Inhibition of pre-penetration development of blast fungus during the infection of resistant rice cultivars

Abstract

Spore germination and appressorium formation of the rice blast fungus, Magnaporthe grisea, were compared during infection of rice cultivars differing for their resistance to strain H5-3. On leaves of the susceptible cv. Sha-tiao-tsao neither spore germination nor appressorium formation were inhibited, in comparison to a control germination test in water on polystyrene. On the resistant cv. Tadukan, both spore germination and appressorium development were strongly inhibited whereas on the partially resistant cv. Shimokita a slight inhibition of germination and significant delay of appressorium formation was observed. Inhibitors of germination were not produced in leaf diffusates of the susceptible cultivar Sha-tiao-tsao. On the contrary, such compounds were excreted by leaves of the two resistant cultivars. The fungitoxicity of diffusate of cv. Tadukan was strong from 5 to 24 h after inoculation. The diffusate of cv. Shimokita was less toxic at 5 h but at 24 h it reached the same level of toxicity as diffusate of cv. Tadukan. Such antifungal plant metabolites accumulating in drops of inoculum may inhibit the pre-penetration development of the rice blast fungus and presumably contribute to disease resistance, including the partial resistance. Plant resistance to fungal diseases might be associated with the inhibition of pathogen development in host tissues during early stages of the infection. The rice blast fungus Magnaporthe grisea does not penetrate readily into leaves of some resistant rice cultivars (Heath et al., 1990; Koga and Kobayashi, 1982) or cannot spread over the first invaded cells (Arase et al., 1983; Peng et al., 1988; Kaur et al., 1984). Inhibition of spore germination and appressoria formation were observed during inoculation of leaf fragments from completely resistant rice cultivars (Lapikova and Dzhavakhiya, 1987). This may be due to plant defence responses (syntheses of lignin, phytoalexins and pathogenesis-related proteins etc.) which may liberate fungitoxic products into the extracellular space. Previously, we found that the partially resistant cultivar Shimokita in combination with strain H53 manifests a reduced infection frequency, smaller lesions and a reduced rate of spore production per lesion (Pasechnik et al., 1995), phenomena typical for partial resistance (Roumen et al., 1993). The prepenetration development of the rice blast fungus on such rice cultivars is not well documented. The aim of the present work was to assess whether fungal development is affected on the leaf surface of cv. Shimokita. The susceptible cv. Sha-tiao-tsao and the completely resistant cv.Tadukan were taken as controls and intact plants rather than leaf fragments were used for the assessment. We used strain H5-3 (race 007) of the rice blast fungus, Magnaporthe grisea (Herbert) Barr. The rice (Oryza sativa L.) cultivars differ in their resistance to race 007: the susceptible cv. Sha-tiao-tsao develops numerous lesions, on the completely resistant cv. Tadukan no visible symptoms are produced and cv. Shimokita exhibits partial resistance with the rare occurrence of compatible-type lesions. Plants were grown in a climatic chamber up to the fourth fully expanded leaf stage (Lapikova et al., 1994). The fourth leaf was used for the analyses. The central part of the leaf was fixed horizontally in an inoculation chamber (Lapikova et al., 1995) and a plastic ring (ID 3 mm, height 1 mm) was placed on the leaf. One drop (10 l)