A Tobacco NtMET1 cDNA Encoding a DNA Methyltransferase: Molecular Characterization and Abnormal Phenotypes of Transgenic Tobacco Plants

A cDNA encoding a DNA methyltransferase, with a predicted polypeptide of 1556 amino acid residues containing all motifs conserved in this enzyme family, was isolated from tobacco plants, and the corresponding gene was designated as NtMET1. RNA blot analysis indicated NtMET1 transcripts to accumulate in dividing tissues of tobacco plants, and they could be detected during the S phase in synchronized dividing BY2 cells. In situ hybridization revealed the transcripts to be localized exclusively in actively proliferating tissues around axillary apical meristem. In order to ascertain physiological roles, transgenic tobacco plants that had the antisense construct were made and examined for phenotypes. Methylation levels of genomic DNA from transgenic plants significantly decreased in comparison with wild-type levels, and distinct phenotypic changes including small leaves, short internodes and abnormal flower morphology were noted. Microscopic observation revealed that leaf structure differed between transgenic and wild-type plants. These results suggest that NtMET1 functions during DNA replication, and that DNA methylation plays an important role in plant morphogenesis.