Introgression of crown rust resistance from diploid oat Avena strigosa into hexaploid cultivated oat A. sativa by two methods: direct crosses and through an initial 2x+4x synthetic hexaploid

Authors: Rines, Howard; Porter, Hedera; Carson, Martin; Ochocki, Gerald

Submitted to: Euphytica International Journal on Plant Breeding
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2007
Publication Date: 4/14/2007
Citation: Rines, H.W., Porter, H.L., Carson, M.L., Ochocki, G.E. 2007. Introgression of crown rust resistance from diploid oat Avena strigosa into hexaploid cultivated oat A. sativa by two methods: direct crosses and through an initial 2x+4x synthetic hexaploid. Euphytica. 158:67-79.

Interpretive Summary: Oat crown rust is the major disease of the cereal oat in the Upper Midwest, the primary region of oat grain production in the U.S. Incorporation of genes into oat for resistance to the pathogen has proven the most economical and ecologically friendly way to protect the crop from disease effects. New sources of resistance genes are constantly needed, however, because resistance genes often become ineffective within a few years as new strains of rust not sensitive to currently used resistance genes become prominent due to selection. Wild oat species related to cultivated oat are a rich source of new resistance genes, but there are often genetic barriers to the ready transfer of genes from wild to cultivated oats. We used two methods to try to transfer resistance genes identified in a wild relative that has only one set of 7 pairs of chromosomes, the physical structures of the genetic material in a plant or animal cell, into cultivated oat whose cells each contain triplicate sets of 7, or 21 total, chromosome pairs. One method used was to make a direct cross between the wild species with 7 pairs of chromosomes and a cultivated oat with 21 pairs of chromosomes and then use special techniques to recover an infrequently formed hybrid embryo and grow it to a plant. The other method involved crossing the resistant wild oat with 7 chromosome pairs by another wild oat species that has 14 chromosome pairs to produce a hybrid plant with 21 chromosome pairs, the same number as in cultivated oat. By crossing the two types of hybrid plants produced by these two methods several times back to cultivated oat, each time selecting for rust disease resistance, we were able to recover plants that were genetically essentially cultivated oat, but with the desired new crown resistance genes incorporated. The oat lines produced containing the newly introduced genes for crown rust resistance, as well as information on the techniques used to produce them, provide oat breeders new materials and tools for producing crown rust resistant oat varieties.

Technical Abstract: New sources of resistance to oat crown rust, Puccinia coronata f. sp. avenae (Eriks.), the major fungal disease of cultivated oat, Avena sativa L. (2n = 6x = 42), are constantly needed due to frequent, rapid shifts in the virulence pattern of the pathogen. Crown rust resistance identified in the diploid oat A. strigosa (Schreb.) (2n = 2x = 14) accession CI6954SP was transferred into cultivated oat using two methods: direct cross of the diploid to a hexaploid cultivar facilitated by embryo rescue, and initial cross of the diploid to a wild tetraploid oat to make a synthetic hexaploid for subsequent crossing to a hexaploid cultivar. Two tetraploids, a crown rust resistant A. murphyi (Ladiz.) accession P12 and a susceptible A. insularis (Ladiz.) accession INS-1, were used in the 2x+4x crosses. Resistant backcross-two derived lines were recovered by both methods. Although the 2x+4x synthetic method did not require the laborious discovery and rescue of an infrequent initial hybrid embryo of the direct cross, the direct cross method provided more rapid backcross recovery of plants with high fertility, full transmission of resistance, and desired plant and seed phenotypes. A suppressor effect, present initially but segregating in backcrosses, appeared to come from the CI6954SP donor and is the same as, or analogous to, suppression by the crown rust resistance gene Pc38. No resistance from A. murphyi P12 was detected in advanced generations, indicating suppression of its resistance in interploidy combinations. That the dominant resistance gene transferred from CI6954SP and a gene Pc94 introgressed earlier from a different A. strigosa accession may be the same or quite similar to one another is indicated by their in-common specificity to suppression of resistance expression, susceptibility to a newly recovered rust isolate, and initial close linkage to the molecular marker SCAR94-2. The introgressed resistance genes from the different sources, even if the same, may have different cultivar genome introgression sites, which would allow tests of dosage effects on resistance expression.