The plant adherin AtSCC2 is required for embryogenesis and sister-chromatid cohesion during meiosis in Arabidopsis

P>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.