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

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 inheritance(1). 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 recessive(2-7). 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.