Shigeyuki Mayama

The role of avenalumin in the resistance of oat to crown rust, Puccinia coronata f. sp. avenae

The relationship between the degree of expression of resistance and the production of avenalumins was investigated in the interactions of 2 physiological races of Puccinia coronata avenae and 21 oat Pc lines each carrying a different single major gene for resistance. Avenalumins accumulated in all incompatible interactions. More rapid and abundant accumulation of avenalumins was found in the more incompatible interactions where fungal growth was more rapidly restricted. Large accumulation of avenalumins coincided with the time of detection of retardation of hyphal growth within leaf tissues. Very little accumulation of avenalumins was found in any compatible interaction. Investigations with the incompatible Shokan 1-race 226 combination showed that the amount of accumulated avenalumins was linearly correlated with infection rate through stomata, and that no avenalumins were detected in areas adjacent to or apart from the infected area. It was thus considered that avenalumins accumulate locally in infected stomatal sites. In highly resistant interactions, the estimated concentration of avenalumins in an infected stomatal site greatly exceeded the inhibitory level for growth of germ tubes from uredospores. Avenalumins could also diffuse into intercellular spaces of infected leaves. It is postulated that the production of avenalumins is regulated as a result of resistance gene expression and is effective against the development of crown rust fungus in oat leaves.

The production of phytoalexins by oat in response to crown rust, Puccinia coronata f. sp. avenae

The antifungal compounds accumulated only in the incompatible host-parasite combinations; the time of rapid accumulation coincided with the time of the retardation of the growth of infection hyphae. Three major compounds were isolated. They are highly hydrophilic nitrogen-containing phenolics. Their antifungal activities against Puccinia coronata avenae and P. graminis tritici were demonstrated. They were regarded as phytoalexins and given the trivial names avenalumins I, II, and III. Avenalumins are the first nitrogen-containing phytoalexins found in plants and they are the first chemically-identified ones found in cereal plants in response to rust fungi.

Production of nematocidal compounds by hairy root cultures of Tagetes patula L.

SummaryMarigold (Tagetes patula L.) hairy roots induced by infection with Agrobacterium rhizogenes produced α-terthienyl when grown in darkness, and an n-hexane extract of the roots showed nematocidal activity. Depending on the hairy root line used, the level of α-terthienyl varied from 15 to 1268 μg per g dry weight, a level that corresponded to 0.15 to 12.7-fold that in intact roots. Analysis by HPLC indicated that the nematocidal activity was due predominantly to α-terthienyl. However, it is suggested that nematocidal compounds other than α-terthienyl are present in hairy roots cultured in the dark for long periods or in the light.

Isolation and structure elucidation of genuine oat phytoalexin, avenalumin I

Abstract The major phytoalexin from oat leaves has been identified as 2-[2-(4-hydroxyphenyDethenyl] -6-hydroxy-4H-3,1-benzoxazin-4-one ( A ).

Microspectrophotometric analysis of the location of ] avenalumin accumulation in oat leaves in response to fungal infection

Abstract Cellular localization of avenalumins accumulated in oat leaves infected by either crown rust through stomata or rice blast fungus through the cuticular layer was demonstrated by microspectrophotometry using the fluorescence emission and u.v. absorption spectra characteristic of avenalumins. Thin sections of incompatible rust-infected leaves revealed collapsed mesophyll cells which emitted autofluorescence and absorbed u.v. light with spectral characteristics close to those of the avenalumins. Autofluorescent and u.v.-absorbing cells were detected only in incompatible rust race-oat combinations. Epidermal cells accumulated quite high levels of avenalumins in response to cuticular infection by the facultative pathogen. Spectrophotometry of the freshly stripped epidermis showed the presence of autofluorescent materials, with spectra characteristic of avenalumins, in infected and the directly adjacent cells. U.v.-absorption spectra close to those of avenalumins were also detected in fluorescent cells. Extraction of epidermal strips with methanol reduced fluorescence emission and u.v.-absorption in fluorescent cells by c . 40% of the original, thus suggesting that avenalumins could be partially eluted out of the fluorescent cells. Epidermal cells which were injected with aqueous solution of avenalumin I became fluorescent. Results suggest that avenalumins accumulate in the fluorescent mesophyll and epidermal cells formed in oat leaves at infection sites of fungal pathogens.

Effects of elevated temperature and α-aminooxyacetate on the accumulation of avenalumins in oat leaves infected with Puccinia coronata f. sp. avenae☆

Abstract The rapid accumulation of avenalumins in incompatible crown rust-infected Shokan 1 oat leaves which occurred at 20 °C was greatly inhibited when infected plants were grown at 25 to 35 °C. At 30 °C, avenalumin accumulation was almost totally inhibited and much greater hyphal growth occurred than at 20 °C. When infected plants were grown at 20 °C for 36 h and then transferred to 30 °C, most of the previously-accumulated avenalumins disappeared rapidly within 48 h and hyphal growth continued. However, no subsequent hyphal growth resulted when the plants were transferred to 30 °C at 72 h after inoculation. Treatment of plants with α-aminooxyacetate, a potent competitive inhibitor of phenylalanine ammonia-lyase, greatly inhibited the rapid accumulation of avenalumins in normally incompatible interactions. This was correlated with an increase in susceptibility of the host plants. Significant inhibition of avenalumin accumulation was observed 48 h after inoculation when inoculation was followed by application of the inhibitor at 10 μ m , but much less inhibition of hypersensitive host cell collapse occurred in the treated leaves. These facts suggest that rapid and large accumulation of the avenalumins is crucial for resistance expression in oat leaves against an incompatible crown rust race.

Role of Phytoalexins in Host Defense Reactions

Phytoalexins are antimicrobial compounds which are synthesized by and accumulate in plants at the infection sites of pathogenic microorganisms. It has been indicated that phytoalexins could be responsible for inhibition of the growth of pathogens in plants and regulation of the plant-parasite interactions (Ebel, 1986; Keen, 1982; Mansfield, 1982). It does not imply, however, that the production of phytoalexins is the only response of the plant in functional defense. Besides the production of low molecular phytoalexins, the activation and accumulation of preformed compounds (Tani et al., 1982), lignification (Asada et al., 1979) and hydroxyproline-rich protein deposition in cell walls (Hammerschmid et al., 1984) are also induced at the sites under stress of infection and are involved in defense reactions of some plants.