MdINT1 enhances apple salinity tolerance by regulating the antioxidant system, homeostasis of ions, and osmosis

Myo-inositol is a versatile compound and plays a vital role in plant growth and stress tolerance. Previously, we found that exogenous application of myo-inositol enhanced the salinity tolerance in Malus hupehensis Rehd. by enhancing myo-inositol metabolism. In this study, we found that the tonoplast-localized myo-inositol transporter 1 (MdINT1) was involved in myo-inositol accumulation and conferred salinity tolerance in apple. MdINT1 is characterized by the highest transcripts among the four apple INT-like genes and could be induced by salt stress at the transcriptional level. Also, it was shown that myo-inositol level was slightly decreased in the leaves of transgenic apple lines over-expressing MdINT1, but was significantly increased in the leaves and roots of MdINT1 silencing line. Interestingly, overexpression of MdINT1 enhanced salinity tolerance by promoting Na+ and K+ balance, antioxidant activity, and accumulation of osmoprotectants in transgenic apple lines. In contrast, under salinity conditions, the MdINT1-mediated protective roles in the antioxidant activity, homeostasis of ions and osmosis were compromised, which in turn increased the risk of salt intolerance in the MdINT1 silencing line.