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It is well documented that the homeodomain-leucine zipper (HD-Zip) transcription factors play diverse roles during plant growth and development. The Class I HD-Zip genes are shown to be involved in the regulation of abiotic stress responses and tolerance. Herein, a novel Class I HD-Zip gene, EsHD1, was isolated from Eutrema salsugineum, and an investigation was performed of its physiological functions in response to drought stress. The analyses of gene expression profiles revealed that the EsHD1 transcripts quickly accumulated upon exposure to various abiotic stress conditions or abscisic acid treatments. Furthermore, the overexpression of EsHD1 in tobacco plants conferred improved tolerance to drought stress. The EsHD1-overexpressing lines had lower levels of reactive oxygen species (ROS), ion leakage, and malondialdehyde, but they manifested higher activities of antioxidant enzymes and the transcription of pathogenic-related genes as compared to wild-type plants under drought stress conditions. Therefore, our findings demonstrated that EsHD1 positively regulated plant drought tolerance via alleviation of ROS-mediated membrane damage.

Salt stress can trigger several physiological responses in plants such as increased accumulation of ABA and antioxidant enzyme activities, which have been closely associated with the tolerance of plants to salt stress. In the present study a novel zeaxanthin epoxidase gene, designated EsABA1, was isolated from Eutrema salsugineum. EsABA1 contained a full-length open reading frame (ORF) of 2001 bp, encoding 666 amino acids. Transcription of EsABA1 rapidly accumulated after exposure to various abiotic stresses, suggesting that the EsABA1 gene was involved in multiple stress responses and tolerance. To further investigate the roles of EsABA1 under salt-stress conditions, transgenic tobacco lines that ectopically overexpressed EsABA1 were obtained. These transgenic lines exhibited more tolerance to salt stress than wild-type (WT) plants, and overexpression of EsABA1 increased the contents of ABA and proline as compared to WT plants under salt-stress conditions. Moreover, the expression levels of stress responsive genes and activities of antioxidant enzymes were much higher in the EsABA1-overexpressing lines than in WT plants. Therefore, our findings indicated that the elevated expression of EsABA1 enhanced the tolerance of transgenic tobacco plants to high salinity stress.