Cell biology of host resistance to the disease caused by pine-wood nematodes

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1. The role of corical resin canals in the early development of a pine wilt disease in Japanese black pine, pinus thunbergii, was studied. A part of the bark of a 2 cm-long segment from a current-year stem was removed by a tangential cut with a razor blade. Both cortex-exposed segments with cut corical resin canals (deignated as +RC-segments) and those without cut resin canals (-RC-segments) were obtained by chance. When a virulent nematode isolate (S6-1) was inoculated onto the cut surface, the surface of the +RC-segments turned brown 4 d after inoculation, and, in some segments, this browning occurred more intensely around cortical resin canals. When segments were cut transversely at the middle, the transverse cut surface of the inoculated +RC-segments was brown and fragile but that of the inoculated -RC-segments was pale green and stable as the non-inoculated controls. Correspondingly, tissue cells including epithelial cells of the cortical resin canal of the +RC-segments were all d … More ead but those of the -RC-segments were alive as the controls. When nematodes were inoculated onto the inner surface (cambium side) of a bark peeling at which cambial xylem cells were exposed, they did not kill the cambial cells. When inoculated on the transverse cut-end of a bark peeling from which nematodes would enter resin canals, nematodes killed all cells in the peeling. The above results suggest that nematodes do not kill directly cortical cells but become harmful to pine cells after living in cortical resin canals.2. We examined then harmfulness of nematodes at different growth stages and found that nematodes those at older stages do not kill pine cells but those at younger stages do.3. We found by experiments using cortex-exposed segments and filter wells that nematodes at younger stages killed pine cells even but not those at older stages, when filter was inserted between them, preventing nematodes from direct contact with pine tissue. This result means that nematodes at younger stages produce and excrete some killing substances but not those at older stages.4. We examined next the difference in resistance aginst nematodes between Pinus species which are more resistant than P.thunbergii, using cortex-exposed segments and bark peelings of young stems. Cortex-exposed segments and bark peelings of young stems of more resistant species were not killed by nematodes at younger stages, indicating that the resistance is derived from resistance of tissue cells of itself.3. We confirmed resistance induction by preinfection with an avilurent strain of nematode, and studied the mechanism of the resistance induction anatomically. Less