Although most plants have both male and female sex organs in the same individual, approximately 5 percent of plant species have separate sexes. This characteristic is called dioecy. The sex determination systems of most dioecious plants resemble the mammalian XY system, but the gene responsible for determining the sex of plants has been a longstanding mystery. We found an unusual gene called OGI (meaning “male tree” in Japanese) in male persimmon trees (Diospyros). Unlike most genes, OGI encodes a very small piece of RNA (ribonucleic acid) that acts as “molecular scissors,” cutting down the expression of another gene called MeGI (meaning “female tree” in Japanese). Discovery of the OGI–MeGI system for sex determination in persimmons is anticipated to contribute not only to our understanding of the evolution of the plant dioecious system, but also to the development of practical orchard applications.