**Gene-Wide Analysis of Aquaporin Gene Family in** *Malus domestica* **and Heterologous Expression of the Gene** *MpPIP2;1* **Confers Drought and Salinity Tolerance in** *Arabidposis thaliana*

#### **Haili Liu 1,**† **, Leilei Yang 1,**† **, Miaomiao Xin <sup>1</sup> , Fengwang Ma <sup>2</sup> and Jingying Liu 1,\***


Received: 24 June 2019; Accepted: 26 July 2019; Published: 29 July 2019

**Abstract:** The aquaporins (AQPs) are a family of integral membrane proteins involved in the transcellular membrane transport of water and other small molecules. A scan of the apple (*Malus domestica*) genome revealed the presence of 42 genes encoding putative AQPs. Based on a phylogenetic analysis of the deduced peptide sequences of the AQPs generated by *Arabidopsis thaliana*, poplar (*Populus trichocarpa*), and rubber (*Hevea brasiliensis*), the apple AQPs were each assigned membership of the five established AQP subfamilies, namely the PIPs (eleven members), the TIPs (thirteen members), the NIPs (eleven members), the SIPs (five members), and the XIPs (two members). The apple AQPs included asparagine-proline-alanine (NPA) motifs, an aromatic/arginine (ar/R) selectivity filter, and the Froger's positions. The heterologous expression of *MpPIP2;1* in *A. thaliana* was shown to enhance the level of tolerance exhibited against both drought and salinity.

**Keywords:** apple; aquaporin; functional analysis; stress tolerance
