Abstract
Eukaryotic organisms exposed to adverse conditions are required to show a certain degree of transcriptional plasticity in order to cope successfully with stress. Epigenetic regulation of the genome is a key regulatory mechanism allowing dynamic changes of the transcriptional status of the plant in response to stress. The Hop stunt viroid (HSVd) induces the demethylation of ribosomal RNA (rRNA) in cucumber (Cucumis sativus) leaves, leading to increasing transcription rates of rRNA. In addition to the clear alterations observed in vegetative tissues, HSVd infection is also associated with drastic changes in gametophyte development. To examine the basis of viroid-induced alterations in reproductive tissues, we analysed the cellular and molecular consequences of HSVd infection in the male gametophyte of cucumber plants. Our results indicate that in the pollen grain, accumulation of HSVd RNA induces a decondensation of the generative nucleus that correlates with a dynamic demethylation of repetitive regions in the cucumber genome that include rRNA genes and transposable elements (TEs). We therefore propose that HSVd infection impairs the epigenetic control of rRNA genes and TEs in gametic cells of cucumber, a phenomenon thus far unknown to occur in this reproductive tissue as a consequence of pathogen infection.
Highlights
The maintenance of genome stability is a constant requirement in living organisms
22 nt sRNAs were present at similar frequencies. These results indicate that Hop stunt viroid (HSVd) accumulation in pollen grains causes changes in endogenous sRNAs, resembling – at least in part – those observed in HSVd-infected cucumber vegetative tissues
In this study we show that both viroid mature forms and vd-sRNAs accumulate in pollen grains of HSVd-infected plants, indicating that during the pathogenesis process the viroid is able to invade this reproductive cell
Summary
The maintenance of genome stability is a constant requirement in living organisms. At the same time, organisms must ensure that a certain level of genome plasticity is available in order to allow for genome rearrangements and mutations that might introduce beneficial traits to cope with stresses. Viroids are pathogenic long non-coding RNAs (lncRNAs), able to infect and systemically invade herbaceous and ligneous plants (Flores et al, 2005; Ding, 2009; Gomez and Pallas, 2013). Constrained by their small (250–400 nts) and non-protein-coding genome, viroids have 5858 | Castellano et al. Evolved into versatile nucleic acids that subvert the plant-cell machinery at diverse functional levels in order to guarantee that their life cycle can be completed within the infected host (Ding, 2009). Some pathogen–host interactions occur without visible plant alterations (latent diseases), viroid infection is frequently associated with phenotypic changes that we recognize as symptoms
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