FUNCTIONAL HOMOLOGS OF FUNGAL METALLOTHIONEIN GENES FROM ARABIDOPSIS

Metallothioneins (MTs) are cysteine-rich proteins required for heavy metal tolerance in animals and fungi. Two cDNAs encoding proteins with homology to animal and fungal MTs have been isolated from Arabidopsis. The genes represented by these cDNAs am referred to as MT1 and MT2. When expressed in an MT-deficient (cup1(Delta)) mutant of yeast, both MT1 and MT2 complemented the cup1(Delta) mutation, providing a high level of resistance to CuSO4 and moderate resistance to CdSO4. Although the MT-deficient yeast was not viable in the presence of either 300 mu M CuSO4 or 5 mu M CdSO4, cells expressing MT1 were able to grow in medium supplemented with 3 mM CuSO4 and 10 mu M CdSO4, and those expressing MT2 grew in the presence of 3 mM CuSO4 and 100 mu M CdSO4. In plants, MT1 mRNA was more abundant in roots and dark-grown seedlings than in leaves. In contrast, MT2 mRNA accumulated more in leaves than in either roots or dark-grown seedlings. MT2 mRNA was strongly induced in seedlings by CuSO4, but only slightly by CdSO4 or ZnSO4. However, MT1 mRNA was induced by CuSO4 in excised leaves that were submerged in medium. These results indicated that Arabidopsis MT genes are involved in copper tolerance. Plants also synthesized metal binding phytochelatins (poly[gamma-glutamylcysteine]glycine) when exposed to heavy metals. The results presented here argue against the hypothesis that phytochelatins are the sole molecules involved in heavy metal tolerance in plants. We conclude that Arabidopsis MT1 and MT2 are functional homologs of yeast MT.