Search for Antifungal Proteins as New Resources of Host Resistance

Incorporation of exotic resistance genes had been a major means in the breeding to rice blast (Pyricularia oryzae) resistance. But such new resistances break down sooner or later.

Table 5.2. Pathogenesis-related proteins found in floral organs

Proteins Plants Organs Reference

RNAses Rosaceae Pistil Sassa et al 1993.8

PR-1 Camellia Pistil Tomimoto et al 1999.13

Chitinase Rice Husk Nakazaki et al 1997.12

Chitinase Rice Pistil Takei et al 1998.14

Thaumatin Rosaceae Pistil Sassa et al 1998.15

In early 1990s, we have initiated a minor study to identify self-incompatibility genes of fruit trees in Rosaceae, and identified a series of pistil-specific RNAses which corresponded to the self-incompatibility genes.8 That work suggested that plants may contain a variety of proteins against disease infection, and that such proteins may be utilized in breeding for host resistance. The proteins for self-incompatibility might be recruited from one of the antifungal proteins in floral organs, which seem to be most vulnerable to fungal infection.9

In the light of recent work to isolate host resistance genes of tomato and rice against bacterial diseases, the host resistance genes seem to function in signal transduction and do not seem to be antifungal agents for invading organisms. At the end of such responses there seem to be a set of proteins which are called pathogenesis-related proteins (PR proteins). They are initially defined as those induced by viral or fungal infection or by some chemicals like salicylic acid and benzothiadiazole.10 They can be involved as antiviral or antifungal agents in a systemic acquired resistance, because such proteins directly inhibit propagation of pathogens. 11 What, then, can be seen if such proteins are incorporated and constitutively expressed in plants ? In fact, the antifungal function of chitinases and PR-1 have been proved by gene transformation or in vitro testing.

In our preliminary search for PR-like proteins, some genes which encode novel PR-1 or chitinases were isolated (Table 5.2).12,13 They are basic types and endogenously induced in the course of development. So, their functions may be different from those expressed by other PR-l proteins, but the isolation of such types of PR proteins, and their subsequent incorporation into rice plants, are expected to reveal a new aspect of host resistance.

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