Brachypodium distachyon line Bd3-1 resistance is elicited by the barley stripe mosaic virus triple gene block 1 movement protein

Abstract

Barley stripe mosaic virus North Dakota 18 (ND18), Beijing (BJ), Xinjiang (XJ), Type (TY) and CV21 strains are unable to infect theBrachypodium distachyonBd3-1 inbred line, which harbours a resistance gene designatedBsr1, but the Norwich (NW) strain is virulent on Bd3-1. Analysis of ND18 and NW genomic RNA reassortants and RNAβ mutants demonstrates that two amino acids within the helicase motif of the triple gene block 1 (TGB1) movement protein have major effects on their Bd3-1 phenotypes. Resistance to ND18 correlates with an arginine residue at TGB1 position 390 (R390) and a threonine at position 392 (T392), whereas the virulent NW strain contains lysines (K) at both positions. ND18 TGB1 R390K (NDTGB1R390K) andNDTGB1T392Ksingle substitutions, and anNDTGB1R390K,T392Kdouble mutation resulted in systemic infections of Bd3-1. ReciprocalNDTGB1 substitutions intoNWTGB1 (NWTGB1K390RandNWTGB1K392T) failed to affect virulence, implying that K390and K392compensate for each other. In contrast, anNWTGB1K390R,K392Tdouble mutant exhibited limited vascular movement in Bd3-1, but developed prominent necrotic streaks that spread from secondary leaf veins. This phenotype, combined with the appearance of necrotic spots in certain ND18 mutants, and necrosis and rapid wilting of Bd3-1 plants after BJ strain (BJTGB1K390,T392) inoculations, show that Bd3-1Bsr1resistance is elicited by the TGB1 protein and suggest that it involves a hypersensitive response.