(1) Whenever the state of Tennessee, or any agency thereof, shall issue any bonds or notes for any public purpose, without regard to when issued, neither the principal of nor the interest on such bonds or notes shall be taxed by this state or by any county or municipality or taxing district of this state, except inheritance, transfer and estate taxes.
(2) Whenever any county or any agency thereof shall issue any bonds or notes for any public purpose, without regard to when authorized, neither the principal nor the interest of such bonds or notes shall be taxed by the state or by any county or municipality in this state.
(3) Whenever any incorporated town or city, or any agency thereof, shall issue any bonds or notes, without regard to when authorized, for any public purpose, neither the principal of nor the interest on such bonds or notes shall be taxed by this state or by any county or municipality of this state.
(4) In the event the courts of this state shall hold that any of this section is invalid and that such bonds or notes are taxable, such holding shall not affect the validity of any bonds or notes so issued.