Cotylenins and fusicoccins, fungal diterpenoid glycosides, are know to have identical, yet unique, plant growth-regulating activities. These compounds widen the stomatal pore, stimulate cell enlargement, break seed dormancy, and stimulate rhizogenesis. Because of these plant-hormone like activities, fusicoccin, a representative of this family, has been widely utilized in plant physiology. During the course of our study on the structure-activity relationships of this class of compounds, 9-deoxy-15-hydroxycotylenol and its 15-methoxymethyl ether were synthesized in order to clarify the role of the 9α-hydroxy group of cotylenol, a common aglycon of cotylenins. Since these cotylenol analogs retained germination-stimulating activity on lettuce seeds, it has been clarified that the 9α-hydroxy group of cotylenol is not essential for its biological activities. This finding is informative in designing useful tools for targeting the 14-3-3 protein, which has recently been identified as the binding protein of fusicoccin.