Rhizomania is the most serious sugar beet disease worldwide in terms of profitability and may cause earnings losses of 50% and more. The disease, which is also referred to as “root madness”, is caused by the “Beet Necrotic Yellow Vein Virus” (BNYVV) and is transmitted by the soil-borne protozoa Polymyxa betae. A BNYVV infection manifests itself in an increased proliferation of the thin roots and secondary roots and in the formation of a greatly reduced root body with reduced sugar content. Infected plants demonstrate a reduced water uptake and are thus more sensitive to dry stress. When the infection spreads to the entire plant, this results in a yellowing of the leaf veins, in necrotic lesions, and yellow flecks on the leaves. Since a curative combatting of the disease is not possible, as is the case with other viral diseases, damage can be prevented only via the cultivation of resistant species. Three major genes against rhizomania are currently being examined in essence: RZ-1 (also referred to as “Holly”), RZ-2 and RZ-3. In addition, further rhizomania resistance genes are described in the literature, although these are of lesser significance. Here, the resistance gene RZ-1 is already incorporated into most breeding lines (seed parent and/or pollinator parent components). It has been found, however, that the resistance conveyed by RZ-1 is insufficient in heavily infected regions or in regions having diverse BNYVV pathotypes (for example Sohi & Maleki, 2004). For this reason, it was already proposed some time ago to combine RZ-1 with, for example, RZ-2 or RZ-3. RZ-2 and RZ-3 originate from Beta vulgaris subsp. maritime sources (WB42, WB41) and genetically map in the same region on chromosome 3 of the sugar beet genome, whereas RZ-1 likewise maps on chromosome 3, but south of RZ-2 and RZ-3. Scholten et al. (1999) determined a distance of 20-25 cM between the RZ major genes RZ-1 and RZ-2. Gidner et al. (2005) found a shorter distance of 5 cM between RZ-1 and RZ-2 and did not conclude that RZ-2 and RZ-3 map on the same locus. Schmidlin et al. (2008) identified differently induced genes by means of expression analysis in infected beets, however these did not correspond to RZ-2 or RZ-3. In the study by Larson et al. (2008), some BNYVV-induced proteins were detected in the sugar beet using the MALDI-TOF-MS method, however the proteins which were encoded by RZ-1, RZ-2 or RZ-3 could not be identified by the scientists. In addition, the sequence region, in particular around this resistance gene, is repetitive, which makes the development of diagnostic markers particularly difficult. Until now, neither high-resolution marker maps nor verified candidate genes were publicly available for the specified rhizomania resistance genes. In addition, the functional background of these resistance genes, i.e. the genetic structure, previously was not fully known. For sustainable cultivation against rhizomania intended to counteract the risk of resistance-breaking BNYVV isolates, it is necessary to constantly identify new resistance genes and to integrate these into the gene pool of the crop plants such as sugar beet.
USPTO Backgrounds
USPTO Backgrounds