**1. Introduction**

As a major component of the primary cell walls of plants [1], pectins are essential for remodeling cell wall and normal cell-cell adhesion during cellular growth [2–5]. D-galacturonic acid (GalA) is the constituent of the capsular polysaccharides and lipopolysaccharides of several bacterial species [6]. In plants, GalA residues, which are the precursor of pectin formation, are contained in the backbone of all pectin polymers [7]. UDP-D-galacturonic acid (UDP-GalA), which is the activated nucleotide sugar form of GalA, is required in the synthesis of GalA-containing polymers. UDP is the abbreviation of uridine diphosphate and it is a nucleotide diphosphate that is made up of a pyrophosphate group, a pentose ribose, and a nucleated base uracil. UDP-GalA is synthesized via 4-epimerization of UDP-D-glucuronic acid (UDP-GlcA), which is a nucleotide sugar that is formed by the reputed inositol oxygenation pathway [8] or by the dehydrogenation of UDP-D-glucose (UDP-Glc) in the upstream [9]. Therefore, enzymes that are related to the formation of UDP-GalA and UDP-GlcA are likely to play critical roles in pectin biosynthesis [7,10].

UDP-D-glucuronic acid 4-epimerase (UGlcAE) is capable of reversibly converting UDP-GlcA and UDP-GalA [6,11]. According to previous literatures [12–15], UDP-D-glucuronic acid 4-epimerase is another name of UDP-D-glucuronate 4-epimerase. Both GAE and UGlcAE are the abbreviations of UDP-D-glucuronic acid 4-epimerase. The abbreviation is unified as UGlcAE in this study.

The UGlcAE, which is a specific membrane-bound 4-epimerase [13], is considered to evolve from some chlamydial bacteria [15]. The UGlcAE is also recognized as a key enzyme in regulating pectin biosynthesis due to its function. In 1958, the isolation of the epimerase was firstly reported [16], and subsequently it was also isolated from *Cyanobacterium anabaena* flos-aquae [17,18] and plants [10,19–22].

Although its function is believed to interconvert UDP-GlcA and UDP-GalA, the UGlcAEs from different organisms have distinct biochemical properties. For example, some UGlcAEs were substrate specific [6,11–14], while the others displayed substrate promiscuity [23]. In addition, UGlcAEs in *Poaceae* species differed from homologs in *Arabidopsis* [13,14], and different UGlcAEs in the same species also shows biochemical properties that varied differentially. For example, UGlcAE3 in *Ornithogalum caudatum* could catalyse the reversible conversion of UDP-GalA and UDP-GlcA; however, OcUGlcAE1 and OcUGlcAE2 did not have this activity [24].

The evolutionary relationship of the *UGlcAE* gene family in plants is not clear, and we hardly know anything about this gene family in tomato (*Solanum lycopersicum*). Tomato, a berry fruit, is considered to be an important economically vegetable worldwide due to its good quality and high yield [25]. With the completion of the whole genome sequencing of tomato [26], it promotes the genome-wide identification of gene families and functional analysis in tomato [27]. Therefore, studies of the *UGlcAE* gene family in tomato could develop potential strategies for improving *Solanum*-related crops genetically and stimulate new research directions, and considering the potentially important functions of the UGlcAE proteins can expand our knowledge of tomato UGlcAE isoforms. In this study, we identified and characterized the tomato *UGlcAE* gene family on a genome-wide scale, and the tissue- and organ-specific expression of *UGlcAE* family under the normal conditions and in response to three hormone treatments were analyzed according to *cis*-acting elements analysis. Analysis of this family not only identifies its members and characteristics in tomato, but it also lays a foundation for future functional analyses of *UGlcAE* genes.
