, biomass, and chlorophyll content under CdCl2 treatment. In Cd-stressed plants, overexpression of ThVHAc1 significantly improved V-ATPase activity and affected the expression of other V-ATPase subunit-encoding genes. Intriguingly, the lower level of ROS accumulation in ThVHAc1-overexpressing lines under CdCl2 treatment demonstrated that ThVHAc1 may modulate Cd stress tolerance by regulating ROS homeostasis. Transient expression of ThVHAc1 in T. hispida further confirmed these findings. Furthermore, promoter analysis and yeast one-hybrid assay revealed that the transcription factor ThWRKY7 can specifically bind to the WRKY cis-element in the ThVHAc1 promoter. ThWRKY7 exhibited similar expression patterns as ThVHAc1 under CdCl2 treatment and improved Cd tolerance, suggesting that ThWRKY7 may be an upstream regulatory gene of ThVHAc1. Therefore, our results show that the combination of ThVHAc1 and its upstream regulator could be used to improve Cd stress tolerance in woody plants.

Root tissue is the primary site of perception for stress from soil, and is the main tissue involved in stress response. Tamarix hispida is a woody halophyte that is highly tolerant to salt and drought stress, but little information available about gene expression in roots in response to abiotic stress. In this study, eight transcriptomes from roots of T. hispida treated with NaHCO for 0, 12, 24 and 48 h (two biological replicates were set at each time point) were built. In total, 47,324 unigenes were generated, and 6,267 differentially expressed genes (DEGs) were identified. There were 2,510, 3,690, and 2,636 genes significantly differentially expressed after stress for 12, 24 and 48 h, respectively. Co-expressed DEGs were clustered into ten classes (P < 0.001). Gene ontology enrichment analysis showed that 13 pathways were highly enriched, such as signal transduction, cell wall, phosphatase activity, and lipid kinase activity, suggesting that these pathways play important roles in the saline-alkaline response. Furthermore, the genes involved in lignin metabolic processes and biosynthesis of proline and trehalose are found closely involved in NaHCO stress response. This systematic analysis may provide an in-depth view of stress tolerance mechanisms in T. hispida.

The MYB transcription factors (TFs) is a plant TF families, which involves in hormone signal transduction, and abiotic stress tolerance, etc. However, there are few studies on the MYB TFs family and its regulatory mechanism in Tamarix hispida. In this study, 14 MYB genes (named ThMYB1 - ThMYB14) were cloned and characterized from T. hispida. The transcription profiles of ThMYBs in T. hispida under different abiotic stress conditions were monitored using qRT-PCR. Most of studied ThMYBs were significantly downregulated and/or upregulated by salt and osmotic stress, ABA, GA3 and JA treatments in at least one organ. Especially, ThMYB13 was induced in the leaves and roots of T. hispida when exposed to NaCl treatment at all study periods, indicating that it may involve in salt stress. To further study ThMYB13 function, ThMYB13 overexpression and knock-down plants and control plants transformed with an empty pROKII were obtained using a transient transformation system. Overexpression of ThMYB13 in T. hispida displayed the lowest O2-, H2O2 and MDA accumulation, minimal cell death, the most stable K+/Na+ ratio and the lowest electrolyte leakage rate among the three kinds of transient expression in T. hispida. Conversely, the RNAi-silencing, transiently transformed plants displayed the opposite physiological changes. Therefore, ThMYB13 might play a role in salt stress tolerance in transgenic T. hispida plants.

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