MGOUN1Encodes anArabidopsisType IB DNA Topoisomerase Required in Stem Cell Regulation and to Maintain Developmentally Regulated Gene Silencing

Abstract

Maintenance of stem cells in the Arabidopsis thaliana shoot meristem is regulated by signals from the underlying cells of the organizing center, provided through the transcription factor WUSCHEL (WUS). Here, we report the isolation of several independent mutants of MGOUN1 (MGO1) as genetic suppressors of ectopic WUS activity and enhancers of stem cell defects in hypomorphic wus alleles. mgo1 mutants have previously been reported to result in a delayed progression of meristem cells into differentiating organ primordia (Laufs et al., 1998). Genetic analyses indicate that MGO1 functions together with WUS in stem cell maintenance at all stages of shoot and floral meristems. Synergistic interactions of mgo1 with several chromatin mutants suggest that MGO1 affects gene expression together with chromatin remodeling pathways. In addition, the expression states of developmentally regulated genes are randomly switched in mgo1 in a mitotically inheritable way, indicating that MGO1 stabilizes epigenetic states against stochastically occurring changes. Positional cloning revealed that MGO1 encodes a putative type IB topoisomerase, which in animals and yeast has been shown to be required for regulation of DNA coiling during transcription and replication. The specific developmental defects in mgo1 mutants link topoisomerase IB function in Arabidopsis to stable propagation of developmentally regulated gene expression.

A DNA2 Homolog Is Required for DNA Damage Repair, Cell Cycle Regulation, and Meristem Maintenance in Plants

“…For example, strong alleles of the mutant of DNA polymerase « showed a sporophytic embryo-defective phenotype (Jenik et al, 2005;Ronceret et al, 2005), while fasciated stems were only found occasionally in the weak alleles (Yin et al, 2009). Similarly, disruption of DNA topoisomerase 1a in combination with reduction of DNA topoisomerase 1b caused seedling lethality, while disruption of DNA topoisomerase 1a alone only caused fasciation Graf et al, 2010). Thus, the phenotype of a mutant may largely depend on the severity and type of DNA damage.…”

“…In addition, fasciated mutants with defects in BRU1/MGO3/TSK, which is thought to be involved in chromatid separation during mitosis (Takeda et al, 2004;Suzuki et al, 2005), are hypersensitive to DNA damage agents, show a release of gene silencing, and are delayed in cell cycle progression. Furthermore, MGO1, which encodes a type-IB DNA topoisomerase, was shown to be involved in stem cell regulation Graf et al, 2010). DNA polymerase « was shown to influence the cell cycle, embryo patterning, epigenetic regulation, somatic homologous recombination, and ABA signaling (Jenik et al, 2005;Yin et al, 2009).…”

Threshold-dependent transcriptional discrimination underlies stem cell homeostasis

“…All transgenic lines were generated in the Landsberg erecta background. wus-7, wus-1, and clv3-2 have been described previously (4,8,34). Preparation for live imaging, Dex treatment, the optics, the microscopy platform (Zeiss 510 LSM), image acquisition, and image reconstruction have been described in earlier studies (6,13,33).…”

WOX13-like genes are required for reprogramming of leaf and protoplast cells into stem cells in the moss Physcomitrella patens

“…during inflorescence meristem growth is less sensitive (Graf et al, 2010). Furthermore, both WUS and WOX5 are essential in regenerating stem cells via tissue culture (Su et al, 2009;Sugimoto et al, 2010).…”