Although rice and the moss Hypnum plumaeforme Wils are taxonomically quite distinct, momilactone A and B have so far been found only in rice and H. plumaeforme. 1-4 Momilactone A and B in rice plants are known to be synthesized as a part of defensive responses and exhibit antibacterial and antifungal activities. 5-7 Rice plants were also found to secrete momilactone A and B from their roots into the rhizosphere and exhibit alleloapthic activities against weed plants. 1,[8][9][10] The plant rhizosphere is a densely populated area in which plant roots must compete with invading root systems of neighboring plants for space, water and mineral nutrients, and with other soil-bore organisms including bacteria and fungi. [11][12][13][14] Therefore, momilactone A and B probably play an important role in rice defense mechanism in the rhizosphere as antimicrobial and allelopathic agents. However, it has not clear that biological meanings of momilactone A and B in H. plumaeforme. H. plumaeforme is often dominative in plant communities and forms large pure colonies. 15,16 H. plumaeforme was grown on MS growth medium and the concentrations of momilactone A and B in the medium were determined as the secretion levels of momilactone A and B from H. plumaeforme. The secretion levels of momilactone A and B were 4.0 and 6.3 μg g -1 dry weight of H. plumaeforme, respectively (Table 1). The endogenous concentration of momilactone A and B in H. plumaeforme was 58.7 and 23.4 μg g -1 dry weight, respectively. 4 Thus, the secretion levels of momilactone A and B, respectively, were 6.8 and 27% of momilactone A and B concentrations in H. plumaeforme. Therefore, although the endogenous concentration of momilactone A in H. plumaeforme was greater than that of momilactone B, the secretion level of momilactone B was much greater than that of momilactone A, which suggests that momilactone B may be selectively secreted into the medium than momilactone A. In addition, biological activity of momilactone B was much greater than that of momilactone A. 10 H. plumaeforme was transplanted on MS growth medium and grown at 25°C with a 12-h photoperiod for 5 days as described previously. 4 During the incubation, additional UV-irradiation (80 min par day, UV, emission peak 253 nm; 10 μmol m -1 s -1 at plant level) was made. Momilactone A and B concentrations in the medium were then determined as the secretion levels by H. plumaeforme. For cantharidin-and jasmonic acid-treatments, H. plumaeforme was transplanted on MS growth medium containing 200 μM cantharidin or 100 μM jasmonic acid, and grown at 25°C with a 12-h photoperiod for 5 days. All manipulations were carried out under sterile conditions. Control plants were incubated MS growth medium for 5 days. Means ± SE from five independent experiments with five assays for each determination are shown.