**1. Introduction**

Longan (*Dimocarpuslongan* Lour.) is an important subtropical fruit tree in the family Sapindaceae, which is grown in many subtropical and tropical countries with most of the production in Southeast

Asia and Australia [1]. Biennial bearing is the most serious problem that affects longan fruit products. Among the factors that affect *D. longan* fruit yield, the difficulty and unstableness to blossom is one of the most challenging problems [2]. Floral bud induction of *D. longan* requires favorable conditions such as a period of low temperature (vernalization), suitable salinity, and dry conditions. To obtain a stable high yield, off-season flowering in longan is achieved by chemical treatment with potassium chlorate (KClO3) application [3,4]. Nevertheless, the induction effect varies in different regions and varieties. Therefore, the study of the molecular regulatory mechanisms of flower induction and abiotic stress tolerance in longan is particularly important for understanding and solving the problems associated with fruit yield. However, due to the long generation time and lack of genome information, knowledge of the molecular regulatory mechanisms of flower induction and abiotic stress tolerance in longan is scarce.

As an important developmental process in the plant life cycle, flowering is directly linked to production whenever seeds or fruits are harvested [5]. The molecular and genetic bases of flowering have been well studied in *Arabidopsis thaliana* [6–8]. There are at least five major flowering pathways in *Arabidopsis*, including the photoperiod, autonomous, vernalization, gibberellin (GA), and aging pathways [9]. These pathways activate or inhibit floral transformation through a series of flower integron genes, such as the flowering locus T (*FT*), flowering locus C (*FLC*), and constans (*CO*) [10]. In addition, several transcription factors (TFs), such as MADS-domain TFs [11], NACs [12], MYBs [13], and DREBs [14], participate in the signaling of flowering regulation. As the seventh largest TF family in flowering plants, many WRKY genes are also involved in the determination of flowering time [15]. For example, in *A*. *thaliana*, the lines over-express *GsWRKY20*, *MlWRKY12*, and *WRKY71* in the flowers earlier than in the wild-type [16–18]. A recent research study found that two WRKY proteins (AtWRKY12 and AtWRKY13) played opposite functions in controlling the flowering time under short-day conditions in *A*. *thaliana* partly through mediating the effect of GA3. The *wrky*12 mutant exhibits late flowering and the *wrky*13 mutant shows earlier flowering than that of the wild-type [19].

Abiotic stresses such as drought, heat, salt, and cold are the major causes of declined crop productivity worldwide. At the molecular level, several TFs, such as AP2/EREBP, NAC, WRKY, bZIP, MYB, and bHLH play a vital role in regulating downstream genes to protect plants from these stresses [20]. As one of the largest TF families in plants, the WRKY TFs also play pivotal roles in regulating many abiotic stress reactions [15]. In *Arabidopsis*, some of the *AtWRKYs* respond strongly to various abiotic stresses, such as salinity, drought, and cold [21–24]. In rice, 11 *OsWRKY* genes showed variable responses to salt, polyethylene glycol (PEG), and cold or heat stresses [25]. Overexpression of *OsWRKY47* increased both the drought tolerance and yield compared with wild-type plants [26]. In mulberry, *Morus013217* and *Morus002784* show high accumulation in response to cold and salt stresses. *Morus005757* shows significant up-regulation in response to dehydration stress, salinity stress, and SA and ABA (Abscisic acid) treatments [27]. Similar results were also found in wheat, common bean [28], grape [29], pineapple [30], soybean [31], moso bamboo [32], *Caragana intermedia* [33], peanut [34], and broomcorn millet [35]. These observations suggest that studying the WRKY gene families may provide valuable insights into the mechanism underlying abiotic stress tolerance in plants. As perennials growing in the subtropical and tropical area, some abiotic stresses, such as drought, heat, salt, and cold often have an adverse effect on the growth and yield of longan. However, given the lack of genome information, the identified and functions of *WRKY* genes in longan are still unknown.

In the present study, we performed a genome-wide identification of WRKY TFs in longan and analyzed their gene structures, conserved motifs, and expression patterns in nine different tissues. This work also determined the expression profiles of longan WRKY (*DlWRKY*) in three flowering stages of two longan cultivars and measured their transcript abundance in response to different phytohormone treatments and various abiotic stresses. This study provides a basis for future studies on *DlWRKY* gene family evolution and function.
