Exploring the Multifunctional Roles of Odontoglossum Ringspot Virus P126 in Facilitating Cymbidium Mosaic Virus Cell-to-Cell Movement during Mixed Infection.

Shu-Chuan Lee,Song-Yi Kuo,Hsuan Pai,Yau-Heiu Hsu,Yun-Lin Song,Na-Sheng Lin,Meng-Hsun He,Ying-Wen Huang,Wen-Chi Chang

doi:10.3390/v13081552

Abstract

Synergistic interactions among viruses, hosts and/or transmission vectors during mixed infection can alter viral titers, symptom severity or host range. Viral suppressors of RNA silencing (VSRs) are considered one of such factors contributing to synergistic responses. Odontoglossum ringspot virus (ORSV) and cymbidium mosaic virus (CymMV), which are two of the most significant orchid viruses, exhibit synergistic symptom intensification in Phalaenopsis orchids with unilaterally enhanced CymMV movement by ORSV. In order to reveal the underlying mechanisms, we generated infectious cDNA clones of ORSV and CymMV isolated from Phalaenopsis that exerted similar unilateral synergism in both Phalaenopsis orchid and Nicotiana benthamiana. Moreover, we show that the ORSV replicase P126 is a VSR. Mutagenesis analysis revealed that mutation of the methionine in the carboxyl terminus of ORSV P126 abolished ORSV replication even though some P126 mutants preserved VSR activity, indicating that the VSR function of P126 alone is not sufficient for viral replication. Thus, P126 functions in both ORSV replication and as a VSR. Furthermore, P126 expression enhanced cell-to-cell movement and viral titers of CymMV in infected Phalaenopsis flowers and N. benthamiana leaves. Taking together, both the VSR and protein function of P126 might be prerequisites for unilaterally enhancing CymMV cell-to-cell movement by ORSV.

Highlights

Mixed infections by plant viruses are commonly found in nature and cause many important viral diseases [1]
The viral synergism caused by odontoglossum ringspot virus (ORSV) and cymbidium mosaic virus (CymMV) isolated from Phalaenopsis [35,36] was confirmed by back-inoculation of purified virions to Phalaenopsis plants alone or mixed (Figure S1)
By aligning all eight fulllength ORSV genome sequences currently available from the NCBI database, we used the conserved 5 and 3 sequences to design the primers ORSV-FP1 and ORSV-RP1 (Figure S2A) for the direct amplification of the complete ORSV genome from total RNA of infected Phalaenopsis leaves by reverse transcription-polymerase chain reaction (RT-PCR)

Summary

Introduction

Mixed infections by plant viruses are commonly found in nature and cause many important viral diseases [1]. The virus–virus and viruses–host interactions in co-infection scenarios may be antagonistic or synergistic depending on the combinations of viral strains, hosts, infection time-points and, in some cases, insect vectors [1,2]. The synergistic interactions arising from mixed infection may result in beneficial effects for one or all viral partners, potentially increasing viral titers and enhancing viral movement and/or symptoms in the host plants [3,4,5,6,7,8]. Virus-encoded suppressors that counteract the plant RNA silencing surveillance system regulate gene expression for proper development in eukaryotes [9] and serve in a major strategic role against virus infection [10,11]. A few studies have shown that viral suppressors of RNA silencing (VSRs) are determinants of viral synergism

Methods

Results

Conclusion