**1. Introduction**

The aquaporins (AQPs) represent a family of integral membrane proteins, and form channels which allow the transport of water and other small molecules across membranes [1]. These proteins are produced by species across the phylogenetic spectrum, from microbes to plants and animals [2]. A typical aquaporin feature six transmembrane (TM1–TM6) helices (H1–H6) and five connecting loops (LA–LE); both their carboxylic and amino terminals lie on the cytoplasmic side, while two half helices formed the seventh TM helix by the opposite LB and LE dipping into the membrane. Given their general conservation across many AQPs, the asparagines-proline-alanine (NPA) motifs, the aromatic/Arginine (ar/R) selectivity filter formed by four residues (F58-H182-C191-R197 in AQP1) [3], and Froger's positions (P1–P5 residues, T116-S196-A200-F212-W213 in AQP1) [4], are considered to be important for function. AQPs are tetrameric proteins and each monomer is functional independently as a channel. Furthermore, the fifth channel, which forms through the middle of tetramer array, has been suggested to conduct gases, like CO<sup>2</sup> [5].

The survival and growth of a plant depends on its ability to maintain a sufficient level of tissue hydration. Proteins referred to as AQPs are known to represent an important component of the regulatory machinery used by plants for this purpose. AQPs have been shown to exert control over germination, since pea seeds imbibed in the presence of the AQP inhibitor mercury do not germinate [6]. The correlation established between the elongation of the *Ricinus communis* seedling hypocotyl and the abundance of the AQP-encoding gene *PIP2-1* has been taken to imply that AQPs also have a role in seedling growth [7]. The product of the tobacco gene *NtAQP1* has been shown to facilitate CO<sup>2</sup> membrane transport, and to contribute both to photosynthesis and stomatal movement [8,9]. The products of the strawberry AQP-encoding genes *FaPIP1;1* and *FaNIP1;1* both appear to be involved in the transport of water into the fruit [10,11]. A number of authors have reported that plant AQPs respond to external stress, triggering physiological adjustments which act to maintain the plant's hydration status [12–16].

Plant genomes encode a substantial number of AQPs: There are 35 such genes in the *Arabidopsis thaliana* genome [17], 72 in the soybean (*Glycine max*) genome [18], and 55 in the poplar (*Populus trichocarpa*) genome [19]. Based on their peptide sequences, higher plant AQPs have been classified into five subfamilies, namely the plasma membrane intrinsic proteins (PIPs), the tonoplast intrinsic proteins (TIPs), the Nod26-like intrinsic proteins (NIPs), the small and basic intrinsic proteins (SIPs), and the uncharacterized intrinsic proteins (XIPs). PIPs, TIPs, NIPs, and SIPs have been found in most higher plants, while to date XIPs have not been identified in *Brassicaceae* and monocots [20,21]. The most abundant of the AQPs are the PIPs and TIPs, most of which are associated with, respectively, the plasma membrane and the vacuolar membrane. Here, the family of apple (*Malus domestica*) AQPs has been characterized at the phylogenetic level, at the level of the chromosomal distribution of their encoding genes and with respect to the content of their functional domains. The effect on drought and salinity tolerance of one of these genes was explored by heterologously expressing it in *A. thaliana*.
