Biology and molecular characterization of necroviruses

Abstract

Three necroviruses, family Tombusviridae, are known to infect Olea europaea L., Olive latent virus 1 (OLV-1), Olive mild mosaic virus (OMMV) and Tobacco necrosis virus D (TNV-D), often asymptomatically. OLV-1 has a wide geographical distribution whereas that of OMMVand TNV-D is not known as their discovery is recent, dating from 2005 and 2009, respectively. OMMV has received special attention because of its origin, likely to have resulted from a recombination event between TNV-D, with which it shares a coat protein (CP) amino acid identity of 86% and OLV-1 sharing an RNA-dependent RNA polymerase amino acid identity of 91%. Due to these properties OMMV and TNV-D are indistinguishable by serology or Reverse Transcription – Polymerase Chain Reaction assays that use primers hybridizing in the CP gene region, rendering ambiguous earlier TNV-D identifications based on such tests. OLV-1 and OMMV are transmitted through the soil to roots of host plants in the absence of any vectors but the transmission rate of OMMV increases to more than double in the presence of Olpidium brassicae zoospores that vector this virus in an in vitro manner. A point mutation leading to an amino acid substitution located in an inner region of the virion is responsible for the loss of biologically assisted transmission, probably by altering the particle conformation rendering the binding site inaccessible to the zoospore receptors. The field occurrences of necrovirus multiple infections are common and increase the chances for recombination events, the epidemiological implications of which are still uncertain. The biological and molecular properties of the necroviruses identified in olive trees are the main focus of this review.