Opening the Door to Epigenetics in PCP

The term “epigenetics” was coined in 1942 by the Britishdevelopmental biologist Conrad H. Waddington and is derivedfrom a combination of “epigenesis” and “genetics”.Waddington dened the term of epigenetics as “the branchof biology that studies the causal interactions between genesand their products, which bring the phenotype into being”(Waddington 1942). He believed that the biological disciplinesof development and genetics should be integrated, despite thefact that these elds were at the time considered to be totallydistinct branches of biology. Waddington had the foresight topredict that fertilized zygotes developed into differentiatedcells, tissues and individuals according to the information pro-vided by their genes, cell-cell communications, and also envir-onmental cues, by using the metaphorical illustration of anepigenetic landscape (Goldberg et al. 2007). Thus during thecourse of development of an organism, cell fate is determinednot only by genes but also by other (epigenetic) factors, whichunderlies the notion of “epigenesis”. A number of epigeneticfactors have now been identied resulting in a more moderndenition of “epigenetics” as heritable changes in gene expres-sion or cellular phenotypes caused by mechanisms other thanchanges in DNA sequence.For over 60 years, plant researchers performing classicalgenetic studies in maize and more modern molecular tech-niques in Arabidopsis have signicantly contributed to thediscovery of “epigenetic” phenomena, such as paramutation(Brink1958),imprinting(Kermicle1970),controloftransposonactivity (McClintock 1984), and gene silencing (e.g. seeBaulcombe 2004; Matzke et al. 2009), long before their under-lyingmolecularmechanismswereevenknown.Thesephenom-ena were considered exceptions to Mendelian genetics sincethe outcomes of genetic segregation often appeared to contestMendel’s predicted laws of inheritance. However, recent dis-coveries of the underlying molecular pathways have alteredthe original perception of these epigenetic phenomena asa layer on top of Mendelian genetics, which plays a majorrole in the control of gene expression. While many aspects ofthese molecular epigenetic controls, namely DNA methylation,histone modications, small RNAs and non-coding RNAs, arecommon to both mammals and plants; plant researchers havecontributedgreatlytothediscoveryofsuchmechanisms.Morerecent studies in Arabidopsis thaliana have been key to reveal-ing the molecular nature of some of these components, whichhave proven to be remarkably relevant to mammals and otherorganisms. Such factors include the chromatin remodelingfactor DDM1, the CG DNA methylation maintenance factorVIM1/UHRF1, the base-excision DNA demethylase DEMETER,and silencing components via small RNAs (Law and Jacobsen2010).