Target of rapamycin (TOR) is an evolutionarily conserved serine/threonine kinase that plays a central role in regulating biological growth, development, and stress responses in eukaryotes. However, the TOR signaling pathway has not been thoroughly studied in apple (Malus domestica). Here, through sequence alignment with homologous genes in Arabidopsis thaliana, 14 conserved members of the TOR signaling pathway, encoded by 28 sequences, were identified in the apple genome. A comprehensive analysis of these members was subsequently performed by integrating their structural features, phylogenetic relationships, and expression profiles under low-nitrogen stress conditions. The results showed that the functional motifs of these members are highly conserved across species, while there are significant differences in the physicochemical properties of each member in apple. Subcellular localization predictions indicated that most members are likely localized to the nucleus; a few may reside in the cytoplasm or chloroplast. Quantitative PCR analysis showed that TOR pathway members are differentially regulated under low-nitrogen stress, suggesting their potential involvement in nitrogen stress response. Furthermore, MdTORs were found to directly interact with several autophagy-related (ATG) proteins in apple plants, in addition to its canonical target ATG13. Collectively, this study systematically characterizes the components of the TOR pathway in the apple genome, examines their expression dynamics under low nitrogen stress, and identifies novel interactions between TOR and ATGs. These research findings broaden our understanding of TOR-regulated autophagic pathways, provide a valuable foundation for future studies into their regulatory mechanisms, and also provide data support for clarifying the responses of TOR signaling pathway members in apple to low-nitrogen stress.
Liu et al. (Sun,) studied this question.