OsMT1a, a type 1 metallothionein, plays the pivotal role in zinc homeostasis and drought tolerance in rice

Metallothioneins (MTs) are small, cysteine-rich, metal-binding proteins that may be involved in metal homeostasis and detoxification in both plants and animals. OsMT1a, encoding a type 1 metallothionein, was isolated via suppression subtractive hybridization from Brazilian upland rice (Oryza sativa L. cv. Iapar 9). Expression analysis revealed that OsMT1a predominantly expressed in the roots, and was induced by dehydration. Interestingly, the OsMT1a expression was also induced specifically by Zn2+ treatment. Both transgenic plants and yeasts harboring OsMT1a accumulated more Zn2+ than wild type controls, suggesting OsMT1a is most likely to be involved in zinc homeostasis. Transgenic rice plants overexpressing OsMT1a demonstrated enhanced tolerance to drought. The examination of antioxidant enzyme activities demonstrated that catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX) were significantly elevated in transgenic plants. Furthermore, the transcripts of several Zn2+-induced CCCH zinc finger transcription factors accumulated in OsMT1a transgenic plants, suggesting that OsMT1a not only participates directly in ROS scavenging pathway but also regulates expression of the zinc finger transcription factors via the alteration of Zn2+ homeostasis, which leads to improved plant stress tolerance.