Nakato Naito

Initiation of resistance and host cell collapse in the hypersensitive reaction of oat leaves against Puccinia coronata avenae

Abstract The relationship between expression of resistance and host cell collapse was studied for the hypersensitive resistance of oat cultivar Shokan 1 infected by incompatible race 226 of Puccinia coronata avenae. Microscopic examination of primary leaves of seedlings treated by several methods involving heat treatment, stem detachment and double inoculation indicated that initial events determining the host resistance occurred between 8 and 12 h after inoculation, when the fungus produced substomatal vesicles. The initial events were followed by the reduction of growth of infection hyphae to half that in susceptible leaves. During the period of prehaustorial growth of the fungus, neither host cell collapse at infected sites nor permeability increase of leaf tissues was observed. The cessation of hyphal growth and haustorial development appeared to be normally concomitant with host cell collapse at 35 h after inoculation. However, study of leaves treated with blasticidin S or ethidium bromide or heat revealed that the limitation of fungal development in resistant reactions was not correlated with the degree of host cell collapse. The development of a normally compatible race was restricted without any collapse of mesophyll cells when the leaves were previously inoculated with the incompatible race and then heated. We propose that in the Shokan 1-race 226 host-parasite system, the initial determinative events for resistance are induced independently from the process of host cell collapse. Host cell collapse, although a normal consequence of the resistant reaction, therefore has no determinative role in the expression of resistance of Shokan 1 leaves to race 226.

Selective enhancement of ribosomal RNA synthesis of crown rust-infected oat leaves by stem excision

Abstract In leaves of Avena sativa infected by a compatible race of Puccinia coronata avenae , a rapid increase of ribosomal ribonucleic acid (rRNA) was found when the stem was excised 4 days after inoculation, at which time the fungus was in the beginning of differentiation toward uredosorus formation. Concentrations increased up to 1·5-fold within 4 h after excision, while in the attached control the rRNA concentration increased gradually and reached the same level 16 h later. Stem excision did not affect the concentration of soluble RNA (sRNA) and deoxyribonucleic acid (DNA). This selective increase of rRNA by stem excision was observed neither in the infected leaves 3 and 5 days after inoculation, nor in the noninoculated control leaves. Experiments using [ 32 P]orthophosphate indicated that the increase of rRNA was due to enhanced rRNA synthesis. No appreciable changes were shown for the synthesis of sRNA, tenaciously bound RNA and DNA. Polyacrylamide gel electrophoretic separation of rRNA species revealed that the increase of rRNA was due to cytoplasmic rRNA and not due to chloroplast rRNA.