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GENETICS, GENOMICS, AND MOLECULAR EVOLUTION

Regulation of Seed Size by Hypomethylation of Maternal and Paternal Genomes¹

Department of Plant and Microbial Biology, University of California, Berkeley, California 94720 (W.X., R.L.F.); Department of Biology, University of Louisiana, Lafayette, Louisiana 70504 (R.C.B., B.E.L.); Section of Plant Biology, Division of Biological Sciences, University of California, Davis, California 95616 (J.J.H.); and Department of Molecular, Cell, and Developmental Biology, University of California, Los Angeles, California 90095 (R.B.G.)

DNA methylation is an epigenetic modification of cytosine that is important for silencing gene transcription and transposons, gene imprinting, development, and seed viability. DNA METHYLTRANSFERASE1 (MET1) is the primary maintenance DNA methyltransferase in Arabidopsis (Arabidopsis thaliana). Reciprocal crosses between antisense MET1 transgenic and wild-type plants show that DNA hypomethylation has a parent-of-origin effect on seed size. However, due to the dominant nature of the antisense MET1 transgene, the parent with a hypomethylated genome, its gametophyte, and both the maternal and paternal genomes of the F₁ seed become hypomethylated. Thus, the distinct role played by hypomethylation at each generation is not known. To address this issue, we examined F₁ seed from reciprocal crosses using a loss-of-function recessive null allele, met1-6. Crosses between wild-type and homozygous met1-6 parents show that hypomethylated maternal and paternal genomes result in significantly larger and smaller F₁ seeds, respectively. Our analysis of crosses between wild-type and heterozygous MET1/met1-6 parents revealed that hypomethylation in the female or male gametophytic generation was sufficient to influence F₁ seed size. A recessive mutation in another gene that dramatically reduces DNA methylation, DECREASE IN DNA METHYLATION1, also causes parent-of-origin effects on F₁ seed size. By contrast, recessive mutations in genes that regulate a smaller subset of DNA methylation (CHROMOMETHYLASE3 and DOMAINS REARRANGED METHYLTRANSFERASES1 and 2) had little effect on seed size. Collectively, these results show that maternal and paternal genomes play distinct roles in the regulation of seed size in Arabidopsis.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Robert L. Fischer (rfischer{at}berkeley.edu).

www.plantphysiol.org/cgi/doi/10.1104/pp.106.088849

^* Corresponding author; e-mail rfischer{at}berkeley.edu; fax 510–642–4995.

Received August 25, 2006; accepted September 19, 2006; published September 29, 2006.

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