15th International Conference on Arabidopsis Research - TAIR

T09-007
Two BRCA2-like genes are needed for homologous
recombination repair in Arabidopsis
Yuichi Ishikawa(1, 2), Kiyomi Abe(1), Keishi Osakabe(1), Masaki Endo(1, 3), Yuji
ito(1), Takashi Kuromori(4), Kazuo Shinozaki(4), Hiroaki Ichikawa(1), Toshiaki
Kameya(2), Seiichi Toki(1)
1-National Institute of Agrobiological Sciences
2-Tohoku University
3-Tsukuba University
4-Riken GSC
The BRCA2 gene was identified as a breast and ovarian cancer susceptibility
gene and has been implicated in the response to DNA damage. The evidence
for a role in DNA repair came from the observation that BRCA2 binds directly
with RAD51, the bacterial RecA homolog, which is required for homologous
pairing and DNA strand exchange during homologous recombination repair.
Furthermore, BRCA2 null mouse embryos that do not survive past day 8 of
embryogenesis are hypersensitive to gamma-irradiation.
Two BRCA2 genes, AtBRCA2L1 and AtBRCA2L2, are present in Arabidopsis
genome (Siaud et al. 2004). Both AtBRCA2L1 and AtBRCA2L2 have four BRC
motifs that interact with Rad51. Yeast two-hybrid assay shows AtBRCA2L1
and AtBRCA2L2 interacts with AtRad51, suggesting that both AtBRCA2L1
and AtBRCA2L2 are involved in homologous recombination repair. The biological
roles of AtBRCA2L1 and AtBRCA2L2 proteins in Arabidopsis were ascertained
by obtaining homozygous mutant plants containing the AtBRCA2L1
or AtBRCA2L2 genomic sequences interrupted by a Ds insertion. Both mutant
plants showed increased sensitivity to gamma-ray radiation, suggesting
that AtBRCA2L1 and AtBRCAL2 have a role in double strand break repair.
We have crossed AtBRCA2L1 and AtBRCA2L2 mutants to produce double
mutants and observed developmental abnormality in double mutant plants.
T09 Genetic Mechanisms (Transcriptional and Chromatin Regulation)
T09-008
An inversion of dominance between epialleles in
polyploid Arabidopsis
Mittelsten Scheid, O.(1), Afsar, K.(2), Paszkowski, J.(3)
1-Gregor Mendel Institute of Molecular Plant Biology, c/o Boku Muthgasse 18, A-1190 Vienna,
Austria
2-Friedrich Miescher Institute for Biomedical Research, Maulbeerstr. 66, CH 4058 Basel, Switzerland
3-Dept. Plant Biology, University of Geneva, 30, Quai Ernest-Ansermet, CH 1211 Geneva 4,
Switzerland
Polyploidy, the presence of more than 2 complete chromosome sets, is frequent
among higher plants, especially in plants cultured for human use. The
success of polyploids may lie in increased genetic redundancy supporting
subsequent diversification. Although doubling the genome does not generate
diversity per se, polyploid formation is associated with rapid genomic rearrangements,
changes in DNA modification and altered gene expression patterns
1.
Several independent tetraploid Arabidopsis thaliana lines, derived from the
same diploid strain uniformly expressing a transgenic locus over many generations,
showed transcriptional silencing of the transgene. These silent states
were stably inherited to tetraploid progeny and diploid derivatives. The loss of
expression after polyploidization appears to be due to epigenetic modification
accompanied by hypermethylation of the silent loci.
Surprisingly, the transcriptionally inactive epialleles reduce the expression of
an active allele if combined in the same tetraploid (but not in a diploid) genome
2. The active allele therefore loses its dominance (with different degrees
of penetrance) and becomes hypermethylated. Moreover, genetic segregation
data indicate that the suppressive effect is lasting even after genetic
separation from the silencing allele, thus resembling paramutation. Therefore,
epigenetic states can be inherited even if the chromosome carrying this information
is not transmitted itself. This leads to functional epigenetic homozygotization
of alleles and, thus, to conversion of new recessive alleles into traits
expressed in early polyploid generations. Such interactions might contribute
to rapid adaptation and evolution of polyploid plants.
1 Osborn et al. (2003) Trends Genet 19:141-147.
2 Mittelsten Scheid et al. (2003) Nat Genet 34: 450-454
15 th <strong>International</strong> <strong>Conference</strong> on Arabidopsis Research 2004 · Berlin