Ribonuclease activity of wheat leaves and rust infection

REMARKABLE changes in the levels and properties of ribonuclease (RNase) occur following the inoculation of a susceptible variety of flax (Linum usitatissimum L., var. Bison) with flax rust uredospores (Melampsora lini (Ehrenb.) Lev, race 3)1. The increase in RNase activity occurs in two distinct phases; an early phase between 2 to 4 d after inoculation (early RNase) and a later phase, beginning 5 d after inoculation (late RNase). These increases in RNase activity are accompanied by substantial changes in the properties of the enzyme including thermal stability, diethyl-pyrocarbonate-sensitivity and substrate preference1. Further studies (A. K. C., M. S., and L. A. S., unpublished) have revealed that these changes in the properties of RNase are due, at least in part, to the formation of new RNase molecules. These have kinetic and catalytic properties that are quite distinct from both of those ‘host’ RNase from healthy flax cotyledons and ‘rust’ RNase from flax rust uredospores or from rust mycelium grown axenically in a chemically defined medium. That changes in late RNase activity are specifically elicited by compatible host-parasite interactions is indicated by the appearance of late RNase which is not detectable in a resistant variety of flax (Bombay) inoculated with race 3 of Melampsora lini1. Furthermore, the characteristic bimodal increase in RNase activity and the accompanying changes in various properties of the enzyme have been found in a number of other rust-infected plants including pine and Ribes (A. E. Harvey, A. K. C., M. S. and L. A. S., unpublished), and mechanical injury of the healthy host tissues causes a significant increase in the level of RNase. The enzyme remains strictly ‘host-type’, however, no change in its physical or catalytic properties being detectable (ref. 1 and A. E. Harvey, A. K. C., M. S., and L. A. S., unpublished). Here we describe the effect of inoculation with uredospores of three races (56, 56A and Australian race 126 ANZ-6, 7) of wheat stem rust (Puccinia graminis (Pers.) f. sp. tritici Eriks. and E. Henn.) in causing quantitative and qualitative changes in the RNase activity of paired, near-isogenic lines of wheat (Triticum aestivum L.) derived from the variety Chinese Spring and carrying the alleles Sr6 and sr6 (refs 2 and 3), respectively. Both lines give susceptible reactions (infection type 4 (ref. 4)) with race 56A, independent of temperature. The homozygous recessive, sr6, imparts susceptibility (infection type 4) to races 56 and 126 ANZ-6, 7. The dominant allele, Sr6, confers a temperature sensitive reaction to these races; resistance is expressed (infection type 0 (ref. 4)) at 19 to 21° C but breaks down (infection type 4) at higher temperatures (25 to 26° C). A few intercellular hyphae are produced at each infection site at 19 to 21° C. These survive for several days and resume growth when infected plants are transferred to 25 to 26° C; but, in our experience, the longer infected plants are held at 19 to 21° C before transfer, the smaller are the number and size of the sporulating pustules to be developed.