Yuichi Honda

Studies on red light-induced resistance of broad bean to Botrytis cinerea: I. Possible production of suppressor and elicitor by germinating spores of pathogen

When detached broad bean leaves were preinoculated with virulent strain B304 of Botrytis cinerea 24 h before a challenge inoculation with strain B304, lesion formation by B304 was significantly inhibited in red light but not in the dark. In leaves that were preinoculated with avirulent strain 021 and then challenged by B304, however, lesion formation was not inhibited even under red light. Such differences in lesion formation after the challenge inoculation with an avirulent strain were also observed with lesions caused by Alternaria alternata, a nonpathogen of broad bean and by avirulent strain 021R in the presence of germination fluid from spores of strains B304 and 021R. These results suggest the possibility that virulent B. cinerea produced a suppressor involved in induced susceptibility and an elicitor involved in resistance induced by red light during spore germination.

DNA fragmentation in Sekiguchi lesion mutants of rice infected with Magnaporthe grisea

Light-dependent activation of the tryptamine pathway in Sekiguchi lesion (sl) mutants (Sekiguchi-asahi, Sekiguchi-koshihikari, and Sekiguchi-sasanishiki) inoculated with Magnaporthe grisea, was demonstrated by a significant increase in tryptophan decarboxylase and monoamine oxidase activities and tryptamine accumulation. Terminal deoxynucleotidyl transferase-mediated dUTP-nick end labeling (TUNEL) staining and gel analysis indicated DNA fragmentation in cells of leaf tissues with Sekiguchi lesions of the three sl mutants. Furthermore, increased DNase activity was also light-dependent in the sl mutants after inoculation with M. grisea. DNA fragmentation was inhibited in leaves when Sekiguchi lesion formation was suppressed by cycloheximide and heat shock pretreatments. These data suggest that Sekiguchi lesion formation in the sl mutants is an apoptosis-like response and that its response is induced by light-dependent activation of the tryptamine pathway, which is responsible for light-enhanced resistance.

Characterization of antifungal glycoprotein in red-light-irradiated broadbean leaflets

Abstract Red-light treatment of broadbean leaflets resulted in the production of antifungal substance(s) against Botrytis cinerea. The antifungal substance(s) was positively charged, as the antifungal constituent was removed by the cation exchanger CM cellulose. Treatment of infection droplets with glycosidases (α-mannosidase, β-galactosidase, β-glucosidase), glycol-specific reagent periodate (NaIO4), and proteinase K completely eliminated antifungal activity, suggesting that both protein and carbohydrate are active components. The protein content of infection droplets was 0.148 mg/ml. The HPLC gel column analysis of infection droplets resulted in four fractions; all the fractions showed antifungal activity.

Suppression and reactivation of UV-induced sporulation by blue light in Bipolaris oryzae

Sporulation inBipolaris oryzae was induced by UV radiation (295 nm), but the number of conidia gradually decreased with increasing duration of UV radiation longer than 1 min. The inductive effect of UV radiation can be nullified by blue light (459 nm) applied immediately before or after inductive UV radiation shorter than 1 min. In contrast, the number of conidia increased with an increasing duration of blue light applied after inductive UV radiation longer than 1 min, but not if it was applied before UV radiation. The present study firstly revealed the possibility of photoreactivation inB. oryzae sporulation.

Effects of some indole-related compounds on the infection behavior of Magnaporthe grisea

Based on their effect on the infection behavior of Magnaporthe grisea, indole-related compounds were classified into three groups. The first group, including tryptophan, indole-3-butyric acid, indole-3-pyruvic acid, and indole-3-acetamide, did not inhibit infection behavior such as spore germination, appressorium formation, or infection hypha formation in M. grisea. The second, including indole acetic acid, indole-3-acetonitrile, oxindole, and tryptamine inhibited all stages of infection behaviors in a dose-dependent manner. The third, including gramine and indole, did not inhibit spore germination or appressorium formation, whereas it did inhibit infection hypha formation in a dose-dependent manner. These results suggest that endogenous or exogenously applied indole-related compounds in the second and third groups may contribute to protection in blast-susceptible plants such as rice and barley.

