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Epigenetics-In the wake of the double helix

Epigenetics-In the wake of the double helix - Epigenetic...

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Epigenetic mechanisms in plants and their implications in plant breeding A THANASIOS S. T SAFTARIS 1, 2 , A LEXIOS N. P OLIDOROS 1 , R ACHEL K OUMPROGLOU 1 , E LENI T ANI 1 , N IVEC K OVACEVIC 2, 3 AND E LENI A BATZIDOU 2 1 Institute of Agrobiotechnology, CERTH, 570 01 Thermi, Greece 2 Department of Genetics and Plant Breeding, AUTh, 540 06 Thessaloniki, Greece 3 Present address: Department of Agronomy, University of Wisconsin-Madison, 1575 Linden Drive, Madison, WI 53706, USA Abstract Higher organisms, including plants, use three systems to initiate and sustain epige- netic gene regulation: DNA methylation, histone modification, and RNA-interfer- ence. Unraveling the relationships between these epigenetic components has led to surprising and rapidly evolving new concepts, showing how they interact and sta- bilize each other. These interacting systems can regulate expression or silencing of genes, resulting in epigenetically controlled phenotypes that can be meiotically or mitotically heritable. In this review we discuss issues relevant to the involvement of epigenetic inheritance as a source of polymorphism generating useful variation for selecting superior genotypes. The role of methylation in hybrid vigor and stability of performance, and aspects of epigenetic transgene silencing in elite transgenic varieties will also be addressed. Introduction “Epigenesis” is a term coined by Aristotle to indicate that the development of an organism evolves through a series of causal interactions between various compo- nents, in antithesis to the theory favored by other Greek philosophers, particularly Democritus and Leucippus, that the fertilized egg represented a preformed minia- ture model of the organism. Preformation was broadly accepted and maintained its hold for millennia until the advent of the microscope and the discovery of the germ layers that settled the issue in favour of epigenesis in the 18 th century. In the 20 th century, Waddintgon (1942) coined the term “epigenetics” in analogy to epigene- sis, in order to indicate that factors above the genotype were also involved in deter- R. Tuberosa and Ronald L. Phillips (eds.), Proceedings of the International Congress “In the Wake of the Double Helix: From the Green Revolution to the Gene Revolution”, 27-31 May 2003, Bologna, Italy , ....................... , ©2004 Avenue media, Bologna.
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158 2 mining a certain phenotype during development. The modern use of the term “epi- genetics” defines all meiotically and mitotically heritable changes in gene expression that are not coded in the DNA sequence itself. Higher organisms, including plants, use three systems to initiate and sustain epi- genetic gene regulation: 1) DNA methylation that historically was the earliest her- itable epigenetic mark to be studied, 2) histone modification, and 3) RNA-inter- ference. While in DNA methylation cytosine methylation is the sole epigenetic mark, histone modifications are numerous involving acetylation, methylation, phosphorylation, ribosylation, ubiquitation of mostly H 3 histone, but also H 4 , H 1, H 2 A as well. These constitute the “histone code” of chromatin epigenetic marks.
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