Helper component
protease (HC-Pro) is a potyvirus-encoded multifunctional protein and a major determinant of
symptom expression in a
susceptible plant. Here, we show the involvement of clover
yellow vein virus (ClYVV) HC-Pro in
necrotic symptom expression in
broad bean (
Vicia faba cv. Wase). In this host, lethal
necrosis was induced by ClYVV no. 30, from which a spontaneous, mosaic-inducing
mutant (MM) was obtained. Mapping with chimeric
viruses between ClYVV no. 30 and MM attributed the
symptom attenuation to two
mutations at the HC-Pro positions 27 (
threonine to
isoleucine) and 193 (
aspartic acid to
tyrosine). Although neither
mutant with the single
amino acid substitution at position 27 or 193 (ClYVV/T27I or D193Y) induced the lethal
necrosis, ClYVV/T27I still retained the ability to induce
necrotic symptoms, but ClYVV/D193Y scarcely did so. The
virus accumulation of ClYVV/D193Y was also lower than that of ClYVV no. 30. The
mutations, T27I and D193Y, are located in a putative
zinc finger domain and in one (
N-terminal) of the two
RNA binding domains, respectively, of HC-Pro. RNA-silencing suppression (RSS) activity of P1/HC-Pro in
Nicotiana benthamiana was weakened by both
mutations. Our results suggest a
correlation between viral
virulence and RSS function and the importance of the two domains in HC-Pro.