Effects of supplementary UV-B radiation on development of damping-off in spinach caused by the soil-borne fungus Fusarium oxysporum

The effects of UV-B radiation (290–320 nm) on development of damping-off of spinach (Spinacia oleracea) caused by the fungusFusarium oxysporum were examined in a growth cabinet. The incidence of disease greatly increased when experimental plants were grown in visible radiation with supplementary UV-B radiation. This increase was suppressed by increasing the irradiation of visible radiation.Fusarium oxysporum was isolated from the roots of all damping-off plants and the roots of some unwilted plants, indicating that spinach infected with the pathogen did not necessarily suffer from damping-off in 15d. Supplementary UV-B radiation suppressed the increase in growth components such as the number of leaves, the plant height and the fresh weight of aboveground plant parts, but did not affect the fresh weight of roots. The ratio of the number of plants infected with pathogen to the total number of plants was over 80% irrespective of light conditions. It was suggested that the defense response of spinach to this pathogen was greatly influenced by the physiological state of aboveground plant parts resulting from supplementary UV-B radiation.

Primary Effect of a Host-Selective Toxin From Magnaporthe Grisea to Mitochondria of Rice Leaves

The effects of the toxin produced by the rice blast fungus Magnaporthe grisea on host (rice and barley) and non-host (finger millet) plants were examined by ultrastructural observation and morphometric measurement. The first toxin-induced changes in rice and barley plants were the loss of the mitochondrial matrix and the disruption of mitochondrial cristae. These modifications in mitochondria were observed 1 hr after toxin exposure and the frequency of modified mitochondria did not increase with time after toxin treatment. Morphological and ultrastructural changes were not observed in other organelles. Toxin effects were observed in epidermal, mesophyll and phloem cells of barley and rice but not in leaves of the non-host plant, finger millet. Further, mitochondrial modifications were observed in epidermal cells of rice cv. Asahi 12 hr after inoculation of M. grisea spores. These results indicated that mitochondrial modification induced by the toxin was a host-specific phenomenon and possibly an essential event for M. grisea to establish basic compatibility with host plants at the species level.

Suppressor- and Elicitor-Activities of Magnaporthe Grisea Toxin in Rice Leaves

The rice blast fungus Magnaporthe grisea (Hebert) Barr (anamorph Pyricularia oryzae Cavara) causes the most serious disease of rice. Many genetic studies have demonstrated that blast resistance of rice plants is controlled by several major gene pairs, and that resistance against the rice blast fungus differs among rice cultivars (Kiyosawa, 1974). Resistance or susceptibility of rice plants to M. grisea is determined by race-cultivar combinations (Yamada et al., 1976). Further, the fungus can also infect other Graminae such as barley, Italian ryegrass, and corn (Asuyama, 1965; Yaegashi 1981). Thus, the rice blast fungus offers an excellent system for studying molecular determinants of host species specificity and cultivar specificity. Host specific-toxins (Nishimura and Kohmoto, 1983; Otani et al., 1995; Scheffer and Yoder, 1983; Walton, 1996; Yoder, 1980) or suppressors (Doke et al, 1980; Oku et al, 1987) are well known as pathogenicity factors of fungal pathogens. In the M. grisea-plant system, however, the pathogenicity factors involved in host susceptibility have not heretofore been demonstrated.

Influence of phototropic response of spore germ tubes on infection process in Colletotrichum lagenarium and Bipolaris oryzae

The germ tubes ofColletotrichum lagenarium showed negative phototropism to UV-blue (300–520 nm) and far-red (>700 nm) regions with maximum in the near ultraviolet (NUV) region, while monochromatic radiations of 575–700 nm (yellow-red region) induced positive phototropism with maximum in the red region. Green light (520–575 nm) was ineffective. Negative phototropism-inducing wavelength regions inhibited germ tube growth and positive phototropism-inducing wavelength regions promoted it significantly.Bipolaris oryzae did not show any phototropic response and light did not affect the germ tube growth. These results indicate that the lens effect, in combination with the light growth reaction and light growth inhibition, is the mechanism of the phototropism of germ tubes ofC. lagenarium. NUV radiation, which induced negative phototropism ofC. lagenarium, promoted appressorium formation, while red light, which induced positive phototropism, suppressed it significantly. In the case ofB. oryzae, light did not affect the infection structure formation. These results indicate that negative phototropism of germ tubes ofC. lagenarium favors the infection process by facilitating the contact of the tips of germ tubes with the host surface, while positive phototropism has the opposite effect.