An Archaebacterial Topoisomerase Homolog Not Present in Other Eukaryotes Is Indispensable for Cell Proliferation of Plants

Abstract

Plants, in contrast to other eukaryotes, possess not only homologs of subunit A (AtSPO11-1, 2, 3) but also of subunit B (AtTOP6B) of the archaebacterial topoisomerase VI. AtTOP6B and AtSPO11-3 are strongly expressed in somatic tissue of Arabidopsis and are able to interact with each other in vitro. A T-DNA insertion in AtTOP6B results in deficient cell proliferation; plants stop growing at the rosette stage, have small crinkled leaves, and die about 4 weeks after germination. Cultured root cells die after a limited number of cell divisions. The mitotic index of the root meristems is strongly reduced. Flow cytometric analysis demonstrates that endoreplication in mutant plants is stopped at the 8C stage; the last cycle is not completed in most cases. Mutant plants show a significant increase in nuclear DNA strand breaks. A T-DNA insertion mutant of AtSPO11-3 has a phenotype that is almost to that of AtTOP6B and the double mutant. Thus, both genes seem to act in vivo as subunits of a functional entity. A loss of this function most likely results in a defect in DNA replication, leading directly, or via the activation of a DNA damage checkpoint, to an arrest of cell division and endoreduplication. The dependence on an archaebacterial topoisomerase VI homolog distinguishes plants from the other eukaryotic kingdoms.

AtREC8 and AtSCC3 are essential to the monopolar orientation of the kinetochores during meiosis

“…The mitotic index was determined as described (Hartung et al, 2002) on roots isolated from five wild-type plants and ten Atscc3-1 plants grown for 21 days in the greenhouse.…”

Spo11 and the Formation of DNA Double-Strand Breaks in Meiosis

“…How the products of these two genes work together to promote recombination is not yet clear, but it is tempting to speculate that DSBs in this organism are made by an obligate heterodimer of different Spo11-related subunits. AtSPO11-3 is dispensable for meiotic recombination, and instead encodes a protein that interacts with a homolog of TopoVIB to constitute a bona fide TopoVI relative (Hartung et al 2002;Sugimoto-Shirasu et al 2002;Yin et al 2002;Sugimoto-Shirasu et al 2005). …”

Regulating DNA Replication in Plants

“…1). Thus, mutations in the A and B subunits of topoisomerase VI, encoded by the ROOT HAIRLESS2 (RHL2) and HYPOCOTYL6 (HYP6) genes, prevent endocycle progression beyond the 8C DNA level (Hartung et al 2002;Sugimoto-Shirasu et al 2002). Likewise, cells of the root hairless1/hypocotyl7 mutant that lack topoisomerase II show abnormal endocycle progression (SugimotoShirasu et al 2005).…”