Maximising the potential of Aegilops ventricosa introgression for Pch1 eyespot resistance and increased grain protein in wheat

Eyespot is the most important disease of the stem base of cereals in the UK causing £12-20 million per annum in lost yield, in addition to significant expenditure on fungicides. Only three sources of resistance to eyespot are known to be present in modern wheat cultivars. The most potent of these is the gene Pch1, which originates from a wild grass. This resistance was introduced into wheat by conventional crossing, replacing a large segment of one of the wheat chromosomes with the equivalent portion from the wild grass (so-called 7DV segment). The two other eyespot resistances (Pch2 and QTL5A) both come from the variety Cappelle Desprez but they both have only moderate effects on eyespot resistance.

Wheat varieties carrying the 7DV segment are highly resistant to eyespot and it has also been observed that varieties containing the 7DV segment have a higher grain protein content. Unfortunately varieties carrying this 7DV segment suffer a yield penalty and it is only relatively recently that Pch1 carrying varieties have been developed that also possess high yield potential. We have shown that most of these varieties carry the full size original segment and so it appears that the negative yield effect of the 7DV segment is compensated by other factors in these new varieties.

Efforts to separate the desirable eyespot resistance and protein content traits from the deleterious yield effect have been seriously hindered by two factors: an apparent reduction in recombination between the native wheat chromosome and the 7DV segment and a lack of suitable DNA markers. However, recent advances in marker technology have made it possible to characterise the 7DV segment and identify lines carrying much smaller segments.

In this project we aim to isolate the Pch1 gene and also identify the part of the7DV segment that confers the increased grain protein content. This will enable plant breeders to develop wheat varieties that carry the desirable parts of the 7DV segment (Pch1 eyespot resistance and the part responsible for increased grain protein content) without the undesirable (yield penalty) parts.

Pch1 lies in a region of 7DV that is similar to that of the moderately effective eyespot resistance gene Pch2 from Cappelle Desprez. These two resistances are potentially due to 'sister' genes and we will determine whether this is the case or whether they are in similar but not identical positions.

The most potent gene for eyespot resistance in modern wheat cultivars is Pch1, which originates from the wheat relative Aegilops ventricosa. This resistance was introduced into wheat by introgression of a chromosomal segment from Ae. ventricosa (7DV segment).

The 7DV segment confers potent eyespot resistance as well as increased grain protein content but varieties carrying this 7DV segment suffer a yield penalty.

Efforts to separate the desirable eyespot resistance and protein content traits from the deleterious yield effect and to locate their relative positions on the 7DV segment have been seriously hindered by two factors: an apparently reduced recombination rate between the native wheat 7D and Ae. ventricosa 7DV regions and a lack of suitable co-dominant DNA markers. However, advances in DNA marker technology have overcome the latter and we have used these to demonstrate recombination within the 7DV segment as a prelude to cloning the Pch1 gene.

Within this project we will exploit the new DNA marker technologies to fine-map the Pch1 locus in a very large population segregating for Pch1. This will be complemented by cytogenetic and mutational approaches to delimit the locus and identify candidate genes for Pch1. Candidates will be validated by transformation into an eyespot susceptible wheat variety.

We will collaborate with plant breeders to phenotype a population of 7DV recombinants to identify the regions of 7DV responsible for increased grain protein content and the the yield penalty.

Pch1 lies in a region of 7DV that is similar to that of the gene Pch2 from Cappelle Desprez located on chromosome 7A of wheat, providing evidence that the two resistances are potentially homoeologues. We will undertake genetic mapping of a large new population segregating for Pch2 and combine this with disease phenotyping to precisely locate the position of Pch2 and establish whether Pch1 and Pch2 are homoeologous.

Eyespot is the most important disease of the stem base of cereals in the UK causing £12-20 million per annum in lost yield, in addition to significant expenditure on fungicides. Furthermore, impending EU legislation on pesticide application will impact on the availability of chemicals suitable for the control of eyespot making the use of host resistance even more important.

We demonstrated previously that, whereas fungicides provided a yield benefit for varieties highly and moderately susceptible to eyespot, no fungicide application produced a yield benefit in varieties carrying the Pch1 eyespot resistance gene. This demonstrates the potency of this resistance whereby its presence removes the need for fungicide application to control eyespot.

Significantly, eyespot facilitates entry of secondary invaders responsible for brown foot rot (e.g. Fusarium species). This indicates that the deployment of Pch1 would also contribute to control of brown foot rot which is, itself an important inoculum source for Fusairum head blight of wheat.

This project will provide plant breeders with the knowledge and plant materials to develop high yielding winter wheat varieties that do not require fungicide application to control eyespot.

The project will also enable plant breeders to efficiently exploit the potential of the 7DV segment to increase grain protein content in future varieties.

In the longer term, the ability of plant breeders to reliably produce high yielding wheat varieties with potent eyespot resistance will benefit growers who will need to make fewer fungicide applications to achieve maximum profitability.

The project will provide a benchmark for the exploitation of 'alien' introgressed chromosomal segments to separate beneficial from deleterious elements. Such a demonstration will encourage plant breeders to deploy the numerous beneficial traits available in grass species in wheat breeding programmes but which they are currently very reluctant to use because of the effects of deleterious linkage drag.