Mercuric-Ion-Induced Gene Expression in Arabidopsis thaliana
Fourteen ecotypes of Arabidopsis thaliana were grown on agar, containing up to 40 μ M HgCl 2 . Two groups could be identified: (1) sensitive ecotypes growing only up to 30 μ M and (2) tolerant ecotypes growing at 40 μ M . The tolerant ecotypes accumulated up to 100 mg mercury/kg fresh weight in their rosettes. Arabidopsis thaliana wild-type Columbia (Col-wt) was analyzed in more detail and showed an exponential mercury accumulation with increasing Hg 2+ -concentrations in the medium. Increasing Hg 2+ -concentrations resulted in a delayed growth of the plants. The roundish shape of the leaves changed to a more elongated form, and the leaf color changed from dark green to bright green. Hg 2+ -induced transcripts of several genes known to be induced by oxidative stress were measured and compared with the induction by ozone and UV-B. Glutathione- S -transferase (GST1) and several pathogenesis-related protein (PR) mRNAs were not induced in plants grown on Hg 2+ -containing medium. However, ozone fumigation increased the level of all these transcripts transiently. Similarly, UV-B irradiation resulted only in a weak mRNA increase. Metallothionein 1a (MT1a) transcript accumulation increased with rising Hg 2+ -concentrations, whereas MT2a mRNA level was unchanged. In contrast, MT2a transcripts were induced by ozone, whereas MT1a transcripts were unaffected. UV-B irradiation had a weak effect on MT1a and no effect on MT2a transcripts. These results indicate that different abiotic stressors led to differential transcript changes of Arabidopsis thaliana.
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