Genome-wide identification, molecular evolution, and functional characterization of fructokinase gene family in apple reveal its role in improving salinity tolerance

Fructokinase (FRK) is a regulator of fructose signaling in plants and gateway proteins that catalyze the initial step in fructose metabolism through phosphorylation. Our previous study demonstrated that MdFRK2 protein exhibit not only high affinity for fructose, but also high enzymatic activity due to sorbitol. However, genome-wide identification of the MdFRK gene family and their evolutionary dynamics in apple are yet to be reported. A systematic genome-wide analysis in this study identified a total of nine MdFRK gene members, which could phylogenetically be clustered into seven groups. Chromosomal location and synteny analysis of MdFRKs revealed that their expansion in the apple genome is primarily driven by tandem and segmental duplication events. Divergent expression patterns of MdFRKs were observed in four source-sink tissues and at five different apple fruit developmental stages, which suggested their potential crucial roles in the apple fruit development and sugar accumulation. Reverse transcription-quantitative PCR (RT-qPCR) identified candidate NaCl or drought stress responsive MdFRKs , and transgenic apple plants overexpressing MdFRK2 exhibited considerably enhanced salinity tolerance. Our results will be useful for understanding the functions of MdFRKs in the regulation of apple fruit development and salt stress response.