The Transfer of Crown Rust Resistance Gene Pc94 from a Wild Diploid to Cultivated Hexaploid Oat
Taing Aung*, James Chong and Mike Leggett
Cereal Research Centre, Agriculture and Agri-Food Canada, 195 Dafoe Road
Winnipeg, MB, Canada, R3T 2M9
Crown rust Puccinia coronata Cda. f.sp. avenae Eriks. is the major foliar disease of cultivated oats in Canada and the United States. Estimates of the reduction of the grain yield, based on yearly reports of Cereal Rust Laboratories U.S. Department of Agriculture, caused by the pathogen vary from 10% to 35% depending on the severity of the infection and the cultivar used. The ability of the crown rust pathogen to produce new biotypes capable of overcoming previously resistant cultivars usually within a few years has led to a continuous search for further sources of resistance. Several wild diploid species of oat particularly, Avena strigosa, confer high degree of resistance to crown rust. Highly effective crown rust resistance gene(s) was identified in a genotype of diploid oat A. strigosa Schreb and the genotype was desiganated as (RL1697) at CRC, Winnipeg. The resistance diploid genome of RL1697 was combined with the hexaploid genome of oat cultivar SunII in the form of a synthetic octaploid (2n = 56). Resistant disomic addition lines were generated in the subsequent backcross progenies from the crosses between the octaploid and SunII recurrent parent. The resistance from the RL1697 chromosomes in the addition lines was introgressed into the SunII genome through induced chromosome pairing procedure. We report on the successful transfer of crown rust resistance from A. strigosa (RL1697) to cultivar SunII.

The diploid oat A.strigosa (RL1697) resistant to crown rust race CR152 was obtained from the Institute of Grassland and Environmental Research, Wales, UK. in 1991. The F1 plants resistant to CR152 were produced from the reciprocal crosses between (RL1697) and A. strigosa {Saia(Wpg)} which is susceptible to CR152. The selfed progeny of the diploid F1 plants segregated in a ratio of 3 R : 1 S, indicating that the RL1697 resistance is inherited as a single dominant gene. Oat cultivar SunII (pistillate parent) was crossed with RL1697 (pollen parent) and the F1 hybrids were treated with 0.2% colchicine to produce three synthetic octaploids (2n = 56). Resistance reaction observed in the synthetic octaploids (2n = 56) indicated that the resistance confer by RL1697 genome when combined with SunII genome is expressed. Resistant plants with 2n = 44, 45 chromosome (disomic addition) were selected from the selfed progenies of the BC1 plants produced from a cross between SunII and the octaploid. The selected resistant chromosome addition lines were cytologically unstable due to addition decay (loss of added RL1697 chromosomes resulted in the loss of the resistance) which indicate that breed-true resistance line cannot be establish from the disomic RL1697 chromosome addition line. . The disomic addition lines were then crossed to another synthetic octaploid (2n=56) derived from a cross between SunII (hexaploid) and A. longiglumis,CW57 (diploid) and the resistant F1 plants so produced were backcrossed to SunII as pistillate parent for five times. The homozygous resistant plants which breed true for resistance in the selfed progenies of BCV populations from heterozygous resistant lines which segregated in a monofactorial ratio of 3 resistant : 1 susceptible were produced. The homozygous and heterozygous plants with 42 chromosomes were selected from the F3 generation of the BCV progenies. The selected resistance lines exhibited no cytological abnormality (no loss or gain of chromosome, no unusual chromosome pairing at meiosis) all the heterozygous resistant lines segregated in 3R:1S ratio and all the homozygous lines breed true for resistance. This confirms the transfer of crown rust resistance gene from A. strigosa (RL1697) to cultivar SunII. One homozygous line which is cytologically stable (2n=42 and meiosis with regular 21 bivalents) and with agronomically desirable characteristics was selected, the line was designated as S42, the resistance gene transferred from RL1697 to SunII is designated as Pc94. Subsequent evaluations showed that Pc94 is highly resistant (0 ; reaction type) to crown rust tested in the green house and under field conditions in Manitoba and in New Zealand.

Introgression of genes from species of lower ploidy (diploids and tetraploids) into the genome of hexaploid oat cultivars is very difficult to achieve by means of regular backcrossing procedures. This is mainly due to the low frequency with which the chromosomes of the wild species pair with those of cultivated oat, consequently reducing the chance of recombination and gene transfer. It is well documented that a genotype of A. longiglumis (CW57) suppress the activity of the gene or genes controlling regular bivalent pairing in A. sativa and induced pairing between non homologous chromosomes that resulted in translocation between two homoeologous chromosomes as well as between unrelated chromosomes. The ability of CW57 gene(s) in inducing pairing between A.strigosa (RL1697) chromosome and A.sativa chromosome probably have affected the transfer of the RL1697 resistance into SunII genetic background.

S42 was used as pollen parent and crossed to 11 North American oat cultivars (AC Belmont, AC Marie, AC Preakness, Caliber, Dal, Derby, Dumont, Jasper, Riel, Robert, Steele). The resistance gene Pc94 is inherited as dominant gene in five oat cultivar (SunII, Calibre, Dal, Derby and Jasper) genetic backgrounds. However, Pc94 is inherited as a recessive gene in seven cultivar (AC Belmont, AC Marie, AC Preakness, Dumont, Riel, Robert and Steele) genetic backgrounds. Since crown rust resistance gene Pc38 is present in all the above seven cultivars and the expression of Pc94 is modified in all these cultivars it is logical to conclude that Pc94 expression is suppressed by Pc38 (J. Chong & T. Aung , see the following article). However, the modification of Pc94 gene expression could also be due to reversal of dominance to recessive nature of the gene in some genetic backgrounds similar to the phenomena reported in wheat with reference to leaf rust resistance genes. While Pc94 is proven to be a highly effective gene, there is still a need to deploy this gene in combination with other Pc genes with the aim to provide adequate protection to the oat crop from the ravages of the disease and to prolong the life of the resistance gene. The S42 genotype with the Pc94 gene has provided a valuable new source of resistance to crown rust for oat improvement programs in Canada.

*Corresponding author (Email: TAUNG@EM.AGR.CA)


Figure Legends:

Fig.1. Crown rust infection on leaves of oat cultivar SunII with and without PC94: Left to right; a pair of leaves with two doses of PC94 gene (homozygous resistant S42), a pair of leaves with one dose of PC94 gene (heterozygous resistant S42) and a pair of leaves without PC94 (susceptible SunII).

Fig.2. Homozygous resistant S42 line tested in crown rust nursery at Glenlea (middle row).

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