Role of the Dicer-like 4 plant protein in OMMV and OLV-1 mixed infections

Abstract

Olive mild mosaic virus (OMMV) and Olive latent virus 1 (OLV-1) are two widely disseminated viruses in Portuguese olive groves, frequently appearing in mixed infections. Previous studies carried out on Nicotiana benthamiana plants revealed that when these two viruses are present in mixed infections, an intensification of symptoms is observed, suggesting a synergistic effect. In this work, we intended to test whether the plant defense mechanism based on RNA gene silencing is involved in the exacerbation of symptoms observed in the OLV-1 and OMMV mixed infections. For this, we evaluated the expression of the gene that encodes the plant protein Dicer-like 4 (DCL4), a primary component of the antiviral defense against RNA viruses. N. benthamiana plants were inoculated by mechanical transmission, using OMMV, OLV-1 and a mixture of OMMV + OLV-1. The expression of the capsid genes of each virus was determined by qPCR, at 5 and 12 days after inoculation, and used to estimate their accumulation levels, as well as the gene encoding DCL4. Analysis of the results showed that the exacerbation of symptoms seen in mixed infections does not translate into a greater viral accumulation. In fact, both viruses showed less accumulation levels when in the presence of the other virus. As for DCL4, it was found that it is more expressed in plant when in presence of both viruses simultaneously, especially 5 days after inoculation. In the presence of OLV-1 only, DCL4 is less expressed at the moment when there is a higher viral expression (at  day 12). In the case of OMMV, DCL4 is more expressed when the expression of the virus is higher. This work seems to suggest that DCL4 may be associated with the silencing of OMMV, while OLV-1 seems to be able to 'bypass' the plant's defense mechanism, but this does not happen when OLV-1 is in the presence of OMMV. This study contributes to increase the knowledge on plant-virus interactions, which is essential for the development of virus-resistant plants, extremely important in the context of sustainable agriculture.