The long and winding road leading to the successful introgression of downy mildew resistance into onion

Abstract

Downy mildew resistance originating from Allium roylei Stearn provides a complete resistance to onions and is based on one, dominant gene. Since A. roylei can successfully be hybridized with onion (A. cepa L.), a breeding scheme aimed at the introgression of this gene was initiated ca. 20 years ago. Several setbacks in this programme were encountered, firstly the identified molecular marker linked to the downy mildew resistance locus became increasingly difficult to use and finally lost its discriminating power and secondly the final step, making homozygous introgression lines (ILs), turned out to be more difficult then was hoped. GISH analysis showed that the chromosomal region harbouring the resistance locus was the only remaining piece of A. roylei in the nuclear background of onion and it also confirmed that this region was located on the distal end of chromosome 3. It was hypothesized that some factor present in the remaining A. roylei region was lethal when homozygously present in an onion genetic background. The identification of an individual with a smaller and more distally located introgression fragment and homozygous ILs in its progeny validated this hypothesis. With the help of these nearly isogenic lines four AFLP Ò markers closely linked to the resistance gene were identified, which can be used for marker-aided selection. The introduction of downy mildew resistance caused by Peronospora destructor into onion is a significant step forward in the development of environmentally-friendly onion cultivars.

Concepts and Strategies of Organic Plant Breeding in Light of Novel Breeding Techniques

“…Organic farmers loose potential yield due to lack of sufficient weed suppression, pest and disease resistance traits in cultivars, such as against downey mildew in onion, late blight in potato and scab in apple [25]. Various pre-breeding programs to enhance such resistances are running and first resistant cultivars are being released [27,28].…”

Agricultural Biodiversity Is Essential for a Sustainable Improvement in Food and Nutrition Security

“…Farms specialize in livestock or crops, reducing the number of species; fields are enlarged, reducing the extent of field margins and hedgerows; soil amendments enhance the uniformity of soils; and monocultures of genetically uniform individuals tend to dominate. Within this framework, agricultural biodiversity is often seen simply as something to conserve as a source of traits that can be used to improve breeds and varieties (see, for example, [1][2][3][4][5][6] for this approach in several different realms). While this is certainly true, we argue that agricultural biodiversity as such is an important asset that delivers substantial benefits in many different realms and that there is increasing evidence that diversity per se needs to be a central element of sustainable agricultural development.…”

Can conventional breeding programmes provide onion varieties that are suitable for organic farming in the Netherlands?

“…Furthermore, they share the opinion that incomplete forms of resistance are not interesting for the conventional market. High levels of resistance against both diseases have been found in Allium species that are related to onion (Allium cepa): leaf blight resistance in Allium fistulosum (Currah and Maude 1984) and downy mildew resistance in Allium roylei (Kofoet et al 1990;van der Meer and de Vries 1990;Scholten et al 2007). Crosses between A. cepa and A. roylei only yield few fertile seeds, but the success rate is still higher than of crosses between A. cepa and A. fistulosum (Kik 2002).…”