Overexpression of a Malus xiaojinensis WRKY transcription factor gene (MxWRKY55) increased iron and high salinity stress tolerance in Arabidopsis thaliana

Apple, as one of the most important economic forest tree species, is widely grown in the world. Abiotic stresses, such as high salinity and iron deficiency, affect apple growth and development. WRKY transcription factors (TFs) are widely involved in the responses of plants to adverse stresses. In the present study, a new WRKY gene was isolated from Malus xiaojinensis and designated as MxWRKY55. Subcellular localization showed that MxWRKY55 was localized to the nucleus. The expression level of MxWRKY55 was highly affected by salt, low-Fe, and high-Fe stresses in M. xiaojinensis seedlings. When MxWRKY55 was introduced into Arabidopsis thaliana, it greatly enhanced the salt and Fe tolerance. When dealt with high- and low-Fe stresses, the overexpression of MxWRKY55 in transgenic A. thaliana resulted in higher levels of root length and fresh weight as well as contents of chlorophyll and Fe than wild type (WT). Increased expression of MxWRKY55 in transformed A. thaliana also contributed to higher contents of chlorophyll and proline and higher activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), while malondialdehyde (MDA) content was lower, especially in response to salt stress. Therefore, these results show that MxWRKY55 plays a positive role in the process of plant resistance to salt, low-Fe, and high-Fe stresses.