• Finding that overexpression of MdVQ37 reduced the salt stress tolerance of transgenic apple plants. • Finding that the growth limitation, ROS production, reduced photosynthetic capacity, and imbalance of ion homeostasis caused by salt stress were aggravated in MdVQ37 -overexpressing transgenic apple lines compared with WT plants. • Finding that compared with WT plants, MdVQ37 -overexpressing transgenic apple lines showed significantly lower transcript levels of genes associated with ion homeostasis and encoding stress-related transcription factors under salt stress. Soil salinity is a major environmental threat to plant growth, development, and yield. Valine-glutamine (VQ) motif-containing proteins (VQ proteins) are involved in plant growth, development, and responses to abiotic and biotic stressors. However, the relationship between salt stress and VQ proteins has not been studied in horticultural crops. In this study, overexpression of the apple VQ family gene MdVQ37 markedly reduced the salt stress tolerance of transgenic apple plants. Physiological and biochemical assays suggested that the growth limitation, reactive oxygen species production, reduced photosynthetic capacity, and imbalance of ion homeostasis caused by salt stress were aggravated in transgenic lines compared with wild-type (WT) plants. In addition, compared with WT plants, MdVQ37 -overexpressing transgenic apple lines showed significantly lower transcript levels of genes associated with ion homeostasis and encoding stress-related transcription factors under salt stress. In summary, we propose that MdVQ37 plays a negative role in the salt stress response of apple, primarily by affecting reactive oxygen species elimination, ion homeostasis, and expression levels of stress-related transcription factors. [ABSTRACT FROM AUTHOR]