(a) A bond authorized by the law, including a refunding bond, that is not payable wholly from ad valorem taxes may be additionally secured by a trust indenture. The trustee may be a bank with trust powers located inside or outside this state.
(b) A trust indenture, regardless of the existence of the deed of trust or mortgage lien on the property, may:
(1) provide for the security of the bonds and the preservation of the trust estate in the manner prescribed by the board;
(2) provide for amendment or modification of the trust indenture;
(3) provide for the issuance of bonds to replace lost or mutilated bonds;
(4) condition the right to spend authority money or sell authority property on the approval of a licensed engineer selected as provided by the trust indenture; and
(5) provide for the investment of authority money.