All original motor vehicle license plates shall be sold by the tax collector or tax commissioner of the several counties. Such officials are designated as agents of the commissioner of motor vehicle safety for the purpose of accepting applications for the registration of motor vehicles and as agents of the state revenue commissioner for purposes of collecting ad valorem taxes in connection with the registration of motor vehicles. The duties and responsibilities incident to the exercise of this designation shall be a part of the official duties and responsibilities of the various tax collectors and tax commissioners.