Maintenance of CpG methylation is essential for epigenetic inheritance during plant gametogenesis

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In mammals, the DNA methyltransferase 1 (Dnmt1) faithfully copies the pattern of cytosine methylation at CpG sites to the newly synthesized strand, and this is essential for epigenetic inheritance1. In Arabidopsis thaliana, several DNA methyltransferases or chromatin modifiers coupled to methylation changes have been characterized, and mutations that cause loss of their function are recessive2,3,4,5,6,7. This is surprising because plant gametogenesis includes postmeiotic DNA replication in haploid nuclei before fertilization. Therefore, the recessive character of the mutations excludes the affected components from a regulatory role in postmeiotic maintenance or modification of epigenetic states. Here we show, however, that depletion of A. thaliana MET1, a homolog of mammalian Dnmt1 (ref. 8), results in immense epigenetic diversification of gametes. This diversity seems to be a consequence of passive postmeiotic demethylation, leading to gametes with fully demethylated and hemidemethylated DNA, followed by remethylation of hemimethylated templates once MET1 is again supplied in a zygote.

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Figure 1: Characterization of met1 mutants.
Figure 2: Segregating phenotypes in T2 progeny.
Figure 3: Maintenance of methylation in somatic tissues of met1 heterozygotes and its loss during gametogenesis.

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We thank H. Vaucheret for the A. thaliana line 6b5, O. Fritsch and B. Hohn for the mutant collection, J. Finnegan for the AsMET1 line, E.J. Richards for the 180-bp repeat probe, K. Afsar and S. Lienhard for technical assistance and S. Adams, B. Hohn, U. Grossniklaus, P. King and C. Spillane for comments on the manuscript. This work was supported by grants from the Swiss Federal Office for Education and Science and the European Union to O.M.S.

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Correspondence to Hidetoshi Saze.

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Saze, H., Scheid, O. & Paszkowski, J. Maintenance of CpG methylation is essential for epigenetic inheritance during plant gametogenesis. Nat Genet 34, 65–69 (2003) doi:10.1038/ng1138

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