Genome-Wide Identification of Pitaya (Hylocereus polyrhizus) TCPs and their Involvement in Flower Development and Abiotic Stress Response

Pitaya is a widely cultivated crop in Southeast Asia with excellent performance against drought stress. The fruits are of great economic value, which draws increasing attention and promotes the plantation extension in China. Presently, a genome-wide identification of the TCP genes was conducted in pitaya. Their physiochemical properties, genome collinearity, phylogenetic relationship, biological functions, cis-acting elements and expression pattern were analyzed. The results showed that pitaya genome codes a total of 24 TCP transcription factors, among which 12 belong to Class I and 12 to Class II. Phylogenetic and collinearity analysis revealed that HpTCPs are phylogenetically close to those of Beta vulgaris; the expansion of pitaya TCPs may be attributed to the segmental and dispersed duplications occurred after the divergence of dicots and monocots. Moreover, the Ka/Ks ratio suggested that HpTCPs are under purifying selection. Gene ontology analysis and subcellular localization prediction indicated that most HpTCPs are located in nucleus involving in the transactivation of downstream genes, which was further verified through transient tobacco transformation experiments and modified yeast two-hybrid assays, performed on randomly selected genes, HpTCP10 and HpTCP24. Investigation of HpTCP promoters detected an abundance of phytohormone- and stress-responsive elements. Transcriptomic data indicated that HpTCPs, especially the Class II members, were predominantly expressed in floral buds and flowers, suggesting a significant association with flower development. Using RT-qPCR, the response to low-temperature and drought stresses was revealed for HpTCPs, among which HpTCP11 and HpTCP23 displayed the most robust up-regulation, indicating their positive involvement in the stress response of pitaya and therefore might be targets for genetic improvement of pitaya.