**1. Introduction**

The growth, development, and survival of plants is constantly challenged by a variety of biotic and abiotic environmental factors. Plants utilize complex molecular mechanisms that regulate patterns of gene expression to protect themselves against these stresses [1,2]. Some key modulators of stress responses have been characterized and have emerged as appropriate targets to enhance abiotic stress tolerance in many plants. They include NAC domain-containing transcription factors, DRE/CRT-binding transcription factors (DREBs/CBFs), mitogen-activated protein kinases (MAPKs), stress-associated proteins (SAPs), and heat shock factor/proteins (HSPs/HSF) [1,3–6]. Among these, the SAPs are a newly identified class of zinc finger proteins (ZFPs) that play crucial roles in various abiotic stress responses by numerous plants [1,2,7].

The SAP gene family members have two special ZF domains: the highly conserved A20 domain, which was first isolated in human umbilical vein endothelial cells with the characterization of a tumor necrosis factor (TNF)-α-inducible protein; and/or the AN1 domain, which is also highly conserved and first identified from *Xenopus laevis* animal hemisphere 1 (AN1) maternal RNA with the delineation of the ubiquitin-like protein [8,9]. The SAP proteins expressed in *Arabidopsis thaliana* (hereafter *Arabidopsis*), rice (*Oryza sativa*), tomato (*Solanum lycopersicum*), and cotton (*Gossypium hirsutum*) have been classified into five groups (I through V) based on results from their phylogenetic analyses [10,11]. One significant feature of plant SAPs is the very frequent occurrence of intronless genes [2]. For example, 11 rice SAP genes, 15 from desert poplar (*Populus euphratica*), and 30 from cotton lack introns and show a remarkably higher percentage of intronless genes [2,6,11].

The roles of SAP genes are increasingly being reported in plants. Transcriptional levels are induced by multiple stresses and provide a positive reinforcement of tolerance to abiotic stress. Rice A20/AN1 protein (OSISAP1/OsSAP1), the first identified plant SAP gene, is induced after different types of stress treatments are applied [1]. The overexpression of *OSISAP1* confers tolerance to dehydration, cold, and salt in transgenic seedlings of tobacco (*Nicotiana tabacum*) [1]. Furthermore, *ZFP177* (*OsSAP9*) and *AtSAP5* are induced by numerous challenges and have significant roles in improving abiotic stress tolerance [12,13]. Similar results have been described for SAPs from maize (*Zea mays*) [14], medicago (*Medicago truncatula*) [15], banana (*Musa* sp.) [16], the halophyte grass *Aeluropus littoralis* [17], and poplar (*Populus alba* × *P. glandulosa*) [18]. These genes also function in biotic stress responses. For example, Tyagi et al. [19] have analyzed the expression patterns of rice SAP gene family members in response to pathogen elicitors, and discovered that *OsSAP1*, *OsSAP8*, and *OsSAP11* are up-regulated. Transgenic tobacco overexpressing *OsiSAP1* shows significantly enhanced basal resistance against infection by the bacterial pathogen *Pseudomonas syringae* pv. *Tabaci* [19].

The SAP genes also help regulate signal transduction and phytohormone synthesis. In rice, the overexpression of *OsDOG* (*OsiSAP11*) [20] and *OsZFP185* (*OsiSAP4*) [21] results in dwarf phenotypes, a decrease in gibberellic acid (GA) contents, and deficient cell elongation. Furthermore, *OsZFP185* negatively regulates the expression of several genes related to abscisic acid (ABA) biosynthesis, and interferes with ABA-mediated tolerance to salt, drought, and cold [21]. Various SAPs can function as E3 ubiquitin ligases, redox sensors, and/or regulators of gene expression under stress [7,13,22,23]. Other novel biological functions for SAP genes will continue to be reported.

Based on their highly conserved A20/AN1 domains, members of the SAP gene family have been identified and characterized in *Arabidopsis* [24], rice [24], maize [14], tomato [10], cotton [11], desert poplar [6], and medicago [25]. Although extensive genomic analyses have provided considerable details about this family in several species, members in apple (*Malus* × *domestica* Borkh.) have not been as thoroughly investigated. Nevertheless, recent completion of the draft genome sequence for apple has enabled genome-wide analyses of its SAP genes [26–28]. Here, we identified SAP members in apple and examined their A20/AN1 domain, protein and gene structures, conserved domains, phylogenetic relationships, chromosomal locations, *cis*-acting elements, and expression patterns for *MdSAPs* cloned in response to water deficits. We also overexpressed *MdSAP10* in *Arabidopsis* and investigated its function. Our results will serve as a basis for exploring the molecular roles of SAPs. By facilitating further studies into their functions in abiotic stress responses, we can continue our efforts to introduce improved apple cultivars.
