Differences in Virulence among PVY Isolates of Different Geographical Origins When Infecting an Experimental Host under Two Growing Environments Are Not Determined by HCPro.

Mongia Makki,Tomás Canto,Francisco Javier del Toro,Francisco Tenllado,Fattouma Djilani-Khouadja,Khouloud Necira

doi:10.3390/plants10061086

Abstract

The contribution of the HCPro factors expressed by several PVY isolates of different geographical origins (one from Scotland, one from Spain, and several from Tunisia) to differences in their virulence in Nicotiana benthamiana plants was investigated under two growing conditions: standard (st; 26 °C and current ambient levels of CO2), and climate change-associated (cc; 31 °C and elevated levels of CO2). In all cases, relative infection symptoms and viral titers were determined. The viral HCPro cistrons were also sequenced and amino-acid features of the encoded proteins were established, as well as phylogenetic distances. Additionally, the abilities of the HCPros of several isolates to suppress silencing were assessed under either growing condition. Overall, viral titers and infection symptoms decreased under cc vs. st conditions. However, within each growing condition, relative titers and symptoms were found to be isolate-specific, with titers and symptom severities not always correlating. Crucially, isolates expressing identical HCPros displayed different symptoms. In addition, all HCPro variants tested displayed comparable silencing suppression strengths. Therefore, HCPro alone could not be the main determinant of the relative differences in pathogenicity observed among the PVY isolates tested in this host, under the environments considered.

Highlights

In compatible infections, viruses overcome antiviral defenses to spread systemically from the point of infection
Characterization of the helper component protease (HCPro) Sequences Encoded by the Potato virus Y (PVY) Isolates Used in this Work
The observed differences could derive from errors in the initially published sequences derived from the use of Taq polymerases of low fidelity in the RT-PCR amplification and/or from manual sequencing, or alternatively, from substitutions arising in these isolates after the time of their initial sequencing and publication, as they may have become more adapted to Nicotiana plants through passages under standard laboratory conditions, since they were initially sampled

Summary

Introduction

Viruses overcome antiviral defenses to spread systemically from the point of infection. In the case of viruses of the genus Potyvirus, the multifunctional, non-structural protein helper component protease (HCPro) [4] has been known, for a long time, to be a pathogenicity determinant: mutations in this protein had the ability to reduce viral titers and attenuate symptoms, and even to prevent viruses from infecting plants systemically [5,6,7]. These observations on HCPro as a determinant of pathogenicity were understood better when they were associated with the silencing-suppression function of this protein [8]. In a collection of lab-generated Tobacco etch virus HCPro mutant variants, their different relative silencing suppression strengths affected virulence in Nicotiana benthamiana plants: decreased silencing suppressor activities correlated with reduced symptoms and titers, but increased silencing suppressor activities did not lead to neither increased symptoms nor titers, relative to non-mutant viruses [9]

Methods

Results

Conclusion