The Tyrells were stewards to House Gardener, the Kings of the Reach, before the Targaryen conquest; after the Gardener line was extinguished in that war, Aegon the Conqueror made the Tyrells overlords in the Reach and Wardens of the South.
The Tyrells were stewards to House Gardener before the Targaryen conquest. House Gardener was the Kings of the Reach. Aegon the Conqueror made the Tyrells overlords in the Reach and Wardens of the South. The Gardener line was extinguished in that war.