Abstract
We compared infection of Nicotiana benthamiana plants by the positive-sense RNA viruses Cucumber mosaic virus (CMV), Potato virus Y (PVY), and by a Potato virus X (PVX) vector, the latter either unaltered or expressing the CMV 2b protein or the PVY HCPro suppressors of silencing, at 25°C vs. 30°C, or at standard (~401 parts per million, ppm) vs. elevated (970 ppm) CO2 levels. We also assessed the activities of their suppressors of silencing under those conditions. We found that at 30°C, accumulation of the CMV isolate and infection symptoms remained comparable to those at 25°C, whereas accumulation of the PVY isolate and those of the three PVX constructs decreased markedly, even when expressing the heterologous suppressors 2b or HCPro, and plants had either very attenuated or no symptoms. Under elevated CO2 plants grew larger, but contained less total protein/unit of leaf area. In contrast to temperature, infection symptoms remained unaltered for the five viruses at elevated CO2 levels, but viral titers in leaf disks as a proportion of the total protein content increased in all cases, markedly for CMV, and less so for PVY and the PVX constructs. Despite these differences, we found that neither high temperature nor elevated CO2 prevented efficient suppression of silencing by their viral suppressors in agropatch assays. Our results suggest that the strength of antiviral silencing at high temperature or CO2 levels, or those of the viral suppressors that counteract it, may not be the main determinants of the observed infection outcomes.
Highlights
Anthropogenic releases of CO2 and of other gases very likely increase global temperatures and may lead to changes in local climates
In the cases of Potato virus Y (PVY) or of the three Potato virus X (PVX) constructs, CP levels decreased at 30°C, with regard to the levels found at 25°C (Fig 1, central western blot panels)
The results obtained showed an increase of Cucumber mosaic virus (CMV) levels, whereas genomic RNAs decreased for PVY and for the three PVX vectors (Fig 2 vs. Fig 1)
Summary
Anthropogenic releases of CO2 and of other gases very likely increase global temperatures and may lead to changes in local climates (http://www.ipcc.ch/). With regard to elevated environment CO2 levels our knowledge on compatible plant-virus interactions it is still limited: CO2 levels of 1000 parts per million (ppm) reduced Potato virus Y (PVY) titers in tobacco fresh weight samples [4], and 750 ppm alleviated the damage this virus caused in the same host [8] The latter levels decreased both symptom severity and virus titers in the infection of tomato by the geminivirus Tomato yellow leaf curl virus and of Tobacco mosaic virus in the same plant [3, 7] respectively, probably because of alterations in the levels of salicylic acid (SA) and jasmonic acid (JA). It has been hypothesized that heat masking (decreased viral titers and symptoms) at high temperature derive from enhanced antiviral gene silencing defenses overcoming the counteracting activities of virus-encoded suppressors of silencing [10, 11, 13]
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