Cell physiological studies on resistance of potato plants to Phytophthora infestans.

VII. Growth of intracellular hyphae of Phytophthora infestans in the living potato plant cells which are resistant or susceptible to the infection of them.

Abstract

In the previous paper, it was shown by means of vital staining that the majority of the intracellular hyphae of Phytophthora infestans in cells of a resistant potato variety were still living at an early stage of necrosis of host cell. The present experiments have been carried out, in order to ascertain, furthermore, whether the growth of intracellular hyphae is inhibited or not, before the hyper-sensitive death of host cell. A petiole of potato leaf was cut longitudinally with a razor, and washed with strearning water in order to eliminate the contents of destroyed cells. The cut surface of the petioles, almost all the cells of which were living, was inoculated with P. infestans. Four and 6 hours after inoculation, microscopical sections of the inoculated materials were prepared and fixed with Bouin or Formalin-acetic-alcohol, and then the length of intracellular hyphae was measured with a micrometer. As the hypersensitive death of the host cells, caused by infection with the incompatible race of P. infestans, occurs about 10-15 hours after inoculation, the values thus obtained may indicate the growth rate of the fungus in living cells. In the materials used in the present experiments, penetration took place about 2-2.5 hours after inoculation. The varieties tested were Irish Cobbler (r), Hokdai No. 10 (R₁), Hokkai No. 17 (R₄), 48005-46 (R₄) and 41089-8 (R₁R₄). These varieties were inoculated with compatible or incompatible races of P. infestans: race 0, race 1, race 4 and race 1, 4. The results obtained are as follows. The differences in the growth rate of the hyphae in the living cells is statistically highly significant between races, but not between varieties. The interaction between varieties and races is not statistically significant. From these results, it may be concluded that, even when the host plant cells show resistance to infection by P. infestans owing to presence of resistant genes R₁ and R₄, the growth of intracellular hyphae of the pathogen is not inhibited, at least, within about 4 hours after penetration. From the accumulated knowledge in this respect, it seems that the most evident cytological phenomenon having close connection with late blight resistance is the time elapsed from penetration of the fungus to the hyper-sensitive death of a host cell.