A single amino acid at N-terminal region of the 2b protein of cucumber mosaic virus strain m1 has a pivotal role in virus attenuation

• Host responses to CMV-m1 infections showed attenuated virulence of CMV-m1.
• Accumulation level of CMV-m1 in Nicotiana benthamiana was significantly lower than that of CMV-Y, although CMV-m1 showed strong cross-protective ability to CMV-Y in the plants.
• Threonine at position 18 of the 2b protein of CMV-m1 plays an important role in virus attenuation.
• The local RNA silencing suppressor activity of the 2b protein of CMV-m1 was a little lower than that of CMV-Y.

Host responses to infection by a mild strain of cucumber mosaic virus, termed CMV-m1, were re-examined in several plant species in comparison with those by a severe strain CMV-Y. Mild systemic symptoms were developed on the six plant species inoculated with CMV-m1. Virus titer in the Nicotiana benthamiana plants infected with CMV-m1 was significantly lower than those infected with CMV-Y, although infection by CMV-m1 interfered with further infection by CMV-Y in the plants. Subsequently, the attenuated virulence of CMV-m1 was analyzed by reassortment and recombination analyses between CMV-m1 and CMV-Y RNAs. The results suggested that the 2b protein of CMV-m1 (m1-2b) is involved in the formation of mild symptoms in N. benthamiana. Furthermore, site-directed mutagenesis demonstrated that Thr18 of m1-2b is responsible for formation of mild symptoms. Local RNA silencing suppressor activity of m1-2b was a little lower than that of severe strain CMV-Y. We discuss the relationship between attenuation of CMV-m1 and the features of m1-2b.