Complex sentence: Formation of virions is not always necessary for localized spread of the virus into neighboring plant cells-ribonucleoprotein particles containing viral genetic material can spread to immediate neighbors through plasmodesmata-however, the capsid protein is required for systemic infection.
Split into simpler sentences: Formation of virions is not always necessary for localized spread of the virus into neighboring plant cells, because ribonucleoprotein particles containing viral genetic material can spread to immediate neighbors through plasmodesmata. However, the capsid protein is required for systemic infection.