Evidence of olive mild mosaic virus transmission by Olpidium brassicae

Abstract

Transmission of three strains of OMMV by an Olpidium sp. was evaluated and compared. The three strains were 1) an OMMV wild type (WT) recovered from olive trees, 2) an OMMV variant (L11) obtained after 15 serial passages of single local lesions induced in Chenopodium murale plants, and 3) a construct OMMV/OMMVL11 in which the coat protein (CP) gene replaced that of the wild type. A single-sporangial culture derived from Chinese cabbage (Brassica pekinensis) used as a bait plant grown in soil of an olive orchard, was identified as Olpidium brassicae based on the size and sequence of the generated amplicon in PCR specific tests. Each of the three virus strains was soil transmitted to cabbage roots in the absence of the fungus at similar rates of 30 to 40%. Separate plant inoculation by O. brassicae zoospores incubated with each viral strain resulted in enhanced transmission of OMMV, reaching 86% of infection whereas that of the other two strains remained practically unaffected at ca. 34%. Binding assays showed that the amount of virus bound to zoospores, estimated spectrophotometrically, was 7% in the case of OMMV, and practically nil in the case of the other two viral strains. Substitution of the coat protein (CP) gene of OMMV by that of the OMMV L11 strain, drastically reduced viral transmissibility in the presence of zoospores to the level of that observed in their absence. Our data shows that OMMV soil transmission is greatly enhanced by O. brassicae zoospores and that the viral CP plays a significant role in this process, most likely by facilitating virus binding and later entrance into the host plant roots.

Plant to Plant Movement

“…There are two distinct stages in the in vitro transmission of these viruses. The virus particles Campbell (1996) and Rochon et al (2004 Varanda et al (2011a). In the release stage, the virus (or infectious material) accesses the root cell cytoplasm following encystment of virus-bound zoospores on root cells.…”

Olive mild mosaic virus transmission by Olpidium virulentus

“…However, acquisition of 2 μg ml −1 of OMMV by O. virulentus zoospores highly raised the level of transmission to 100 % when compared to that of the 50 μg ml −1 observed in the absence of the fungus (Table 1). This shows that O. virulentus seems to be even a more efficient vector of OMMV than its other vector, O. brassicae, where 5 μg account for a 86 % efficiency of transmission (Varanda et al 2011b). This is particular interesting as to our knowledge this is the first report of a virus that is transmitted by two different species of Olpidium sp.…”

“…(HerreraVasquez et al 2009). Molecular identification has allowed the determination of O. brassicae as a vector of OMMV (Varanda et al 2011b) and O. virulentus as a vector of Tobacco stunt virus, Pepino mosaic virus, of a dianthovirus causing the necrotic streak of fique and of Mirafiori lettuce big-vein virus and Lettuce big-vein associated virus (Sasaya and Koganezawa 2006;Alfaro-Fernandez et al 2010;Gonzalez et al 2010;Maccarone et al 2010).…”

“…The recovery of the fungi through Chinese cabbage plants (Brassica campestris, Pekinensis group) from a field where horticultural crops had grown intensively (north of Portugal), as well as the observation of fungal structures in roots and isolation of single sporangium were performed as described previously (Varanda et al 2011b). A single sporangium, was placed in a pot containing a mixture of sterile sand and vermiculite where 5 day old cabbage seedlings were growing and maintained in a growth chamber at 16-22°C with a 16 h photoperiod for 1 month (Varanda et al 2011b). …”

“…Amplicons were purified using GFX PCR DNA purification kit (GE Healthcare) and directly sequenced in both directions. Transmission assays were performed as in Varanda et al (2011b). Viral concentrations ranging from 0.07 to 150 μg ml −1 were tested.…”