A spectrum of HRT-dependent hypersensitive responses elicited by the 52 amino acid N-terminus of turnip crinkle virus capsid protein and its mutants

• The N-terminal, 52-amino acid (AA) R domain of TCV is sufficient for eliciting HRT-mediated HR in Nicotiana benthamiana and Arabidopsis.
• Single AA substitutions within this region cause varying degrees of perturbation to HRT-mediated HR and resistance.
• Mutants R6A, R8A, and G14A abolishes, weakens, and enhances HR and resistance, respectively, creating a continuing spectrum of defense responses.
• The conformational dynamics of R domain may play a critical role in conditioning HRT-mediated resistance.

The capsid protein (CP) of turnip crinkle virus (TCV) is the elicitor of hypersensitive response (HR) and resistance mediated by the resistance protein HRT in the Di-17 ecotype of Arabidopsis. Here we identified the N-terminal 52-amino-acid R domain of TCV CP as the elicitor of HRT-dependent HR in Nicotiana benthamiana. Mutating this domain at position 6 (R6A), but not at positions 8 (R8A) or 14 (G14A), abolished HR in N. benthamiana. However, on Di-17 Arabidopsis leaves only R8A R domain elicited visible epidermal HR. When incorporated in infectious TCV RNAs, R8A and G14A mutations exerted dramatically different effects in Di-17 plants, as R8A caused systemic cell death whereas G14A led to complete restriction of the mutant virus. This continual spectrum of HR and resistance responses elicited by various R domain mutants suggests that the CP–HRT interaction could be perturbed by conformational changes in the R domain of TCV CP.