Identification and marker-assisted selection of genes for reducing the susceptibility of new winegrape cultivars to fungal pathogens
Abstract
Worldwide winegrape production is highly dependent on the frequent use of fungicides which are costly for growers and have negative impacts on the environment. These issues could be minimised, or even eliminated, by the breeding of new winegrape varieties with reduced susceptibility to powdery mildew, downy mildew and botrytis. We have employed marker-assisted selection technology, in combination with the rapid flowering microvine mutant, to rapidly introgress powdery and downy mildew resistance genes from wild Chinese Vitis species into V. vinifera breeding lines which will be used to develop second generation mildew resistant premium winegrape varieties with increased durability of resistance in the field.
Summary
The two most economically important grapevine diseases worldwide are powdery mildew (PM) caused by the fungus Erysiphe necator (syn. Uncinula necator) and downy mildew (DM) caused by the oomycete Plasmopora viticola. The cultivated winegrape, Vitis vinifera, has little or no genetic resistance to these pathogens. As a result, control of these diseases is entirely dependent on the widespread application of fungicides. In addition to the economic cost of disease control, there is also increasing pressure to reduce agrochemical use for the control of plant pathogens on crops grown for human consumption.
This project has continued the research initiated in the previous Wine Australia project CSP 0904 Advanced grapevine genetics for varietal improvement in which the RUN1/RPV1 locus from the wild North American grapevine Muscadinia rotundifolia, that confers strong resistance to both powdery and downy mildew, was successfully introgressed into premium winegrape varieties by marker-assisted selection. A major aim of this current project has been to develop germplasm resources and genetic markers that will facilitate the development of second generation mildew resistant winegrape varieties in which PM and DM resistance loci from different wild species have been combined to increase the durability of the resistance in the field.
To achieve this, we have targeted the introgression of mildew resistance loci from wild Chinese Vitis species. However, in order to remove any potentially deleterious quality traits that may also be introgressed from these wild species and to meet Australian quarantine requirements regarding the permitted release of hybrids generated from imported wild Vitis species, we have undertaken a backcrossing program to reduce the component of the genome from the wild species. To speed up this backcrossing program we have successfully employed marker-assisted selection (MAS) in combination with a unique rapid grapevine breeding system based on the V. vinifera Pinot Meunier microvine mutant.
We have successfully generated microvine lines containing the REN4 PM resistance locus from V. romanetii and the RPV12 DM resistance locus from V. amurensis and these two loci have further been combined within the same microvine breeding line. It is anticipated that these REN4/RPV12 microvine lines will be used as parents in future crosses with selected first generation premium winegrape varieties containing the RUN1/RPV1 locus to produce the second generation mildew resistant winegrape varieties with dual PM and DM resistance loci. As part of this project, we have also fine mapped the chromosomal position of the REN4 and RPV12 loci and this has led to the identification of tightly linked DNA markers that will be used to verify the inheritance of all four mildew resistance loci in the future selection of the second generation mildew resistant winegrape varieties.
Botrytis bunch rot also continues to be a major problem for winegrape production. This is especially the case in cool climate regions where there is a high chance of rain around harvest, which can lead to serious bunch rot infections epidemics. As there are no known single dominant resistance genes to B. cinerea in any known plant species we cannot use the same strategy for generating disease-resistant varieties as we have used for powdery and downy mildew. Winegrape varieties with tight bunches, such as Riesling and Chardonnay, are the most susceptible to botrytis bunch rot, most likely due to prolonged water retention within the bunch after rain events. Previous studies have shown that a reduction in bunch compactness will significantly reduce the incidence and severity of botrytis bunch rot in the vineyard.
Thus, the second major aim of this project has been to identify major quantitative trait loci (QTLs) responsible for regulating internode length during berry cluster development. This would enable the development of genetic markers that can be incorporated into our marker-assisted selection process to identify progeny that will have more open bunches. In this way, the second generation vines will not only have increased resistance to powdery and downy mildew but also reduced susceptibility to bunch rot.
Our results confirmed that there is a high genetic component for the heritability of rachis internode development in lateral branches which has a major influence on bunch architecture. However, with the mapping populations available to us, we were unable to identify major QTLs for bunch architecture. One possible explanation for this outcome is that the mapping populations we employed were too small i.e. ranging in size from 56 - 101 individuals. Using populations with > 1000 individuals would have a much higher rate of success, because internode development is likely to be controlled by many genes. CSIRO has approximately 500 varieties of table and wine grapes in its germplasm collection, which display significant variation in berry size, bunch architecture and fruit quality traits. Thus, a genome-wide association analysis may be more suitable approach to identifying loci and markers linked to bunch architecture than biparental mating.