Many fungitoxic compounds in plants occur in vivo as inactive glycosides. The active aglycone appears only as a result of pathological or mechanical damage presumably due to the action of a host specified glycosidase activity. For the b-glucosidase specified by the gene glu (Pryor, MNL 50:15, 1976) this supposed role can be tested by determining if plant resistance is still expressed in an individual homozygous for the Null allele glu-N, and thus lacking b-glucoside activity. Lines carrying different genes for resistance to the rust Puccinia sorghi were derived from the International Monogenic lines developed by Hooker. The test individuals were produced among the progeny of the following back-cross:

Rp glu-A X rp glu-N

Rp glu-N rp glu-N

and the results are presented in Table 1. Although the data are small the conclusion is clear. The expression of resistance was unaffected by the absence of glucosidase activity. The degree of resistance in the two resistance classes (Table 1) was indistinguishable. This conclusion applied to 3 alleles at the Bp locus and to the Rp4 and Rp5 loci.

Table 1.

The genetic data in two crosses deviate significantly from the expected 1:1 segregation of alleles at the Rp and glu loci. However the number of progeny is small.

On the other hand the segregation patterns of the resistance loci on chromosome 10 with the glu locus deviate very markedly in three of the four crosses, suggesting that Rp and glu may be linked. A pooled estimate (Table 1) from the relevant crosses indicates about 33% recombination between Rp and glu.

A. J. Pryor

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