Haustoria of Cuscuta japonica, a Holoparasitic Flowering Plant, Are Induced by the Cooperative Effects of Far-Red Light and Tactile Stimuli

Abstract

When seedlings of Cuscuta japonica were grown with Vigna radiata (the host plant) in a flower pot for 6 d under white light and then irradiated with far-red or blue light (ca. 6 [ano photons m~2 s1), the seedlings parasitized V. radiata. However, no parasitism of the seedlings was observed under red or white light or in darkness. The parasitic behavior of seedlings of C. japonica was observed even if an acrylic rod was used as a substitute for the host plant. Upon incubation under far-red light, the seedling twined tightly around the rod and developed haustoria towards it. Haustoria also developed when apical and subapical regions of seedlings were held between two glass plates that were about 0.7 mm apart and were irradiated with far-red light. However, no haustoria were induced by either the hold or irradiation alone. These results indicate that parasitism of Cuscuta japonica is controlled by the cooperative effects of two physical signals, far-red light and appropriate tactile pressure. Our findings suggest that parasitism by the genus Cuscuta involves a novel strategy.