Mo. Rev. Stat. § 161.052: Vacancies in office.

Any vacancy occurring in the term of office of any board member shall be filled by appointment by the governor, by and with the advice and consent of the senate, for the unexpired term. If a vacancy occurs while the general assembly is not in session, the governor shall make a temporary appointment until the next session of the general assembly, when he shall nominate some person to fill the office.