Genome-Wide Identification and Expression Analysis of WRKY Transcription Factor Genes in Passion Fruit (Passiflora edulis)

The WRKY gene family is an important class of transcription factors in higher plants that play key roles in secondary metabolism, phytohormone signaling, plant defense responses, and abiotic stress responses. The WRKY gene family has not been systematically studied in Passiflora edulis, a tropical fruit with edible and medicinal values. In this study, we performed a genome-wide analysis of passion fruit and identified 58 candidate PeWRKY genes distributed unevenly on nine chromosomes of passion fruit. Phylogenetic and gene structure analyses showed that members of the PeWRKY gene family could be categorized into three groups: class I (9), class II (40), and class III (9). Promoter and target gene prediction analyses indicated that the PeWRKY gene may be involved in various biological processes, including growth and development, metabolism, hormones, and stress responses, by regulating multiple target genes. The Ka/Ks ratios of PeWRKY indicated that PeWRKY may have undergone strong purification selection in the passion fruit genome. Tissue expression analysis showed that the PeWRKY gene was expressed in different tissues of Passiflora edulis, where the number and expression of the PeWRKY gene were more significant in the roots. Quantitative real-time polymerase chain reaction (qRT-PCR) showed that the relative expression of all 10 candidate PeWRKY genes was significantly up-regulated in leaf tissues after 48 h of drought stress compared with the control, and only PeWRKY55 and PeWRKY30 genes were down-regulated. In addition, at the time of biotic stress treatment up to 24 h, most PeWRKY genes were up-regulated after treatment, except the PeWRKY40 gene, which showed down-regulation. Three genes, PeWRKY02, PeWRKY30 and PeWRKY58, showed significant up-regulation of their expression after treatment up to 48 h. This study provides a valuable reference for the functional characterization of WRKY genes in passion fruit and other plants.