Breeding oat for resistance to the crown rust pathogen Puccinia coronata f. sp. avenae: achievements and prospects

Crown rust, caused by Puccinia coronata f. sp. avenae (Pca), is a significant impediment to global oat production. Some 98 alleles at 92 loci conferring resistance to Pca in Avena have been designated; however, allelic relationships and chromosomal locations of many of these are unknown. Long-term monitoring of Pca in Australia, North America and elsewhere has shown that it is highly variable even in the absence of sexual recombination, likely due to large pathogen populations that cycle between wild oat communities and oat crops. Efforts to develop cultivars with genetic resistance to Pca began in the 1950s. Based almost solely on all all-stage resistance, this has had temporary benefits but very limited success. The inability to eradicate wild oats, and their common occurrence in many oat growing regions, means that future strategies to control Pca must be based on the assumption of a large and variable prevailing pathogen population with high evolutionary potential, even if cultivars with durable resistance are deployed and grown widely. The presence of minor gene, additive APR to Pca in hexaploid oat germplasm opens the possibility of pyramiding several such genes to give high levels of resistance. The recent availability of reference genomes for diploid and hexaploid oat will undoubtedly accelerate efforts to discover, characterise and develop high throughput diagnostic markers to introgress and pyramid resistance to Pca in high yielding adapted oat germplasm.

Genetic association of crown rust resistance gene Pc68, storage protein loci, and resistance gene analogues in oats

Segregating F(3) families, derived from a cross between oat cultivar Swan and the putative single gene line PC68, were used to determine the association of seed storage protein loci and resistance gene analogues (RGAs) with the crown rust resistance gene Pc68. SDS-PAGE analysis detected three avenin loci, AveX, AveY, and AveZ, closely linked to Pc68. Their diagnostic alleles are linked in coupling to Pc68 and were also detected in three additional lines carrying Pc68. Another protein locus was linked in repulsion to Pc68. In complementary studies, three wheat RGA clones (W2, W4, and W10) detected restriction fragment length polymorphisms (RFLPs) between homozygous resistant and homozygous susceptible F(3) DNA bulks. Four oat homologues of W2 were cloned and sequenced. RFLPs detected with two of them were mapped using F(3) and F(4) populations. Clone 18 detected a locus, Orga2, linked in repulsion to Pc68. Clone 22 detected several RFLPs including Orga1 (the closest locus to Pc68) and three RGA loci (Orga22-2, Orga22-3, and Orga22-4) loosely linked to Pc68. The diagnostic RFLPs linked in coupling to Pc68 were detected by clone 22 in three additional oat lines carrying Pc68 and have potential utility in investigating and improving crown rust resistance of oat.

Genetic and physical mapping of Lrk10-like receptor kinase sequences in hexaploid oat (Avena sativa L.)

Oat receptor-like kinase gene sequences, homologous to the Lrk10 gene from wheat (Triticum aestivum L.), were mapped in oat (Avena sativa L.). PCR primers designed from the wheat Lrk10 were used to produce ALrk10 from oat. Two DNA sequences, ALrk1A1 and ALrk4A5, were produced from primers designed from coding and noncoding regions of ALrk10. Their use as RFLP probes indicated that the kinase genes mapped to four loci on different hexaploid oat 'Kanota' x 'Ogle' linkage groups (4_12, 5, 6, and 13) and to a fifth locus unlinked to other markers. Three of these linkage groups contain a region homologous to the short arm of chromosome I of wheat and the fourth contains a region homologous to chromosome 3 of wheat. Analysis with several nullisomics of oat indicated that two of the map locations are on satellite chromosomes. RFLP mapping in a 'Dumont' x 'OT328' population indicated that one map location is closely linked to Pg9, a resistance gene to oat stem rust (Puccinia graminis subsp. avenae). Comparative mapping indicates this to be the region of a presumed cluster of crown rust (Puccinia coronata subsp. avenae) and stem rust resistance genes (Pg3, Pg9, Pc44, Pc46, Pc50, Pc68, Pc95, and PcX). The map position of several RGAs located on KO6 and KO3_38 with respect to Lrk10 and storage protein genes are also reported.

