siRNAs Derived from Cymbidium Mosaic Virus and Odontoglossum Ringspot Virus Down-modulated the Expression Levels of Endogenous Genes in Phalaenopsis equestris

Lan, Han-hong; Wang, Cui-mei; Chen, Shuang-shuang; Zheng, Jian-ying

NA silencing-directed antiviral defense is unclear and would be the topic in future. RdRP6 from O. sativa plays role in defense against rice viruses (Hong et al., 2015; Jiang et al., 2012) . In our study, RdRP5 and RdRP1 were up-regulated in infected P. equestris, implying their counterpart roles in host response to CymMV and ORSV infection. In general, these results suggested the complicated interactions between CymMV/ORSV and P. equestris RNA silencing pathway, which likely are modulated by the intertwined effects of multiple RNAi components. Target identification is very important to understand deeply the functions and implications of viral siRNAs in its interaction with host. In this study, we used the sRNA target analysis server psRNATarget (Dai and Zhao, 2011) to predict putative target genes of P. equestris by CymMVand ORSV-derived siRNAs. Because of the large number of putative genes targeted by siRNAs, only those gene sequences with more than 12 continuous base-pairing with vsiRNAs were selected for subsequent analysis. In total, 21 and 33 putative target genes were predicted by CymMVand ORSV-derived siRNAs, respectively (Table 2 ). Annotation of the 54 genes predicted was done referring to the De novo transcriptome assembly databases in the P. equestris (Niu et al., 2016) . Among of those annotated target genes, many were related to disease/stress response, including nine R genes which were annotated as NBS-LRR or NBS protein genes, one F-box/LRR or F-box domaincontaining protein gene, three ubiquitination-related protein genes, four receptor-like protein kinase (RLK) genes, and one universal stress protein (USP) gene (Table 2) . A majority of NBS-LRR protein genes play important roles in pathogen recognition and host defense against a variety of pathogens including viruses, bacteria and fungi (DeYoung and Innes, 2006; McHale et al., 2006) . Plant F-box proteins are involved in stress responses. It also has been shown that F-box proteins and RNAi pathway could modulate each other in plants; even more, some plant viruses encode an F-box-containing viral silencing suppressors (VSRs) to down-regulated RNAi pathway through the autophagy (Derrien et al., 2012; Earley et al., 2010; Jones-Rhoades et al., 2006; Kim and Delaney, 2002) . RLKs also participate in pathogen recognition and resistance (Morris and Walker, 2003) . USPs can increase adversity tolerance of plants (Sinha et al., 2016) . Besides, our results revealed that 4 target genes (DCL1, argonaute16, RdRP-SHL2, and piRNA biogenesis protein EXD1) were annotated as core components of RNAi response (Table 2) , suggesting RNAi re- Interplay between CymMV/ORSV and Phalaenopsis equestris by sRNA-seq 517 sponse may be targeted and its expression levels be further suppressed by vsiRNAs generated in P. equestris infected with CymMV and ORSV, which is similar to other hostpathogen systems (Xu and Zhou, 2017) . Many intracellular pathogens, including viruses, parasites, and bacteria, invade, replicate, spread within and exit their host cells in a cytoskeleton-dependent manner (Gouin et al., 2005; Greber and Way, 2006; Gruenheid and Finlay, 2003) . In this study, 9 target genes (mainly including myosin, networked 1A, pentatricopeptide repeat-containing protein, and vimentin) were annotated as cytoskeleton-related proteins (Table 2) , implying they play important roles in CymMV and ORSV infections in P. equestris. Moreover, several chloroplastor mitochondria-related genes were annotated