Fifteen years after transfer to hops, hop stunt viroid-grapevine (HSVd-g) was replaced by HSVd-hop (HSVd-h), a sequence variant that contains changes at five different positions. HSVd-g54 is a laboratory mutant derived from HSVd-g that differs from its progenitor by a single G to A substitution at position 54. While infection by HSVd-h induces only mild stunting in cucumber (Cucumis sativus L.), HSVd-g54 induces much more severe symptoms in this indicator host. Comparison of transcriptome profiles of cucumber infected with HSVd-h or HSVd-g54 with those of mock-inoculated controls obtained by whole transcriptome shotgun sequencing revealed that many genes related to photosynthesis were down-regulated following infection. In contrast, genes encoding RNA-dependent RNA polymerase 1 (CsRDR1), especially CsRDR1c1 and CsRDR1c2, as well as those related to basal defense responses were up-regulated. Expression of genes associated with phytohormone signaling pathways were also altered, indicating that viroid infection initiates a complex array of changes in the host transcriptome. HSVd-g54 induced an earlier and stronger response than HSVd-h, and further examination of these differences will contribute to a better understanding of the mechanisms that determine viroid pathogenicity.

China accounts for over 50% of apple production worldwide. Very recently, a novel ilarvirus, Apple necrotic mosaic virus (ApNMV), was isolated from apple trees showing mosaic symptoms in Japan. This study compared different types of mosaic symptoms observed in apple trees in China under field conditions. Complete nucleotide sequences were obtained for six isolates of ApNMV. The genomic components varied in size from 3,378 to 3,380 nt (RNA1), 2,778 to 2,786 nt (RNA2), and 1,909 to 1,955 nt (RNA3), respectively. Although nucleotide sequence similarities with subgroup 3 ilarviruses were low (49.2 to 64.3%), results of phylogenetic analysis indicated that Chinese ApNMV isolates were clustered in subgroup 3 together with Prunus necrotic ring spot virus (PNRSV) and Apple mosaic virus (ApMV). Apple mosaic disease occurred widely in apple producing areas of China with a very high percentage (92.1%, 268 out of 291) of symptomatic trees being infected with ApNMV but not with ApMV. The data suggested that ApNMV might be the main pathogen causing apple mosaic disease in China. The genomes of the six studied Chinese ApNMV isolates demonstrated substantial sequence diversity. Here, we demonstrated a strong association of ApNMV with the mosaic disease of apple trees in China.

RNA interference, or RNA silencing, is an important defence mechanism against viroid infection in plants. Plants encode multiple DICER‐LIKE (DCL) proteins that are key components of the RNA silencing pathway. However, the roles of different DCLs in defence responses against viroid infection remain unclear. Here, we determined the function of tomato DCL2b (SlDCL2b) in defence responses against potato spindle tuber viroid (PSTVd) infection using SlDCL2b loss‐of‐function tomato mutant plants. Compared with wild‐type plants, mutant plants were more susceptible to PSTVd infection, developing more severe symptoms earlier and accumulating higher levels of PSTVd RNAs. Moreover, we verified the feedback mechanism for the regulation of SlDCL2b expression by miR6026. Functional blocking of tomato miR6026, by expressing its target mimics, can enhance resistance to PSTVd infection in tomato plants. These findings deepen the current understanding of RNAi‐based resistance against viroid infection and provide a potentially new strategy for viroid control.

Peach (Prunus persica) is one of the most popular stone fruit in the world. In April 2017, three flat peach and eleven nectarine samples showing leaf yellowing and mosaic symptoms were collected in Pinggu district in Beijing, Peoples Republic of China. To identify viruses that infect peach trees, symptomatic leaves from three flat peach trees (cv. Rui Pan 19) were mixed well into one pool (designated as RP19) for high-throughput sequencing (HTS). Total RNA of leaf samples was extracted using the TRIzol reagent (Tiangen Biotech Co., Ltd, Beijing) following manufacturer’s instructions. The cDNA library was constructed after the removal of ribosomal RNA by Epicentre Ribo-zeroTM rRNA Removal Kit (Epicentre, USA). Sequencing was performed on an Illumina Hiseq 4000 platform and generated 150 bp paired-end reads. Approximately 32.3 million reads were obtained after trimming, and the resulting reads (after removal of host reads) were assembled using Trinity platform (Grabherr et al. 2013). The obtained 9,206 cont...