Release of momilactone A and B from rice plants into the rhizosphere and its bioactivities

Momilactone A and B are present in rice plants and are secreted into the rice rhizosphere throughout the crop growth and development. Their secretion rates increased till flowering initiation and declines thereafter. Although momilactone A concentration in rice shoots and roots was greater than momilactone B but secretion rate of later was greater than former; this suggested that momilactone B may be selectively secreted into the rice rhizosphere than momilactone A. Momilactone A inhibited the root and shoot growth of Echinochloa crus-galli and Echinochloa colonum > 10 μM concentrations, while, momilactone B caused inhibition above 1 μM. Comparing the I50 values (concentrations required for 50% inhibition of plant growth), the inhibitory activities of momilactone B on the growth of E. crus-galli and E. colonum were 4.7- to 19.2-folds greater than momilactone A, which indicates that momilatone B was more inhibitory to growth than momilactone A. The momilactone A and B were slightly inhibitory to the growth of rice seedlings but most inhibitory to grassy weeds (E. crus-galli and E. colonum). Besides, no visible damage to rice seedlings was observed from momilactone A and B. These results suggest that rice plants may inhibit the growth of their neighbouring plants through secretion of momolactone A and B into the rhizosphere, without serious toxicity to rice plants. Thus, momilactone A and B may play an important role in rice defence mechanism in the rhizosphere to prevent competition from invading root systems of neighboring plants. However, the role of momilactone B for the defence mechanism may be greater than momilactone A, because growth inhibitory activity and secretion level of momilactone B was much greater than momilactone A.