A Center Controller reported an aircraft descended below their assigned altitude and flew below the Minimum Vectoring Altitude.

Narrative

Aircraft X was in level flight at 9000 ft. awaiting his approach instructions into ZZZ airport. Once traffic was clear; I issued the following clearance; 'Aircraft X maintain 9000 until ZZZZZ; cleared straight in RNAV XX Approach into ZZZ airport.' Aircraft properly read back the clearance including the altitude. I noticed shortly after his mode C indicating 8600 ft. then asked to verify altitude and/or proximity to ZZZZZ. He said he was before ZZZZZ but descending to 8000 ft. because of the approach plate. The MVA below the aircraft was 9000ft. so I issued a low-altitude alert to the aircraft and advised him of the MVA in his area. He apologized and climbed back up from 8400 to 9000 ft. and then continued on the approach. The issue was clearly just a mistake on the pilot's part; forgetting his altitude to maintain and probably just misreading the approach plate showing 8000 feet as the minimum approach altitude at ZZZZZ. I don't think there's really any significant change that can be made other than to caution pilots to be more careful when reading approach plates and listening to ATC instructions.