Comparative analysis of virus pathogenicity and resistance-breaking between the P- and A-type from the beet necrotic yellow vein virus using infectious cDNA clones

Abstract

The A-type of beet necrotic yellow vein virus (BNYVV) is widely distributed in Europe and is one of the major virus types causing rhizomania disease in sugar beet. The closely related P-type is mainly limited to a small region in France (Pithiviers). Both virus types possess four RNAs (RNA1–4), but the P-type harbours an additional fifth RNA species (RNA5). The P-type is associated with stronger disease symptoms and resistance-breaking of Rz1, one of the two resistance genes which are used to control BNYVV infection. These characteristics are presumably due to the presence of RNA5, but experimental evidence for this is lacking. We generated the first infectious cDNA clone of BNYVV P-type to study its pathogenicity in sugar beet in comparison to a previously developed A-type clone. Using this tool, we confirmed the pathogenicity of the P-type clone in the experimental host Nicotiana benthamiana and two Beta species, B. macrocarpa and B. vulgaris. Independent of RNA5, both the A- and the P-type accumulated in lateral roots and reduced the taproot weight of a susceptible sugar beet genotype to a similar extent. In contrast, only the P-type clone was able to accumulate a virus titre in an Rz1-resistant variety whereas the A-type clone failed to infect this variety. The efficiency of the P-type to overcome Rz1 resistance was strongly associated with the presence of RNA5. Only a double resistant variety, harbouring Rz1 and Rz2, prevented an infection with the P-type. Reassortment experiments between the P- and A-type clones demonstrated that both virus types can exchange whole RNA components without losing the ability to replicate and to move systemically in sugar beet. Although our study highlights the close evolutionary relationship between the two virus types, we were able to demonstrate distinct pathogenicity properties that are attributed to the presence of RNA5 in the P-type.