Heterologous expression of a Fraxinus velutina SnRK2 gene in Arabidopsis increases salt tolerance by modifying root development and ion homeostasis

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Abstract
Key message FvSnRK2182 is involved in regulating the growth and stress response. Abstract SnRK2 family members are positive regulators of downstream signals in the abscisic acid (ABA) signaling pathway, playing key roles in the plant responses to abiotic stresses. Fraxinus velutina Torr. is a candidate phytoremediator of saline–alkali areas, and is a valuable research subject because of its adaptability in saline soil. We identified a SnRK2 gene in F. velutina (named FvSnRK2182), which was significantly upregulated under salt stress. A bioinformatics analysis showed that FvSnRK2182 has a Ser/Thr kinase domain typical of the SnRK2 subfamily. Compared with wild-type (WT) Arabidopsis, its heterologous expression in Arabidopsis resulted in higher auxin content during seed germination and seedling growth, leading to longer primary roots and more lateral roots. The transgenic lines were better able to tolerate treatments with NaCl (100 mM) and/or ABA (0.2 and 0.5 µM), producing a greater biomass than the WT plants. Under NaCl treatment, the shoots of the transgenic lines had lower Na+ contents and higher K+ contents than the WT plants, and the genes encoding the ion transport–related proteins SOS1, HKT1, NHX1, and AKT1 were significantly upregulated. In addition, the expression of the genes functioning downstream of SnRK2 in the ABA signaling pathway (Rboh, AREB4, ABF2, and ABF3) were significantly upregulated in transgenic lines under NaCl stress. These results showed that expressing FvSnRK2182 in Arabidopsis significantly increased their resistance to ABA and salt stress by regulating root development and maintaining ion homeostasis, which suggests that FvSnRK2182 may be involved in regulating the growth and stress response of F. velutina.

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