Probable O-linked N-acetylglucosamine transferase (OGT) involved in various processes such as gibberellin (GA) signaling pathway and circadian clock. OGTs catalyze the addition of nucleotide-activated sugars directly onto the polypeptide through O-glycosidic linkage with the hydroxyl of serine or threonine. Probably acts by adding O-linked sugars to yet unknown proteins. Acts as a repressor of GA signaling pathway to inhibit hypocotyl elongation. Functions with GIGANTEA (GI) in pathways controlling flowering, circadian cotyledon movements and hypocotyl elongation. Acts as a light-regulated promoter of elongation via its interaction with GI. Acts as an activator of cytokinin signaling. Required with SEC for gamete and seed development (PubMed:12136030). Its OGT activity has been proved in vitro but not in vivo (Ref.5, PubMed:11457967, PubMed:12136030, PubMed:15155885, PubMed:15608330, PubMed:8799194). Possesses O-fucosyltransferase activity on specific serine and threonine residues (PubMed:28244988). Mediates O-fucosylation of the DELLA protein RGA, a repressor of the GA signaling pathway (PubMed:28244988). O-fucosylation enhances RGA activity by promoting RGA binding to key transcription factors in brassinosteroid and light-signaling pathways (PubMed:28244988). Regulates root hair patterning upstream of the transcription factor WER, independently of DELLA proteins and GA signaling (PubMed:32928908). Involved in abscisic acid (ABA) signaling partly through functional ABAR (PubMed:34712252). Mediates O-fucosylation of CPN20 that may depress ABA responses during seed germination and seedling development (PubMed:34712252). Involved in the modulation of the pace of the circadian clock by mediating O-fucosylation of APRR5, one of the core circadian clock components (PubMed:31899321). O-fucosylation promotes APRR5 proteolysis (PubMed:31899321). L-seryl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-(N-acetyl-beta-D-glucosaminyl)-L-seryl-[protein] + H(+) + UDP L-threonyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-(N-acetyl-beta-D-glucosaminyl)-L-threonyl-[protein] + H(+) + UDP GDP-beta-L-fucose + L-seryl-[protein] = 3-O-(alpha-L-fucosyl)-L-seryl-[protein] + GDP + H(+) GDP-beta-L-fucose + L-threonyl-[protein] = 3-O-(alpha-L-fucosyl)-L-threonyl-[protein] + GDP + H(+) Optimum pH is 8.0. Protein modification; protein glycosylation. Homomultimer; via its TPR repeats. Interacts with GI (PubMed:15155885). Interacts with TCP14 and TCP15 (PubMed:22267487, PubMed:15155885). Interacts (via N-terminus) with APRR5 (PubMed:31899321). Interacts with CPN20 (PubMed:34712252). A number of isoforms are produced. According to EST sequences. Widely expressed. Present throughout the plant (at protein level). Detected in all organs of the plant and at all stages of the life cycle. Detected 1 day after germination in the radicle just before its emergence from the seed. At 2.5 and 3 days after germination, expression in the young seedling is highest in the cotyledons and the root tip. At 3, 4 and 5 days, expression is also detectable in the hypocotyl. At 10 days of age, expression in the first pair of true leaves is reduced relative to the rest of the seedling. 2 days later, this difference disappears and the expression level is again fairly similar throughout the aboveground portion of the plant, with a higher intensity in the vegetative apex. This developmental regulation is not detected in leaves developing at later nodes. Older plants also display uniform expression throughout the vegetative organs, but this expression is less intense. In older seedlings, expression is observed throughout the root, particularly at the tip of the primary root and in lateral roots. Expression is also observed in trichomes and senescing leaves, and in inflorescence internodes, flowers. Expression is observed in the seeds and carpels of fully elongated siliques. Lower expression is also observed in expanding siliques and in the developing seeds in these siliques. Expression is also detected in the embryo of maturing seeds (after the disappearance of the endosperm) (at protein level). The TPR repeats mediate protein-protein interactions and are essential for its function. Expression of such repeats in plants accelerate flowering. Altered root epidermis morphology and root hair patterning. Belongs to the glycosyltransferase 41 family. O-GlcNAc transferase subfamily.