Characterization of an Arabidopsis thaliana DNA hypomethylation mutant

We have recently isolated two Arabidopsis thaliana DNA hypomethylation mutations, identifying the DDM1 locus, that cause a 70% reduction in genomic 5-methylcytosine levels [1]. Here we describe further phenotypic and biochemical characterization of the ddm1 mutants. ddm1/ddm1 homozygotes exhibited altered leaf shape, increased cauline leaf number, and a delay in the onset of flowering when compared to non-mutant siblings in a segregating population. Our biochemical characterization investigated two possible mechanisms for DNA hypomethylation. In order to see if ddm1 mutations affect DNA methyltransferase function, we compared DNA methyltransferase activities in extracts from wild-type and ddm1 mutant tissues. The ddm1 mutant extracts had as much DNA methyltransferase activity as that of the wild-type for both the CpI and CpNpG substrates suggesting that the DDM1 locus does not encode a DNA methyltransferase. Moreover, the ddm1 mutations did not affect the intracellular level of S-adenosylmethionine, the methyl group donor for DNA methylation. The possibility that the DDM1 gene product functions as a modifier of DNA methylation is discussed.