[대학원 생명과학과 세미나 안내]
연사 : 최연희 교수(서울대학교 생명과학부)
연제 : Active DNA demethylation during Arabidopsis reproduction
일시 : 2018년 5월 18일 (금) 오후 5시
장소 : 하나과학관 A동 B131호
초청교수 : 김옥매 교수
Abstract
Cytosine methylation is a major epigenetic modification. Its ability to modify DNA accessibility can result in transcriptional silencing of gene and transposons, maintenance of genome integrity, and some other important regulatory functions such as genomic imprinting. DNA methylation is stably maintained during mitotic cell division both in animals and plants. However, during reproduction and differentiation, it is subject to cycles of erasure and resetting in animals. By contrast, such a big wave of DNA methylation change, so called epigenetic reprogramming, does not occur in plant embryos. Rather, they use different strategy to maintain genome integrity in the next generations using unique double fertilization in flowering plants. One sperm cell fuses with an egg cell to form an embryo and the other sperm cell fuses with a central cell to from an endosperm, an extraembrynic nutritive seed tissue. Interestingly, hypomethylation was observed only in the endosperm, not in the embryo. DEMETER (DME) DNA glycosylase initiates cytosine demethylation via base excision DNA repair pathway. Prior to fertilization, DME demethylase is expressed in the companion cells of the egg and sperm cells, central cell and the vegetative cell, respectively. Therefore, it is suggested that extensive maternal allele demethylation in the endosperm is inherited from the central cell and DME might be responsible for it. However, due to the technical inaccessibility of a single central cell being buried within many layers of sporophytic maternal tissues, it has long been a challenge to isolate pure central cell in Arabidopsis. We systematically optimized each step to separate central cells from the female maternal tissues using manual dissection of the pistils, then, protoplasting technique, followed by Isolation of Nuclei TAgged in specific Cell Types (INTACT) technique to purify central cell nuclei, resulting in a purity of 75-90% The yield was sufficient to undertake downstream whole genome-wide DNA methylation analysis. We find that DNA demethylation is indeed initiated in central cells, which requires DEMETER activity in Arabidopsis. On the contrary, CG methylation efficiency is elevated in female gamete cells compared with nonsexual tissues. Our results demonstrate that locus-specific, active DNA demethylation in the central cell is the origin of maternal chromosome hypomethylation in the endosperm.