Download Identifying leaf rust resistance in diverse accessions and cultivars of

Identifying leaf rust resistance in diverse accessions and cultivars of wheat
Kathryn Turner
Leaf rust, caused by Puccina triticina Eriks, is one of the most common diseases
affecting wheat, consistently reducing yields by 5-15% with higher losses occurring in some
years (Kolmer, 1996). In the hard red spring wheat growing regions of the United States,
farmers are spraying fungicides annually to mitigate crop losses, but genetic resistance can
provide protection at lower cost to farmers and risk to the environment. Breeding for resistance
to leaf rust is the most cost
effective method with an
estimated 27:1 benefit to cost ratio
(Marasas et al., 2004). Race
specific resistance genes have
been readily defeated when
released individually, while
combinations of race specific and
race non-specific resistance genes
have been more durable
(McIntosh p25). The race nonspecific resistance gene Lr34 has
been highly effective throughout
wheat-growing regions of the
world, increasing the level of
resistance when combined with
race specific genes (Kolmer, 1996). In Minnesota and the greater hard red spring wheat growing
region of the United States, the combination of Lr16, Lr23, and Lr34 has been highly resistant
(Kolmer et al. 2007) for more than 15 years. By identifying new sources of genetic resistance
and mapping the location of resistance genes, we can effectively incorporate these genes in
breeding efforts. By identifying genes in cultivars that have historically been highly resistant, we
can better understand and utilize durable genetic resources for leaf rust resistance.
Two projects were developed to identify, map, and effectively utilize leaf rust resistance
genes. The first compares biparental and association mapping methods to identify resistance in
diverse germplasm. The objectives are to identify and map new leaf rust resistance genes, better
characterize and map known genes, and determine whether association or biparental mapping
approaches are more effective for identifying leaf rust resistance loci in a diverse set of
germplasm. Association mapping has several advantages over biparental mapping, including
increased mapping resolution and reduced research time by utilizing historic recombination
rather than developing new mapping populations, and the ability to detect a greater number of
alleles at a particular locus (Yu and Buckler, 2006). However, with the exception of studies by
Crossa et al. in 2007 and Maccaferri et al. in 2010, association mapping has not been widely
applied for mapping leaf rust resistance. In 2011 and 2012, 3,200 diverse accessions from the
National Small Grains Collection in Aberdeen, Idaho were rated for resistance in leaf rust
screening nurseries in St. Paul and Crookston, MN. One replication and repeated checks every
100 experimental lines were used. Thirty accessions were selected on field resistance and
maturity to develop biparental mapping populations by crossing to the susceptible cultivar
‘Thatcher’. From fall 2011 to spring 2012, these 30 selected accessions were screened in the
greenhouse as seedlings with 10 leaf rust races and as adults using a field mixture of isolates and
a single race virulent in the seedling screenings. Pedigree information and marker screening was
also used to identify possible known genes present in the accessions. From the results of these
screenings, nine recombinant inbred line mapping populations were selected for potential to have
novel race specific or race non-specific genes. Biparental populations with candidate novel race
specific genes will be mapped initially using bulk segregant analysis, using at least 10
homozygous resistant and 10 homozygous susceptible F2:3 families. Candidate race non-specific
resistance genes will be mapped using at least 150 F6 lines. For association mapping analysis,
selected lines with similar maturity will be planted in St. Paul and Crookston in 2013.
Association analysis will use data collected as a part of the Triticeae Coordinated Agriculture
Project (TCAP). Data will include greenhouse seedling screening with Race 1, a field mixture,
and a race with specificity to common durum resistance and field severity response ratings in
2011, 2012, and 2013. All 3,200 lines will be genotyped using the Illumina 9,000 SNP chip for
wheat as a part of the larger TCAP. The effectiveness of association mapping will be compared
to the biparental mapping populations in terms of the number and identity of resistance loci
detected. We are also interested in whether one method may be more useful for identifying
novel as well as race specific or race non-specific genes.
The second project involves identifying leaf rust resistance genes in historically resistant
hard red spring wheat cultivars to better characterize the durable resistance present in our
breeding germplasm. Our objectives are to determine the number and identity of genes in
‘Ulen’, ‘RB07’, and ‘AC Taber’ and to identify markers associated with the resistance genes to
be used for marker assisted selection. Ulen was postulated to have Lr23, Lr10, and an additional
unknown resistance gene (Anderson et al., 2006). RB07 and Faller both have the cloned gene
Lr21 which conferred near immunity in the field until 2010 when a new race of leaf rust was
detected with virulence to Lr21 (Kolmer et al., 2011). Surprisingly, Faller had higher levels of
susceptibility than RB07, which remained highly resistant. AC Taber is a widely grown
Canadian wheat cultivar, released in 1990 with Lr14a, Lr13, and an uncharacterized resistance
gene “LrTb” showing resistance in adult plants (Liu and Kolmer, 1997). Biparental populations
have been developed to evaluate RB07 x Faller, Ulen x Thatcher, and AC Taber x Thatcher
through seedling resistance screening and field trials in order to map unknown genes.
References
Anderson, J.A., R.H. Busch, D.V. McVey, J.A. Kolmer, Y. Jin, G.L. Linkert, J.V. Wiersma, R. Dill-Macky, J.J.
Wiersma, AND G.A. Hareland. 2006. Registration of ‘Ulen’ Wheat. Crop Science 46: 979-980.
Crossa, J., Burgueño, J., Dreisigacker, S., Vargas, M., Herrera-Foessel, S. A., Lillemo, M., Singh, R. P., Trethowan,
R., Warburton, M., Franco, J., Reynolds, M., Crouch, J. H., and Ortiz, R. (2007). Association analysis of
historical bread wheat germplasm using additive genetic covariance of relatives and population structure.
Genetics, 177(3), 1889-1913.
Kolmer, J. A. (1996). Genetics of resistance to wheat leaf rust 1. Annual Review of Phytopathology, 34(1), 435-455.
Kolmer, J. A., Jin, Y., and Long, D. L. (2007). Wheat leaf and stem rust in the United States. Crop and Pasture
Science, 58(6), 631-638.
Kolmer, J. A. and J. A. Anderson. 2011. First Detection in North America of Virulence in Wheat Leaf Rust
(Puccinia triticina) to Seedling Plants of Wheat with Lr21. Plant Disease. 95(8):1032.
Liu, J. Q., and Kolmer, J. A. (1997). Genetics of leaf rust resistance in Canadian spring wheats AC Domain and AC
Taber. Plant disease, 81(7), 757-760.
Maccaferri, M., Sanguineti, M. C., Mantovani, P., Demontis, A., Massi, A., Ammar, K., Kolmer, J., Czembor, J.,
Breiman, A., and Tuberosa, R. (2010). Association mapping of leaf rust response in durum wheat.
Molecular Breeding, 26(2), 189-228.
Marasas, C.N., M. Smale, and R.P. Singh. 2004. The Economic Impact in Developing Countries of Leaf Rust
Resistance Breeding in CIMMYT-Related Spring Bread Wheat. Economics Program Paper 04-01. Mexico,
D.F.: CIMMYT.
McIntosh, R. A., Wellings, C. R., & Park, R. F. (1995). Wheat rusts: an atlas of resistance genes. CSIRO
PUBLISHING.
Yu, J., & Buckler, E. S. (2006). Genetic association mapping and genome organization of maize. Current Opinion in
Biotechnology, 17(2), 155-160.