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Cymbidium mosaic virus (CymMV) and Odontoglossum ringspot virus (ORSV) are the two most prevalent viruses infecting orchids and causing economic losses worldwide. Mixed infection of CymMV and ORSV could induce intensified symptoms as early at 10 days post-inoculation in inoculated Phalaenopsis amabilis, where CymMV pathogenesis was unilaterally enhanced by ORSV. To reveal the antiviral RNA silencing activity in orchids, we characterized the viral small-interfering RNAs (vsiRNAs) from CymMV and ORSV singly or synergistically infecting P. amabilis. We also temporally classified the inoculated leaf-tip tissues and noninoculated adjacent tissues as late and early stages of infection, respectively. Regardless of early or late stage with single or double infection, CymMV and ORSV vsiRNAs were predominant in 21- and 22-nt sizes, with excess positive polarity and under-represented 5'-guanine. While CymMV vsiRNAs mainly derived from RNA-dependent RNA polymerase-coding regions, ORSV vsiRNAs encompassed the coat protein gene and 3'-untranslated region, with a specific hotspot residing in the 3'-terminal pseudoknot. With double infection, CymMV vsiRNAs increased more than 5-fold in number with increasing virus titres. Most vsiRNA features remained unchanged with double inoculation, but additional ORSV vsiRNA hotspot peaks were prominent. The potential vsiRNA-mediated regulation of the novel targets in double-infected tissues thereby provides a different view of CymMV and ORSV synergism. Hence, temporally profiled vsiRNAs from taxonomically distinct CymMV and ORSV illustrate active antiviral RNA silencing in their natural host, Phalaenopsis, during both early and late stages of infection. Our findings provide insights into offence-defence interactions among CymMV, ORSV and orchids., (© 2019 The Authors. Molecular Plant Pathology published by British Society for Plant Pathology and John Wiley & Sons Ltd.)

Orchids display unique phenotypes, functional characteristics and ecological adaptations that are not found in model plants. In this study, we aimed to characterize the microRNA (miRNA) transcriptome and identify species- and tissue-specific miRNAs in Phalaenopsis aphrodite. After data filtering and cleanup, a total of 59,387,374 reads, representing 1,649,996 unique reads, were obtained from four P. aphrodite small RNA libraries. A systematic bioinformatics analysis pipeline was developed that can be used for miRNA and precursor mining, and target gene prediction in non-model plants. A total of 3,251 unique reads for 181 known plant miRNAs (belonging to 88 miRNA families), 23 new miRNAs and 91 precursors were identified. All the miRNA star sequences (miRNA*), the complementary strands of miRNA that from miRNA/miRNA* duplexes, of the predicted new miRNAs were detected in our small RNA libraries, providing additional evidence for their existence as new miRNAs in P. aphrodite. Furthermore, 240 potential miRNA-targets that appear to be involved in many different biological activities and molecular functions, especially transcription factors, were identified, suggesting that miRNAs can impact multiple processes in P. aphrodite. We also verified the cleavage sites for six targets using RNA ligase-mediated rapid amplification of 5' ends assay. The results provide valuable information about the composition, expression and function of miRNA in P. aphrodite, and will aid functional genomics studies of orchids.