Involved in the synthesis of plastoquinone-9. Can use both homogentisic acid and 2,5-dihydroxyphenylacetic acid gamma-lactone as prenyl acceptors, and solanesyl diphosphate > farnesyl diphosphate > geranylgeranyl diphosphate >> phytyl diphosphate as prenyl donors. Do not catalyze the decardoxylation of homogentisate uncoupled from prenylation. all-trans-nonaprenyl diphosphate + H(+) + homogentisate = 2-methyl-6-all-trans-nonaprenylbenzene-1,4-diol + CO2 + diphosphate Inhibited by haloxydine (3,5-dichloro-2,6-difluoro-4-haloxypyridine). A number of isoforms are produced. According to EST sequences. Seeds overexpressing HST accumulate increased levels of tocopherol. Belongs to the UbiA prenyltransferase family. Was initially thought to have a homogentisate phytyltransferase activity and to be involved in tocopherol biosynthesis.