Involved in the synthesis of tocopherol (vitamin E). Catalyzes the condensation of homogentisate and phytyl diphosphate to form dimethylphytylhydrquinone. Low activity with geranylgeranyl diphosphate as substrate, but no activity with farnesyl diphosphate or solanesyl diphosphate. Tocopherol functions to limit lipid oxidation during seed desiccation, quiescence and germination and early seedling development. Protects thylakoid membrane lipids from photooxidation and is required for low-temperature adaptation. H(+) + homogentisate + phytyl diphosphate = 2-methyl-6-phytyl-1,4-benzene-1,4-diol + CO2 + diphosphate Cofactor biosynthesis; tocopherol biosynthesis. Reduced seed longevity, severe seedling growth defects during germination and high levels of lipid hydroperoxides and hydroxy fatty acids. Seeds and plants overexpressing HPT1 accumulate increased levels of tocopherol. Belongs to the UbiA prenyltransferase family.