EVIDENCE FOR COMPLEMENTARY GENE ACTION CONFERRING RESISTANCE TO PUCCINIA GRAMINIS AVENAE IN AVENA SATIVA

Plants derived from a cross between the oat cultivar Kyto (Avena sativa L.) and A. sterilis L. were highly resistant to oat stem rust (Puccinia graminis Pers.) in the adult plant stage. This resistance was much superior to that of either parent. This is the first example of transgressive segregation in this host-parasite system. The genetic mechanism of this resistance is not completely understood but the resistance appears to be conferred by gene Pg-12 from Kyto interacting with a second gene. An alternate hypothesis is that a suppressor gene in Kyto prevents the resistance conditioned by Pg-12 from being expressed in the adult plant stage. This type of resistance is highly effective against all but two of the oat stem rust races known to occur in North America.

RESISTANCE TO PUCCINIA GRAMINIS AVENAE AND P. CORONATA AVENAE IN THE WILD AND CULTIVATED AVENA POPULATIONS OF IRAN, IRAQ AND TURKEY

Tests of over 1400 Avena accessions, comprising eight species, from Iran, Iraq and Turkey have shown that resistance to oat stem rust caused by Puccinia graminis Pers. f. sp. avenae Eriks and E. Henn. occurs infrequently in the Avena populations of the region and was found only in A. barbata Pott ex Link and A. sterilis L. Resistance to oat crown rust caused by P. coronata Cda f. sp. avenae Eriks, was common in A. barbata from Turkey and in A. sterilis from all three countries. The inheritance of resistance conferred by two genes from the region, Pg-15 and Pc-54 is described. Gene Pg-15, the first stem rust resistance gene found in this region, is partially dominant and independent of the Pg-2, Pg-9, Pg-12 and Pg-13 loci. It conferred resistance to races that are important in North America. Gene Pc-54 was also the first crown rust resistance gene to be identified from this region. It was usually recessive, and allelic or closely linked with gene Pc-35.

INHERITANCE OF RESISTANCE TO PUCCINIA CORONATA AVENAE AND ITS ASSOCIATION WITH SEED CHARACTERISTICS IN FOUR ACCESSIONS OF AVENA STERILIS

The inheritance of resistance to oat crown rust Puccinia coronata Cda. f. sp. avenae Eriks. was studied in four accessions of Avena sterilis L. Three of the accessions, CAV 4963, CAV 1358 and CAV 1376, originated from Israel, and one, CAV 1964, from Algeria. Seedling rust tests on F2 backcross families indicated that a single recessive gene, Pc-55, in CAV 4963 conditioned seedling resistance to 10 of 12 crown rust isolates tested. In CAV 1964, a single dominant gene Pc-56 conferred resistance in both the adult and seedling stages to all crown rust isolates tested except race 239, while a second dominant gene conditioned resistance to only two of the twelve cultures used. From adult and seedling tests it appeared that the resistance in CAV 1358 and CAV 1376 was conditioned by a number of recessive minor additive genes. The genes Pc-55 and Pc-56 are not allelic with the A. sterilis derived genes Pc-35, Pc-38, Pc-40, Pc-45, Pc-46, Pc-47, Pc-48, and Pc-50. Genes Pc-39 and Pc-55 are either very closely linked or allelic and Pc-56 is not closely linked to either Pc-39 or Pc-55. The usefulness of genes Pc-55 and Pc-56 was demonstrated in tests which showed that both genes were effective against 99.8 and 94.5%, respectively, of all crown rust cultures isolated in Canada in 1974 and 1975. The genes for seed color and awn character did not appear to be linked to the crown rust resistance genes. In CAV 4963, CAV 1358 and CAV 1376 the genes for grey color and wild type awns appeared to be linked with recombination values of about 2, 23 and 18%, respectively.