Isolation and Characterization of SYN1, a RAD21-like Gene Essential for Meiosis in Arabidopsis

Abstract

The proper pairing, recombination, and segregation of chromosomes are central to meiosis and sexual reproduction. The syn1 mutation was previously identified as a synaptic mutant in a T-DNA-tagged population of plants. SYN1 has been isolated and found to exhibit similarity to Schizosaccharomyces pombe RAD21 and RAD21-like proteins, which are required for chromosome condensation and sister chromatid cohesion during mitosis. Plants homozygous for syn1 are male and female sterile and show defects in chromosome condensation and pairing beginning at leptonema of meiosis I. Fragmentation of the chromosomes was observed at metaphase I. Alternative promoters produced two SYN1 transcripts. One transcript was expressed at low levels in most tissues, whereas the other was expressed only in prebolting buds. DNA blot analyses suggest that Arabidopsis contains a small RAD21 gene family. Consistent with the DNA blot data, a second Arabidopsis RAD21 -like gene has been identified. These results suggest that different RAD21-like proteins play essential roles in chromosome condensation and pairing during both meiosis and mitosis. INTRODUCTIONThe normal segregation of chromosomes during meiosis in most eukaryotic organisms is dependent on the successful pairing of homologous chromosomes during the zygotene stage of prophase I. After DNA replication, each chromosome enters meiosis as two chromatids. During leptonema of meiotic prophase I, the dispersed chromosomes condense to form long thin threads. During zygonema, homologous chromosomes align, and the synaptonemal complex, an elaborate proteinaceous structure that holds homologs closely apposed along their lengths, is formed. At pachynema, nonsister chromatids of the paired chromosomes recombine, forming chiasmata, which become visible as the chromosomes start to desynapse during diplonema. During meiosis I, sister chromosomes must be attached so that they can orient properly and achieve reductive division. During anaphase I, sister chromosome cohesion is released in preparation for the second meiotic division. Therefore, the formation and proper maintenance of sister chromatid cohesion are essential for the proper alignment and segregation of chromosomes during meiosis (Miyazaki and Orr-Weaver, 1994).A considerable amount of information is available on the events associated with meiosis in a number of eukaryotic systems, including plants (Dawe, 1998). However, much less is known about the genes required during meiosis in plants than in several other systems, including yeast (Roeder, 1995), Drosophila (Orr-Weaver, 1995), and Caenorhabditis elegans (Zetka and Rose, 1995). Based on similarities to meiotic genes identified in other organisms, several genes expressed during meiosis have been identified in Arabidopsis (Sato et al., 1995;Klimyuk and Jones, 1997), lily (Kobayashi et al., 1994), and wheat (Ji and Langridge, 1994). In addition, meiotic mutants have been identified in numerous plants species, including Arabidopsis (Dawson et al., 1993;Chaudhury et al., 1994;He et al., 1996...

A homologue of the yeast HOP1 gene is inactivated in the Arabidopsis meiotic mutant asy1

“…No hybridisation signal was detectable; moreover, a subsequent experiment in which bud poly(A)+ RNA was probed also failed to produce any signal (data not shown). These results are reminiscent of those obtained with other Arabidopsis meiotic genes: SYN1 (Bai et al 1999) and MS5 (Glover et al 1998); however, in these cases low-level expression has been detected using RT-PCR. A similar approach was therefore employed to investigate ASY1 expression.…”

“…b An identical blot washed at low stringency, revealing the presence of an additional ASY1-like gene (ASY2) in the Arabidopsis genome The ASY1 gene was found to be expressed in nonmeiotic tissues rather than being specifically induced by meiosis as HOP1 is in yeast (Hollingsworth et al 1990). Two other meiotic genes from Arabidopsis are also expressed in non-meiotic tissues, namely SYN1 (Bai et al 1999) and MS5 (also known as Jag18 and Pollenless3) (Glover et al 1998;Sanders et al 1999;Luong, Caryl and Franklin, unpublished observations), although a meiosis-specific splice variant of SYN1 was reported (Bai et al 1999). In addition when the expression patterns of the Arabidopsis homologues of the yeast meiotic gene Dmc1 (Doutriaux et al 1998) and the Drosophila meiotic gene Pelota (Caryl, Jones and Franklin, unpublished observations) were examined these were also found to be expressed in non-meiotic tissues.…”

Mechanism of haploidy-dependent unreductional meiotic cell division in polyploid wheat

“…Why did sister kinetochores orient differently in LDN and LDN polyhaploid? Kinetochore orientation in meiosis is regulated by a complex genetic network with multiple genes/proteins, including the cohesin Rec8 (Watanabe and Nurse 1999;Watanabe et al 2001;Bai et al 1999;Pasierbek et al 2001;Cai et al 2003;Yokobayashi et al 2003;Bannister et al 2004;Parra et al 2004;Chelysheva et al 2005;Golubovskaya et al 2006;Zhang et al 2006), monopolin complex (Toth et al 2000), Polo-like kinase Cdc5 (Clyne et al 2003;Lee and Amon 2003), and many others (Toth et al 2000;Lee et al 2002;Shonn et al 2002;Clyne et al 2003;Lee and Amon 2003;Yokobayashi and Watanabe 2005;Petronczki et al 2006;Hauf et al 2007;Monje-Casas et al 2007). Both LDN and LDN polyhaploid have the same A and B genomes, but at different ploidy levels.…”

“…Sister chromatids are longitudinally connected to each other by a protein complex, called cohesin, until AII in meiosis. Rec8 is the meiotic cohesin component highly conserved in eukaryotes, including yeast, plants, animals, and humans (Bai et al 1999;Parisi et al 1999;Watanabe and Nurse 1999;Pasierbek et al 2001;Cai et al 2003;Yokobayashi et al 2003;Bannister et al 2004;Parra et al 2004;Chelysheva et al 2005;Golubovskaya et al 2006;Zhang et al 2006). Meiotic cohesin is established at the S phase of meiosis (Uhlmann and Nasmyth 1998).…”

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

“…Arabidopsis SMC1, SMC3, and SCC3 were identified in somatic and meiotic tissues (Chelysheva et al 2005;Lam et al 2005;Liu et al 2002). SYN1 mediates cohesion during meiosis (Bai et al 1999;Bhatt et al 1999;Cai et al 2003). SYN2 and SYN3, mainly expressed in meristematic tissues, seem to be mitotic α-kleisins (Dong et al 2001).…”