Releases the supercoiling and torsional tension of DNA introduced during the DNA replication and transcription by transiently cleaving and rejoining one strand of the DNA duplex. Introduces a single-strand break via transesterification at a target site in duplex DNA. The scissile phosphodiester is attacked by the catalytic tyrosine of the enzyme, resulting in the formation of a DNA-(3'-phosphotyrosyl)-enzyme intermediate and the expulsion of a 5'-OH DNA strand. The free DNA strand then rotates around the intact phosphodiester bond on the opposing strand, thus removing DNA supercoils. Finally, in the religation step, the DNA 5'-OH attacks the covalent intermediate to expel the active-site tyrosine and restore the DNA phosphodiester backbone (By similarity). Can complement a TOP1-deficient yeast mutant (PubMed:16669064). Plays a critical role in the maintenance of a regular pattern of organ initiation. Topoisomerases I enzymes (TOP1A and TOP1B) are essential for plant survival (PubMed:12215507). Functions together with the stem cell maintenance gene WUSCHEL (WUS) in stem cell regulation. Required to maintain developmentally regulated gene repression. Functions synergistically with chromatin remodeling factors (PubMed:20228247). Is required for the repression of WUS expression in flower development. Plays a role in polycomb group (PcG) protein-mediated histone H3 trimethylation on 'Lys-27' (H3K27me3) at the WUS gene locus. H3K27me3 induces transcriptional repression of WUS. May assist AGAMOUS (AG) in recruiting PcG proteins to WUS locus. Reduces nucleosome density, especially at genes that are targets of PcG proteins (PubMed:25070639). Plays a role in epigenetic silencing. Involved in RNA-directed DNA methylation (RdDM) by promoting Pol V transcription to generate long non-coding RNA transcripts. Is dispensable for Pol IV-mediated small interfering RNA (siRNA) biogenesis. Promotes transposable element (TE) silencing at endogenous RdDM target loci through histone H3 dimethylation of 'Lys-9' (H3K9me2). Promotes the production of Pol V-dependent long non-coding transcripts that facilitate the recruitment of siRNA-AGO4 and AGO4 occupancy at TEs (PubMed:24992598). ATP-independent breakage of single-stranded DNA, followed by passage and rejoining. Interacts with DEK3. A number of isoforms are produced. According to EST sequences. Expressed in inflorescence meristems. Expressed in primordia of sepals, petals, stamens, carpels and ovules. Expressed in midstage embryos. Defects in phyllotaxis and plant architecture. Morphological abnormalities of several organs (PubMed:12215507). Defects in cellular development and organization of both the shoot and the root meristem (PubMed:20228247). Eukaryotic topoisomerase I and II can relax both negative and positive supercoils, whereas prokaryotic enzymes relax only negative supercoils. Belongs to the type IB topoisomerase family.