Mo. Rev. Stat. § 139.070

Any person who has a lien by mortgage or otherwise, upon any real property upon which the taxes have not been paid, may pay such taxes and the interest, penalty and costs thereon; and the receipt of the county collector shall constitute an additional lien upon such land, to the amount therein stated; and the amount so paid and the interest thereon at the rate specified in the mortgage or other instrument shall be collectible with, or as a part of, and in the same manner as the amount secured by the original lien.