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

Abstract

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.