O-linked N-acetylglucosamine transferase (OGT) that mediates O-glycosylation of capsid protein (CP) of virus in case of infection by Plum pox virus (PubMed:16014901). OGTs catalyze the addition of nucleotide-activated sugars directly onto the polypeptide through O-glycosidic linkage with the hydroxyl of serine or threonine (PubMed:16014901). Probably acts by adding O-linked sugars to yet unknown proteins. Its OGT activity has been proved in vitro but not in vivo (PubMed:12136030). Required with SPY for gamete and seed development (PubMed:16014901). Mediates O-glycosylation of the DELLA protein RGA, a repressor of the gibberellin (GA) signaling pathway (PubMed:26773002). O-glycosylation by SEC inhibits RGA binding to four of its interactors PIF3, PIF4, JAZ1, and BZR1 that are key regulators in light, jasmonate, and brassinosteroid signaling pathways, respectively (PubMed:26773002). Activates ATX1 through O-GlcNAc modification to augment ATX1-mediated H3K4me3 histone epigenetic modification at FLC locus, thus preventing premature flowering (PubMed:30150325). 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 Protein modification; protein glycosylation. Interacts with TCP14 and TCP15 (PubMed:22267487). Interacts with ATX1 (PubMed:30150325). Early flowering phenotype due to reduced histone H3 'Lys-4' trimethylation (H3K4me3) of FLC thus leading to lower FLC expression. Belongs to the glycosyltransferase 41 family. O-GlcNAc transferase subfamily.