Phototropism of Conidial Germ Tubes of Botrytis cinerea and Its Implication in Plant Infection Processes

The germ tubes of Botrytis cinerea showed negative phototropism to near ultraviolet (NUV) and blue (300 to 520 nm) light followed by far-red (700 to 810 nm), whereas red light (600 to 700 nm) induced positive phototropism significantly. Minimum germ tube growth occurred during exposure to negative phototropism-inducing wavelengths, whereas it was maximum under positive phototropism-inducing wavelengths. NUV radiation and blue light that induced negative phototropism of B. cinerea promoted infection-hypha formation on both onion scale and broad bean (Vicia faba) leaf epidermal strips, whereas positive phototropism-inducing red light suppressed it, resulting in a high proportion of germ tubes without infection hyphae. In broad bean leaf infection, the number of infection points and area of necrosis per drop of conidial suspension were higher under NUV radiation and blue light than that of a dark control or leaflets pretreated with NUV radiation and blue light. In contrast, lower numbers of infection points and very small necrotic lesions developed under red light. In the case of red-light-pretreated leaflets, the number of infection points developed were higher, but areas of necrosis did not increase significantly compared with leaflets kept under red light without pretreatment. These results show the importance of phototropism of conidium germ tubes in plant infection.