Cohesin gene defects may impair sister chromatid alignment and genome stability in Arabidopsis thaliana

Abstract

In contrast to yeast, plant interphase nuclei often display incomplete alignment (cohesion) along sister chromatid arms. Sister chromatid cohesion mediated by the multi-subunit cohesin complex is essential for correct chromosome segregation during nuclear divisions and for DNA recombination repair. The cohesin complex consists of the conserved proteins SMC1, SMC3, SCC3, and an alpha-kleisin subunit. Viable homozygous mutants could be selected for the Arabidopsis thaliana alpha-kleisins SYN1, SYN2, and SYN4, which can partially compensate each other. For the kleisin SYN3 and for the single-copy genes SMC1, SMC3, and SCC3, only heterozygous mutants were obtained that displayed between 77% and 97% of the wild-type transcript level. Compared to wild-type nuclei, sister chromatid alignment was significantly decreased along arms in 4C nuclei of the homozygous syn1 and syn4 and even of the heterozygous smc1, smc3, scc3, and syn3 mutants. Knocking out SYN1 and SYN4 additionally impaired sister centromere cohesion. Homozygous mutants of SWITCH1 (required for meiotic sister chromatid alignment) displayed sterility and decreased sister arm alignment. For the cohesin loading complex subunit SCC2, only heterozygous mutants affecting sister centromere alignment were obtained. Defects of the alpha-kleisin SYN4, which impair sister chromatid alignment in 4C differentiated nuclei, do apparently not disturb alignment during prometaphase nor cause aneuploidy in meristematic cells. The syn2, 3, 4 scc3 and swi1 mutants display a high frequency of anaphases with bridges (~10% to >20% compared to 2.6% in wild type). Our results suggest that (a) already a slight reduction of the average transcript level in heterozygous cohesin mutants may cause perturbation of cohesion, at least in some leaf cells at distinct loci; (b) the decreased sister chromatid alignment in cohesin mutants can obviously not fully be compensated by other cohesion mechanisms such as DNA concatenation; (c) some cohesin genes, in addition to cohesion, might have further essential functions (e.g., for genome stability, apparently by facilitating correct recombination repair of double-strand breaks).

The STRUCTURAL MAINTENANCE OF CHROMOSOMES 5/6 Complex Promotes Sister Chromatid Alignment and Homologous Recombination after DNA Damage inArabidopsis thaliana

“…Sister chromatids of Arabidopsis are often aligned in a random manner along chromosome arms in interphase nuclei of meristematic and differentiated cells with a 4C or higher DNA content (Schubert et al, 2006. Accumulating evidence suggests that sister chromatid alignment in Arabidopsis, as in yeast and mammals, is mediated by cohesins (Cai et al, 2003;Schubert et al, 2009). Mutants of cohesin genes, smc1 (titan8) and smc3 (titan7), show aberrant seed development in Arabidopsis (Liu et al, 2002).…”

“…Disturbed S Phase Cohesion in syn1 Mutants Impairs Establishment of DSB-Mediated Cohesion SYN1, a REC8/SCC1 homolog, is required in Arabidopsis for sister chromatid cohesion in meiotic as well as in somatic cells (Schubert et al, 2009). Because SYN1 does not contain a Ser residue that becomes phosphorylated in response to DSB formation and is conserved among RAD21/SCC1 homologs (Heidinger-Pauli et al, 2008), SYN1 is likely to be involved in the establishment of S phase cohesion but not of DSB-induced cohesion.…”

“…Because the Arabidopsis homolog SYN1 is transcribed in somatic cells, and unchallenged syn1 mutant nuclei show reduced sister chromatid alignment and occasional separation of sister centromeres (4.5 to 9.2% of 4C nuclei from different syn1 mutants showed up to 18 centromere-specific FISH signals, while only 1.5% of wild-type 4C nuclei showed up to 12 centromere-specific signals. ), SYN1 is thought to be involved in S phase cohesion in somatic cells (Schubert et al, 2009). The syn2/rad21.1 mutant is deficient in DSB repair (da Costa-Nunes et al, 2006;Kozak et al, 2009), though it aligns sister chromatids in unchallenged conditions as frequently as the wild type does (Schubert et al, 2009), suggesting a role of SYN2 in DSB-responsive cohesion.…”

“…), SYN1 is thought to be involved in S phase cohesion in somatic cells (Schubert et al, 2009). The syn2/rad21.1 mutant is deficient in DSB repair (da Costa-Nunes et al, 2006;Kozak et al, 2009), though it aligns sister chromatids in unchallenged conditions as frequently as the wild type does (Schubert et al, 2009), suggesting a role of SYN2 in DSB-responsive cohesion. We assume that the S phase cohesion is prerequisite to irradiation-enhanced cohesion establishment by SYN2-containing cohesins.…”

Reduced activity ofArabidopsischromosome-cohesion regulator geneCTF7/ECO1alters cytosine methylation status and retrotransposon expression

“…2 This multisubunit cohesin protein complex, formed by proteins of the STRUCTURAL MAINTENANCE OF CHRO-MOSOMES (SMC) family and associated non-SMC factors, is essential for regulating the higher-order chromosomal structure in eukaryotes. 3 Canonical SMC proteins are about 1000 amino acids long and are loaded onto chromosomes during telophase and G 1 phase before DNA replication. 4 Eukaryotic cohesin complexes involve a catalytic heterodimer formed by two SMC proteins that interact via a hinge domain, which opens to allow loading.…”