Ectopic expression of a Malus hupehensis Rehd. myo- inositol oxygenase gene (MhMIOX2) enhances tolerance to salt stress

Myo-inositol oxygenase (MIOX), an essential monooxygenase enzyme, catalyzes the conversion of myo-inositol (MI) into D-glucuronic acid (D-GlcUA). The MIOX genes have been isolated and studied extensively in model plants and crops, but the characteristics and roles of MIOXs in woody plants remain largely unknown. In this study, two MhMIOXs cloned from Malus hupehensis Rehd. were characterized. Both MhMIOX1 and MhMIOX2 expression were significantly induced throughout the whole ABA and NaCl stress treatment, especially under NaCl treatment. To gain insight into the roles of MhMIOX1/2 in MI metabolism, we heterologously expressed MhMIOX1/2 in poplar and Arabidopsis. The results showed that only MhMIOX2 overexpression caused the decrease of MI content in transgenic plants. Either the transgenic poplar or Arabidopsis lines overexpressing MhMIOX2 showed better growth performance than the wild type (WT) upon salt stress treatment. Furthermore, compared to the WT, the H2O2 and MDA content obviously decreased, and the activity of ROS-scavenging enzymes (POD, APX and CAT) was strongly elevated in transgenic poplar under salt stress conditions, which suggest MhMIOX2 overexpression enhancs the tolerance of the transgenic plants to salt stress. Besides, there was no obvious difference in ascorbate (AsA) content between the WT and transgenic poplar under normal condition, indicating MhMIOX2 is important for MI oxidative metabolism, but not for AsA production in poplar and Arabidopsis.

Automated summary

In this study, two MhMIOXs were cloned from Malus hupehensis Rehd. and their expression was significantly induced under NaCl stress. Heterologous expression of MhMIOX1/2 in poplar and Arabidopsis showed that MhMIOX2 overexpression decreased MI content and improved salt stress tolerance. Overexpression of MhMIOX2 also led to decreased H2O2 and MDA content, as well as elevated activity of ROS-scavenging enzymes in transgenic poplar under salt stress conditions.