Abstract
RNA-dependent RNA polymerase 6 (RDR6) is one of the key factors in plant defense responses and suppresses virus or viroid invasion into shoot apical meristem (SAM) in Nicotiana benthamiana. To evaluate the role of Solanum lycopersicum (Sl) RDR6 upon viroid infection, SlRDR6-suppressed (SlRDR6i) ‘Moneymaker’ tomatoes were generated by RNA interference and inoculated with intermediate or lethal strain of potato spindle tuber viroid (PSTVd). Suppression of SlRDR6 did not change disease symptoms of both PSTVd strains in ‘Moneymaker’ tomatoes. Analysis of PSTVd distribution in shoot apices by in situ hybridization revealed that both PSTVd strains similarly invade the basal part but not apical part including pluripotent stem cells of SAM in SlRDR6i plants at a low rate unlike a previous report in N. benthamiana. In addition, unexpectedly, amount of PSTVd accumulation was apparently lower in SlRDR6i plants than in control tomatoes transformed with empty cassette in early infection especially in the lethal strain. Meanwhile, SlRDR6 suppression did not affect the seed transmission rates of PSTVd. These results indicate that RDR6 generally suppresses PSTVd invasion into SAM in plants, while suppression of RDR6 does not necessarily elevate amount of PSTVd accumulation. Additionally, our results suggest that host factors such as RDR1 other than RDR6 may also be involved in the protection of SAM including pluripotent stem cells from PSTVd invasion and effective RNA silencing causing the decrease of PSTVd accumulation during early infection in tomato plants.
Highlights
Viroids are single-stranded, circular RNA molecules known as the smallest plant pathogens, ranging from 246 to 401 nucleotides in length [1, 2]
Because 91B plants was confirmed to be SlRDR6-suppressed plants as expected, we examined the growth of SlRDR6i plants under our experimental conditions before starting the Potato spindle tuber viroid (PSTVd) infection assay
RNA-dependent RNA polymerase 6 (RDR6), one of the key factors in RNA silencing, generally maintains and amplifies RNA silencing by mediating generation of dsRNA, which is the precursor of secondary siRNA
Summary
Viroids are single-stranded (ss), circular RNA molecules known as the smallest plant pathogens, ranging from 246 to 401 nucleotides (nt) in length [1, 2]. They are non-coding RNA pathogens and replicate autonomously depending on the transcriptional machinery of the invaded host cells [3, 4]. The members of family Avsunviroidae form branched secondary structures and replicate in the chloroplast of invaded cells through a symmetric rolling-circle mechanism and exhibit self-slicer activity by hammerhead ribozyme motifs contained in their own strands of both polarities. Mild strains of PSTVd tend to have lower accumulation in an infected host than severe strains, while the severity of disease symptoms does not necessarily correlate with the accumulation of PSTVd [8,9,10]
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