Momilactone-A synthase (, momilactone A synthase, OsMAS) is an enzyme with systematic name 3beta-hydroxy-9beta-pimara-7,15-diene-19,6beta-olide:NAD(P)<sup>+</sup> oxidoreductase. This enzyme catalyses the following chemical reaction:
The rice phytoalexin momilactone A is a diterpenoid secondary metabolite that is involved in the defense mechanism of the plant. Momilactone A is produced in response to attack by a pathogen through the perception of elicitor signal molecules such as chitin oligosaccharide, or after exposure to UV irradiation.