Abstract
Key messageThe combined effects of enhanced total transgene expression level and allele-specificity combination in transgenic allele-pyramided Pm3 wheat lines result in improved powdery mildew field resistance without negative pleiotropic effects.Allelic Pm3 resistance genes of wheat confer race-specific resistance to powdery mildew (Blumeria graminis f. sp. tritici, Bgt) and encode nucleotide-binding domain, leucine-rich repeat (NLR) receptors. Transgenic wheat lines overexpressing alleles Pm3a, b, c, d, f, and g have previously been generated by transformation of cultivar Bobwhite and tested in field trials, revealing varying degrees of powdery mildew resistance conferred by the transgenes. Here, we tested four transgenic lines each carrying two pyramided Pm3 alleles, which were generated by crossbreeding of lines transformed with single Pm3 alleles. All four allele-pyramided lines showed strongly improved powdery mildew resistance in the field compared to their parental lines. The improved resistance results from the two effects of enhanced total transgene expression levels and allele-specificity combinations. In contrast to leaf segment tests on greenhouse-grown seedlings, no allelic suppression was observed in the field. Plant development and yield scores of the pyramided lines were similar to the mean scores of the corresponding parental lines, and thus, the allele pyramiding did not cause any negative effects. On the contrary, in pyramided line, Pm3b × Pm3f normal plant development was restored compared to the delayed development and reduced seed set of parental line Pm3f. Allele-specific RT qPCR revealed additive transgene expression levels of the two Pm3 alleles in the pyramided lines. A positive correlation between total transgene expression level and powdery mildew field resistance was observed. In summary, allele pyramiding of Pm3 transgenes proved to be successful in enhancing powdery mildew field resistance.
Highlights
In agricultural practice, two important strategies to control crop diseases are the use of pesticides and the use of diseaseresistant cultivars
We included in the field trial non-transformed Bobwhite and spring wheat cultivars Asosan, Chul, and Kolibri, which carry the endogenous alleles Pm3a, Pm3b, and Pm3d, respectively
We planted spreader rows between each line of plots consisting of powdery mildew susceptible wheat plants infected with Bgt isolate 96224. This isolate was originally collected in Switzerland and since cultivated in the laboratory. It was chosen for field infection, because previously conducted leaf segment tests in the laboratory showed avirulence of the isolate on wheat lines carrying any of the alleles Pm3a, Pm3b, Pm3d, or Pm3f (Brunner et al 2010, 2011, 2012)
Summary
Two important strategies to control crop diseases are the use of pesticides and the use of diseaseresistant cultivars. A common approach is the introgression of resistance genes found in the gene pool of the crop species of interest by multiple steps of backcrossing (Visscher et al 1996; Hillel et al 1990; Tanksley and Nelson 1996). A faster and more precise method of introgression is the genetic transformation of high yielding crop cultivars with resistance genes of interest (Narusaka et al 2013; Rodriguez-Moreno et al 2017; Lacombe et al 2010; Mondal et al 2016). Resistance gene Pm3 from wheat confers resistance to powdery mildew The Pm3 alleles encode nucleotide-binding domain; leucine-rich repeat (NLR) type of receptors which are able to recognize effectors from Bgt and
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