Effect of sugar beet genotype on the B eet necrotic yellow vein virus P 25 pathogenicity factor and evidence for a fitness penalty in resistance‐breaking strains

Summary B eet necrotic yellow vein virus ( BNYVV ), vectored by P olymyxa betae , causes rhizomania in sugar beet. For disease control, the cultivation of hybrids carrying Rz1 resistance is crucial, but is compromised by resistance‐breaking ( RB ) strains with specific mutations in the P 25 protein at amino acids 67–70 (tetrad). To obtain evidence for P 25 variability from soil‐borne populations, where the virus persists for decades, populations with wild‐type ( WT ) and RB properties were analysed by P 25 deep sequencing. The level of P 25 variation in the populations analysed did not correlate with RB properties. Remarkably, one WT population contained P 25 with RB mutations at a frequency of 11%. To demonstrate selection by Rz1 and the influence of RB mutations on relative fitness, competition experiments between strains were performed. Following a mixture of strains with four RNAs , a shift in tetrad variants was observed, suggesting that strains did not mix or transreplicate. The plant genotype exerted a clear influence on the frequency of RB tetrads. In Rz1 plants, the RB variants outcompeted the WT variants, and mostly vice versa in susceptible plants, demonstrating a relative fitness penalty of RB mutations. The strong genotype effect supports the hypothesized Rz1   RB strain selection with four RNAs , suggesting that a certain tetrad needs to become dominant in a population to influence its properties. Tetrad selection was not observed when an RB strain, with an additional P 26 protein encoded by a fifth RNA , competed with a WT strain, supporting its role as a second BNYVV pathogenicity factor and suggesting the reassortment of both types.