Effect of three suppressors on the expression of powdery mildew resistance genes in barley

Three recessive mutagen-induced alleles that partially suppress the phenotypic expression of the semidominant powdery mildew resistance gene Mla12 have been studied. When each suppressor is present in homozygous condition, the infection type 0, conferred by gene Mla12 when homozygous, is changed to intermediate infection types. The three suppressor lines were crossed with seven near-isogenic lines with different powdery mildew resistance genes and one, M100, was crossed with nine additional lines. Seedlings of parents and from the F1and F2 generations were tested with powdery mildew isolates that possessed the appropriate avirulence and virulence genes. The segregation of phenotypes in the F2 generation disclosed that the three suppressors affected the phenotypic expression of three resistance genes, whereas that of four resistance genes remained unaffected. The suppressor in mutant M100 affected the phenotypic expression of 9 of the 10 additional resistance genes present. It is suggested that the three suppressors are mutationally modified genes involved in host defence processes. This implies that different resistance genes employ different, but overlapping, spectra of defence processes, or signal transduction pathways. Key words : barley, Hordeum vulgare, powdery mildew, Erysiphe graminis hordei, mutation, resistance, suppressor.

Genetic analysis of barley mutants with modifications of powdery mildew resistance gene Ml-a12

Fifteen mutants with increased powdery mildew susceptibility (infection types between 0–1 and 3–4) were crossed with the Ml-a12 resistant mother line 'Sultan-5' (infection type 0) and the susceptible variety 'Carlsberg II'. Analysis of the material revealed that 13 mutants had mutational modifications of the Ml-a12 gene. Two mutants had a suppressor-mutant gene that modified the phenotypic expression of gene Ml-a12. One suppressor gene was recessive, the other was semidominant. The possible function of gene Ml-a12 is discussed.Key words: barley, Hordeum vulgare, powdery mildew, Erysiphe graminis hordei, mutation, resistance, suppressor.

Modification of barley powdery mildew resistance gene Ml-a12 by induced mutation

Kernels from a barley line, 'Sultan-5', with powdery mildew resistance gene Ml-a12 were treated with mutagens. Among 10 381 M1 spikes progeny tested with Ml-a12 avirulent powdery mildew, 25 segregated mutants with infection types between 0–1 n and 3–4cn. The resistance of the mutants is race specific in the sense that it is expressed only with powdery mildew cultures that are Ml-a12 avirulent but not with an Ml-a12 virulent culture. Genetic analysis of 10 mutants revealed that 9 had mutant genes that were allelic to gene Ml-a12, and one had a recessive mutant gene inherited independently of Ml-a12 on which it acted as a suppressor. The high mutation frequency in gene Ml-a12 and the gradual inhibition of the expression of gene Ml-a12, by mutation or suppression, strongly supports the suggestion that the gene function is associated with incompatibility rather than with compatibility.Key words: barley, Hordeum vulgare, powdery mildew, Erysiphe graminis hordei, mutation, resistance, suppressor.

Cytological studies of the early stages of powdery mildew in barley and wheat. XI. Autofluorescence and haloes at penetration sites of appressoria of Erysiphe graminis hordei and Erysiphe pisi on barley coleoptiles

Cytological and physiological comparisons between autofluorescence and haloes which appeared at penetration sites of appressoria of Erysiphe graminis and Erysiphe pisi on barley coleoptiles were made using dye stains and fluorescence microscopy. Haloes induced by these fungi on barley coleoptiles were successfully detected by various dyes when the inoculated coleoptiles were treated with 70% ethanol at 80 °C for 30 min before staining. Responses of haloes on coleoptiles to basic and acid dyes were very similar to those on barley leaves which have been reported earlier. In time-course studies, haloes were not detected until at least 15 min after initiation of cytoplasmic aggregation. The occurrence of haloes was suppressed by supplemental Ca2+, but unaffected by K+, Na+, and Li+. Similar results were obtained in haloes induced by both fungi. Autofluorescent regions remained in situ after the inoculated coleoptile cells were plasmolyzed, suggesting that this phenomenon might be closely associated with host cell walls. As reported earlier, autofluorescence preceded the appearance of cytoplasmic aggregates by 1–10 min and, moreover, its appearance was enhanced by divalent cations such as Ca2+, Mg2+, and Mn2+, but not by monovalent cations such as K+, Na+, and Li+. Based on these results and earlier studies, it was concluded that the autofluorescence and haloes represented different responses in host cell walls to the penetration activities of both pathogenic and nonpathogenic fungi.