WELSH under-19 international Luke is a second-year professional at Home Park and made his debut last season, when he became the first player born in the 21st century to have played for the Argyle first-team.
Jephcott grew in prominence through the 2018-19 season and was given his debut in the October defeat to Burton Albion in Sky Bet League One.
Luke would go on to make 10 appearances last season and was handed a new contract in the summer of 2019.
He joined Truro City on loan prior to the start of the current campaign and was then recalled in January. On returning to Home Park, Luke announced his first-team arrival with successive braces in the wins at Scunthorpe and Carlisle.