Transient expression of the coat protein of Apple chlorotic leaf spot virus inhibits the viral RNA accumulation in Nicotiana occidentalis

The coat protein of Apple chlorotic leaf spot virus (ACLSV-CP) plays a crucial role in infectivity and efficient viral RNA accumulation in host cells (J. en. Virol, 88, 2007). In this study, the effect of ACLSV-CP on viral RNA accumulation in Nicotiana occidentalis was investigated. The CP, CPm40 (an amino acid (aa) substitution of Ala to Ser at aa position 40), CPm75 (a substitution of Phe to Tyr at aa position 75), and CPm40m75 (two aa substitutions at positions 40 and 75) of ACLSV (P205) were transiently expressed in N. occidentalis leaves by agroinfiltration. Immunoblot analysis showed that CP and CPm40m75 accumulated in infiltrated tissues, in contrast to CPm40 and CPm75 which were not detected, suggesting that the stable accumulation of CP is important for effective viral RNA accumulation. However, co-agroinfiltration of an infectious ACLSV cDNA clone (pBICLSF) or pBICLSF-based CP mutants (pBICLCPm40, pBICLCPm75, and pBICLCPm40m75) with a vector expressing CP (pBE2113-CP) showed no viral genomic RNA accumulations were found in any leaves infiltrated with these constructs. The inhibition of ACLSV-RNA accumulation was found only in leaves co-expressed with CP protein, but not with a frame-shift mutant of CP, a movement protein (P50), and a frame-shift mutant of P50.