Disease resistance. Leaf Rust Resistance.

Lr51

Background information

Triticum speltoides Taush (2n = 14, S genome) is an attractive source of high levels of resistance to leaf, stem and stripe rust of wheat (1). Leaf rust resistance genes Lr28, Lr35, Lr36,and Lr47 were derived from this species (3). Crosses between T. speltoides and hexaploid wheat (T. aestivum L. 2n=42, ABD genomes) followed by selection of resistant progeny frequently resulted in translocations between the T. speltoides and common wheat chromosomes because many T. speltoides genotypes have the ability to promote homoeologous chromosome pairing when hybridized with wheat (1).

Leaf rust resistance gene Lr51, formerly known as LrF7, was transferred from Triticum speltoides to common wheat by J. Dvorak (1). Lr51 is located within a 15-30-cM segment of Triticum speltoides chromosome 1S translocated to the long arm of chromosome 1B of bread wheat (2). In tests for resistance to Puccinia triticina race 5, J. Dvorak showed that plants homozygous for Lr51 were highly resistant with hypersensitive flecks, whereas heterozygous plants showed slightly lower levels of resistance with small pustules surrounded by necrosis and chlorosis, indicating incomplete dominance (1). A more recent study (2) showed that Lr51 is resistant to the current predominant races of leaf rust (P triticina) in the U.S. (see table below).

Lr51 has not been frequently used for breeding, in spite of the good levels of resistance it confers. Probably because the presence of large T. speltoides chromosome segments in a wheat background was associated with likely negative effects.

Cultivar / BC₆F₂ Line	Predicted Lr51	Rust race	Infection types
Neepawa	Absent	THBJ	33+
Neepawa-F-7-3	Present	THBJ	;
Express	Absent	THBJ	2+3
Express-Lr51	Present	THBJ	;
Neepawa	Absent	MCDS	33+
Neepawa-F-7-3	Present	MCDS	;
UC1037	Absent	MCDS	33+
UC1037-Lr51	Present	MCDS	;
Kern	Absent	MCDS	33+
Kern-Lr51	Present	MCDS	;

Infection types of BC₆F₂ lines. BC₆F₂ lines homozygous for the presence of the T. speltoides chromosome 1S translocation or for the non-translocated T. aestivum chromosome 1B were infected with two races of P. triticina. The presence of Lr51 was inferred from the banding pattern of two RFLP markers, XAga7 and Xmwg710. Infection types: ' 0' = immune; ';' = necrotic flecks; '1' = necrotic flecks and small uredinia; '2' = small uredinia with chlorosis; '3' = moderate size pustules without chlorosis or necrosis; '4' =Large uredinia without necrosis or chlorosis. Symbols '- 'and '+' inicate smaller or larger uredinia.

Methods

Helguera et al. (2) developed a CAPS marker for the T. speltoides segment carrying the Lr51 gene. To get the detailed protocol, follow this link.

Available germplasm

The table above shows several hard red spring that already have the Lr51 gene transferred. The donor parent for these crossings was the line F-7-3 (derived from Neepawa*6/Triticum speltoides F-7). Another translocated line containing the resistance gene is F-7-12. However, this line is not suitable for breeding because its translocations pattern is more complex, and could be the reason for the scarce use of Lr51

References

1. Transfer of leaf rust resistance from Aegilops speltoides to Triticum aestivum. Dvorak J. In: Canadian Journal of Genetics and Cytology,1977, 19:133-141.

2. PCR markers for Triticum speltoides leaf rust resistance gene Lr51 and their use to develop isogenic hard red spring wheat lines. Helguera M, Vanzetti L, Soria M, Khan IA, Kolmer J, Dubcovsky J. In: Crop Science. In press. [abstract]

3. Catalogue of Gene Symbols for Wheat. McIntosh RA, Yamazaki Y, Devos KM, Dubcovsky J, Rogers WJ, Appels R. In: Proceedings of the 10th International Wheat Genetics Symposium. Volume 4. Instituto Sperimentale per la Cerealicoltura, Rome, Paestum, Italy.2003