A collection of tetraploid wheats as a source of new alleles of the powdery mildew resistance gene Pm3.

Powdery mildew is one of the devastating wheat diseases and is caused by the biotrophic fungus Blumeria graminis f.sp. tritici. The identification of natural sources of resistance and breeding for resistant varieties is the most effective way to control this disease, as chemical control is expensive. To date, more than 40 Pm resistance genes have been characterized, while only one of these genes, Pm3, has been cloned. Pm3 is localized on the short arm of wheat chromosome 1A and is present in 17 functional allelic forms (Pm3a to Pm3g, Pm3k to Pm3t). The Pm3 alleles confer race-specific resistance to different wheat powdery mildew races. The objectives of this work were to study the behaviour of powdery mildew resistance Pm3 gene in a collection of tetraploid wheats and to isolate the complete genomic sequence of Pm3 gene in susceptible and resistant genotypes in order to identify new functional alleles. The search for new Pm3 alleles was carried out on a collection of 233 tetraploid wheat genotypes (Triticum turgidum L.). The collection comprises 128 cultivars of durum wheat (ssp. durum), as well as 19 accessions of ssp. turgidum, 20 of ssp. turanicum, 20 of ssp. polonicum, 12 of ssp. carthlicum, 20 of ssp. dicoccum, and 14 of ssp. dicoccoides. The entire set of these accessions was phenotypically characterized for resistance against wheat powdery mildew by screening with one powdery mildew isolate (O2). This screening led to the identification of 34 resistant accessions (14,6% of the total set). The tetraploid wheat collection was analysed for the Pm3 haplotype by screening for the presence of a Pm3-like gene with a STS marker. This STS marker amplifies a 946 bp fragment from the 5' non- coding region of Pm3b and is diagnostic for the presence of a Pm3-like gene. A total of 129 accessions, both resistant and susceptible, out of 233 (55.4%) tested were identified with a likely presence of a Pm3 gene. Subsequently a set of 21 resistant and susceptible lines showing the presence of a Pm3-like gene was screened for the presence of Pm3a to Pm3g alleles. A positive PCR reaction was observed for the Pm3b allele. Five of these lines, either resistant and susceptible, were further analysed with 11 PCR primer combination covering all the Pm3b sequence. Pm3b allele from tetraploid wheat had a size of 4,442 bp, comprising two exons of 4,156 and 86 bp respectively, and an intron of 200 bp. Sequences alignment compared with the Pm3b sequence, used as reference, indicated that the Pm3 genes from the five tetraploid wheat lines contain several “indels” and SNPs in the form of highly polymorphic sequence blocks. The present study demonstrates that allelic variation occurs in functional genes and represents the first step for the identification and sequencing of new functional Pm3 alleles in tetraploid wheat genotypes.