Differences in Hypersensitive Reaction among Rice Cultivars Carrying Various Resistance Genes to the Blast Fungus, Pyricularia oryzae.

Abstract

Using several rice cultivars including near isogenic lines of Nipponbare with various resistance genes, the role of hypersensitive cell death in resistance to infection by Pyricularia oryzae was investigated in order to establish whether hypersensitive death of infected cells played the same role in expression of each resistance gene of the host. A close correlation was found between the percentage of dead cells 12 h after inoculation and the degree of resistance evaluated by the number of lesions 7 days after inoculation. Cultivars with a higher rate of hypersensitive cell death were considered to be more highly resistant. Toride 1, which has Pi-z(t), however, showed a lower rate of cell death, in spite of no visible lesions that would permit classification as highly resistant. Heat shock treatment caused a remarkable reduction in cell death 12 h after inoculation, resulting in the infection hyphae being developed well into the epidermal cells of the resistant cultivar. This indicated that reduction of resistance against blast fungus by thermal treatment was related to reduction in ability to show hypersensitive cell death. Seedlings of several cultivars and isogenic lines of Nipponbare, which carried various resistance genes and which had been heated, were sprayed with a spore suspension of an avirulent fungus strain. Many lesions were observed on heated leaves of the resistant cultivars carrying the effective resistance genes Pi-k, Pi-ta or Pi-z. No visible lesions were, however, observed on heated leaves of Toride 1 and Nipponbare-z(t) having the resistance gene Pi-z(t), suggesting that the role of hypersensitive cell death in the resistance mechanisms of cultivars having Pi-z(t) differed from that of other resistance genes.