Mo. Rev. Stat. § 456.2-205: Enforceability of mediation or arbitration provisions.

1. Subject to the exception in subsection 2 of this section, a provision in a trust instrument requiring the mediation or arbitration of disputes between or among the beneficiaries, a fiduciary, a person granted nonfiduciary powers under the trust instrument, or any combination of such persons is enforceable.

2. A provision in a trust instrument requiring the mediation or arbitration of disputes relating to the validity of a trust is not enforceable unless all interested persons with regard to the dispute consent to the mediation or arbitration of the dispute.