(a) District bonds, including revenue bonds, that are 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 a deed of trust or mortgage lien on property, may:
(1) contain any provisions prescribed by the board for the security of the bonds and the preservation of the trust estate;
(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 district money or sell district property on the approval of a licensed engineer selected as provided by the trust indenture; and
(5) provide for the investment of district money.