RED LIGHT-INDUCED HOST RESISTANCE AND THE INVOLVEMENT OF ANTIFUNGAL SUBSTANCE(S)
Y HONDA, SZ ISLAM and S ARASE
Faculty of Life and Environmental Science, Shimane University, Matsue 690-8504, Japan
Background and objectives
It has been reported that irradiation of soyabean plants by ultraviolet radiation induced phytoalexin (glyceollin) accumulation which inhibited the penetration and infection of Phytophthora megasperma to host tissues. Short-wave UV radiation also induces the production of the phytoalexin pisatin in pea pods. Production of phytoalexins in UV-irradiated rice leaves was also reported . However, the possibility that visible light of long wavelengths might induce host resistance has not been reported. The objective of this study was to investigate red light-induced resistance of broad bean against Botrytis cinerea  and the possible involvement of antifungal substance(s).
Results and conclusions
Large necrotic areas developed under inoculum droplets containing B. cinerea on broad bean leaflets under both NUV radiation and blue light. By contrast, necrotic lesion development was significantly suppressed under yellow and red lights. Maximum infection hypha formation was recorded under NUV radiation and blue light where 91 and 83% of appressoria, respectively, produced infection hyphae. Under yellow and red light, infection hypha formation was significantly inhibited and browning of cells under or around the appressoria was observed. Both cell browning and inhibition of infection hypha formation were not observed under other light treatments. In particular, under NUV radiation, blue, green and far-red lights or in the dark, over 70% of appressoria formed infection hyphae.
The germination rate of B. cinerea in inoculation droplets on broad bean leaflets kept under red light for 48 h was 8.2%, but 48% of spores germinated in control water droplets. In contrast, spore germination in the dark was 92.3 and 80.5% on the leaf surface and in controls on glass, respectively, without significant difference.
On detached epidermal strips, inhibition of infection hypha formation by B. cinerea was not observed under red light where 73-79% of appressoria produced infection hyphae, while 77-80% were recorded in the dark.
These results indicate that irradiation of broad bean leaflets with red light activated the production of antifungal substance(s) in the leaf tissues, which seem to be responsible for induced resistance of broad bean against B. cinerea. Further, it was demonstrated that the accumulation of antifungal substance(s) in leaf tissues was promoted by infection of B. cinerea.
This antifungal substance(s) was of high molecular weight, water soluble, heat stable and fungus specific. Cellulose thin-layer chromatography (TLC) of infection droplets of B. cinerea or control water droplets, recovered from inoculated broad bean leaflets kept under red light for 48 h, displayed two inhibition zones at the origin point and at an approximate Rf value of 0.60. On the contrary, relatively faint inhibition zones were observed in cellulose TLC of water droplets. This anti-fungal substance(s) is less active against B. fabae than B. cinerea. The known phytoalexins from Vicia faba, the wyerone derivatives, were not detected within droplets.
1. Kato H, Kodama T and Akatsuka T, 1994. Phytopathology 36, 299-301.
2. Islam SZ, Honda Y and Arase S, 1998. Journal of Phytopathology (in press).