Terrestrial plant genomes are widely expanded in genes that encode glycosyltransferases, which facilitate the attachment of sugar moieties onto proteins, lipids, or function in the biosynthesis of polysaccharides. Specifically, in Arabidopsis thaliana, 39 genes are predicted to encode protein O-fucosyltransferases (POFTs). In contrast to POFTs in metazoan systems, very little is known about these enzymes in plants, and biochemical evidence for POFT activity for even one of these Arabidopsis genes remains elusive. Nonetheless, this family of enzymes play fundamental roles in essential plant processes, including angiosperm sexual reproduction, termed double fertilization. Importantly, this process is responsible for the production of the majority of our food crops. During double fertilization, the male gametophyte (pollen) germinates to produce a sperm cell trafficking structure called the pollen tube that physically penetrates through the pistil tissues to deliver its gametes to a distant ovule located deep within the ovary. Whereas the process of pollen tube penetration through the pistil has been anatomically well-described, the genetic regulation remains poorly understood. In this dissertation, we identify one novel member of the Arabidopsis putative POFT family, O-FUCOSYL TRANSFERASE 1 (AtOFT1), which plays a key role in pollen tube penetration through the stigma–style interface. oft1 mutant pollen tubes have a reduced ability to elongate past the style, leading to a nearly 2000-fold decrease in oft1 pollen transmission efficiency and as much as a 10-fold reduction in seed set. We demonstrate that AtOFT1 is localized to the Golgi apparatus, indicating its potential role in cellular glycosylation events. Furthermore, we show AtOFT1 and other similar Arabidopsis genes represent a novel clade of sequences related to metazoan POFTs, and that mutation of residues that are important for O-fucosyltransferase activity compromise AtOFT1 function in vivo. Finally, we catalytically assess two other putative POFT family members and show that they utilize the metazoan POFT substrate, GDP-fucose. The results of this study elucidate a physiological function for AtOFT1 in pollen tube penetration during double fertilization, expands our biochemical knowledge of this hypothesized gene family in Arabidopsis, and highlights the potential significance of protein O-glycosylation events in plant systems
