Abstract
BackgroundSalicylic acid (SA) regulates multiple anti-viral mechanisms, including mechanism(s) that may be negatively regulated by the mitochondrial enzyme, alternative oxidase (AOX), the sole component of the alternative respiratory pathway. However, studies of this mechanism can be confounded by SA-mediated induction of RNA-dependent RNA polymerase 1, a component of the antiviral RNA silencing pathway. We made transgenic Nicotiana benthamiana plants in which alternative respiratory pathway capacity was either increased by constitutive expression of AOX, or decreased by expression of a dominant-negative mutant protein (AOX-E). N. benthamiana was used because it is a natural mutant that does not express a functional RNA-dependent RNA polymerase 1.ResultsAntimycin A (an alternative respiratory pathway inducer and also an inducer of resistance to viruses) and SA triggered resistance to tobacco mosaic virus (TMV). Resistance to TMV induced by antimycin A, but not by SA, was inhibited in Aox transgenic plants while SA-induced resistance to this virus appeared to be stronger in Aox-E transgenic plants. These effects, which were limited to directly inoculated leaves, were not affected by the presence or absence of a transgene constitutively expressing a functional RNA-dependent RNA polymerase (MtRDR1). Unexpectedly, Aox-transgenic plants infected with potato virus X (PVX) showed markedly increased susceptibility to systemic disease induction and virus accumulation in inoculated and systemically infected leaves. SA-induced resistance to PVX was compromised in Aox-transgenic plants but plants expressing AOX-E exhibited enhanced SA-induced resistance to this virus.ConclusionsWe conclude that AOX-regulated mechanisms not only play a role in SA-induced resistance but also make an important contribution to basal resistance against certain viruses such as PVX.
Highlights
Salicylic acid (SA) regulates multiple anti-viral mechanisms, including mechanism(s) that may be negatively regulated by the mitochondrial enzyme, alternative oxidase (AOX), the sole component of the alternative respiratory pathway
Additional evidence for a role for RNA silencing in SA-mediated defense arose from studies showing that viral silencing suppressor proteins, for example the cucumber mosaic virus (CMV) 2b protein or HC-Pro encoded by potyviruses, interfere with SA-mediated signaling and SA biosynthesis [25,26,27,28]
Construction and characterization of stably transformed N. benthamiana plants with modified alternative respiratory pathway capacity To increase or decrease alternative respiratory pathway capacity, N. benthamiana plants were transformed with, respectively, either a wild-type tobacco Aox1a cDNA, or a mutant sequence, Aox-E [45]
Summary
Salicylic acid (SA) regulates multiple anti-viral mechanisms, including mechanism(s) that may be negatively regulated by the mitochondrial enzyme, alternative oxidase (AOX), the sole component of the alternative respiratory pathway. Studies of this mechanism can be confounded by SA-mediated induction of RNA-dependent RNA polymerase 1, a component of the antiviral RNA silencing pathway. RDR1 may contribute to defense through regulation of host mRNAs encoding other defensive factors, for example those involved in jasmonic acid-induced defenses [19,20,21,22,23,24]. Additional evidence for a role for RNA silencing in SA-mediated defense arose from studies showing that viral silencing suppressor proteins, for example the cucumber mosaic virus (CMV) 2b protein or HC-Pro encoded by potyviruses, interfere with SA-mediated signaling and SA biosynthesis [25,26,27,28]
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