The plant adherin AtSCC2 is required for embryogenesis and sister-chromatid cohesion during meiosis in Arabidopsis Jose Sebastian, Maruthachalam Ravi †‡ , Sebastien Andreuzza ‡ , Aneesh P. Panoli, Mohan P. A. Marimuthu and Imran Siddiqi * Centre for Cellular & Molecular Biology, Uppal Road, Hyderabad 500007, India Received 16 January 2009; accepted 6 February 2009; published online 23 March 2009. * For correspondence (fax +91 40 2716 0591; e-mail imran@ccmb.res.in). † Current address: Section of Plant Biology, University of California, Davis 1, Shields Ave, Davis, CA 95616, USA. ‡ These authors contributed equally to the work. SUMMARY Adherin plays an important role in loading the cohesin complex onto chromosomes, and is essential for the establishment of sister-chromatid cohesion. We have identified and analyzed the Arabidopsis adherin homolog AtSCC2. Interestingly, the sequence analysis of AtSCC2 and of other putative plant adherin homologs revealed the presence of a PHD finger, which is not found in their fungal and animal counterparts. AtSCC2 is identical to EMB2773, and mutants show early embryo lethality and formation of giant endosperm nuclei. A role for AtSCC2 in sister-chromatid cohesion was established by using conditional RNAi and examining meiotic chromosome organization. AtSCC2-RNAi lines showed sterility, arising from the following defects in meiotic chromosome organization: failure of homologous pairing, loss of sister-chromatid cohesion, mixed segregation of chromosomes and chromosome fragmentation. The mutant phenotype, which included defects in chromosome organization and cohesion in prophase I, is distinct from that of the Arabidopsis cohesin mutant Atrec8, which retains centromere cohesion up to anaphase I. Immunostaining experiments revealed the aberrant distribution of the cohesin subunit AtSCC3 on chromosomes, and defects in chromosomal axis formation, in the meiocytes of AtSCC2-RNAi lines. These results demonstrate a role for AtSCC2 in sister- chromatid cohesion and centromere organization, and show that the machinery responsible for the establishment of cohesion is conserved in plants. Keywords: meiosis, chromosome organization, cohesin, conditional RNAi, gametogenesis. INTRODUCTION The establishment of sister-chromatid cohesion during DNA replication and its controlled release at the metaphase to anaphase transition is necessary for the proper segregation of chromosomes at mitosis and meiosis. Sister-chromatid cohesion is mediated by cohesins, which are complexes of proteins that bind to chromosomes and hold sister chro- matids together (reviewed in Nasmyth, 2001). The cohesin complex belongs to the class of structural maintenance of chromosome (SMC) complexes that have important chro- mosomal functions (Cobbe and Heck, 2004; Nasmyth and Haering, 2005). The cohesin complex comprises four core subunits, Scc1, Scc3, Smc1 and Smc3, which are conserved from yeast to mammals. The N- and C-terminal portions of Smc1 and Smc3 associate to form ATPase domains, and interact with Scc1 to form a ring. It has been proposed that cohesion is effected by the two sister chromatids being encircled by a cohesin ring, and that the opening of the ring requires ATP hydrolysis (Arumugam et al., 2003; Gruber et al., 2003). The association of cohesins with chromosomes depends on a class of proteins called adherins. Adherin mutants in fungi and animals show defects in sister-chromatid cohesion, meiotic recombination, DNA repair and gene regulation (reviewed in Dorsett, 2004). In Saccharomyces cerevisiae, the adherin complex is composed of two subun- its, Scc2 and Scc4, which are conserved from fungi to humans (Ciosk et al., 2000; Watrin et al., 2006). The Scc2 subunit contains multiple HEAT repeats, which are consid- ered to be involved in protein–protein interactions (Neuwald and Hirano, 2000). Several lines of evidence suggest that adherins are directly involved in loading cohesins onto chromosomes. Firstly, cohesin has been found to load onto chromosomes at adherin-binding sites before translocating away to other chromosomal locations (Lengronne et al., ª 2009 The Authors 1 Journal compilation ª 2009 Blackwell Publishing Ltd The Plant Journal (2009) 59, 1–13 doi: 10.1111/j.1365-313X.2009.03845.x