Abstract
BackgroundIn plants, RNA silencing plays a fundamental role as defence mechanism against viruses. During last years deep-sequencing technology has allowed to analyze the sRNA profile of a large variety of virus-infected tissues. Nevertheless, the majority of these studies have been restricted to a unique tissue and no comparative analysis between phloem and source/sink tissues has been conducted. In the present work, we compared the sRNA populations of source, sink and conductive (phloem) tissues in two different plant virus pathosystems. We chose two cucurbit species infected with two viruses very different in genome organization and replication strategy; Melon necrotic spot virus (MNSV) and Prunus necrotic ringspot virus (PNRSV).ResultsOur findings showed, in both systems, an increase of the 21-nt total sRNAs together with a decrease of those with a size of 24-nt in all the infected tissues, except for the phloem where the ratio of 21/24-nt sRNA species remained constant. Comparing the vsRNAs, both PNRSV- and MNSV-infected plants share the same vsRNA size distribution in all the analyzed tissues. Similar accumulation levels of sense and antisense vsRNAs were observed in both systems except for roots that showed a prevalence of (+) vsRNAs in both pathosystems. Additionally, the presence of overrepresented discrete sites along the viral genome, hot spots, were identified and validated by stem-loop RT-PCR. Despite that in PNRSV-infected plants the presence of vsRNAs was scarce both viruses modulated the host sRNA profile.ConclusionsWe compare for the first time the sRNA profile of four different tissues, including source, sink and conductive (phloem) tissues, in two plant-virus pathosystems. Our results indicate that antiviral silencing machinery in melon and cucumber acts mainly through DCL4. Upon infection, the total sRNA pattern in phloem remains unchanged in contrast to the rest of the analyzed tissues indicating a certain tissue-tropism to this polulation. Independently of the accumulation level of the vsRNAs both viruses were able to modulate the host sRNA pattern.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-1327-5) contains supplementary material, which is available to authorized users.
Highlights
In plants, RNA silencing plays a fundamental role as defence mechanism against viruses
Subsequent loading of vsRNAs to Argonaute (AGO)-containing complexes or RISCs directs the degradation of both genomic and Herranz et al BMC Genomics (2015) 16:117 subgenomic viral RNAs [17,18]. This antiviral silencing response can be amplified by cellular RNA-dependent RNA polymerases (RDRs) which use as templates cleaved viral RNAs to generate new Doublestranded RNA (dsRNA) substrates for Dicer enzymes [19]
Our results indicated that whereas Melon necrotic spot virus (MNSV) vsRNAs derived from whole genomic RNA, Prunus necrotic ringspot virus (PNRSV) vsRNAs mapped at a higher percentage to RNA3
Summary
RNA silencing plays a fundamental role as defence mechanism against viruses. We compared the sRNA populations of source, sink and conductive (phloem) tissues in two different plant virus pathosystems. A series of grafting experiments demonstrated that RNA silencing has a systemic nature and that the long-distance transmission of the putative sequence-specific signaling molecule must be transported through the phloem [20]. Whereas some lines of evidence have indicated a role for sRNAs with different sizes in long-distance transmission of RNA silencing [27,28,29], other studies have suggested that the signal could be either a sRNA precursor or produced from dsRNA by a DCL-independent mechanism [30]
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