Genetic modification of alternative respiration in Nicotiana benthamianaaffects basal and salicylic acid-induced resistance to potato virus X
© Lee et al; licensee BioMed Central Ltd. 2011
Received: 13 December 2010
Accepted: 28 February 2011
Published: 28 February 2011
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. 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.
Antimycin 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.
We 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.
Salicylic acid (SA) is an important defensive signal in plants that is required for elicitor-triggered immunity and the establishment of systemic acquired resistance (SAR)[1–8]. Plants exhibiting SAR possess an enhanced state of protection against a broad spectrum of pathogens, including viruses, oomycetes, fungi and bacteria [3, 5]. SA inhibits various phases of the viral life cycle including replication, cell-to-cell movement and systemic movement. However, the precise effects of SA can differ between various host-virus combinations [9–16].
RNA silencing (also known as post-transcriptional gene silencing) is thought likely to contribute to SA-induced virus resistance although it is unlikely to be the only mechanism involved . RNA silencing is a sequence-specific mechanism regulating the synthesis, stability, and translatability of mRNA molecules that is guided by small RNA molecules in the size range 21-26 nt [reviewed in ref. ]. The involvement of RNA silencing in SA-induced virus resistance was first suggested by the discovery that SA can induce expression of a component of the RNA silencing machinery, RNA-directed RNA polymerase 1 (RDR1) . RDR1 may also contribute to defense through regulation of host mRNAs encoding other defensive factors, for example those involved in jasmonic acid-induced defenses [19–24]. 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 [25–28].
There is evidence that mitochondrial signaling processes regulate some aspects of SA-induced virus resistance [discussed by [1, 3]]. Reactive oxygen species (ROS) are constantly generated within mitochondria as by-products of respiratory electron transport chain activity [29–31]. Perturbation in this ROS pool or in mitochondrial redox can function in intracellular signal transduction and, through the poorly understood process of mitochondrial retrograde regulation, affect the pattern of nuclear gene expression [31–35]. This form of signaling is influenced by the alternative oxidase (AOX). AOX is a mitochondrial enzyme that is the sole component of the alternative respiratory pathway. The functions of the alternative respiratory pathway include negative regulation of mitochondrial ROS production and maintenance of primary metabolism under stress conditions [30, 36–40].
Evidence supporting an additional role for mitochondrial signaling and AOX in virus resistance includes observations that non-toxic levels of respiratory inhibitors such as antimycin A or cyanide induce resistance against several plant viruses [9, 12, 13, 15, 16, 41, 42], and that SA, which is a weak cytochrome pathway inhibitor, induces Aox1a gene expression [36, 43, 44]. Murphy and associates  found that expression of an AOX coding sequence by a TMV-derived expression vector enhanced its spread in N. benthamiana plants. Gilliland and colleagues  found that in directly inoculated leaves, SA- and antimycin A-induced resistance to TMV was transiently enhanced in transgenic plants that had decreased alternative respiratory pathway capacities. However, in transformed tobacco plants that had increased alternative respiratory pathway capacities due to constitutive expression of an Aox1a transgene, the induction of resistance to TMV by antimycin A was inhibited, while SA-induced resistance was not .
To explain these results, it was proposed that SA and antimycin A both stimulate a signaling pathway that is negatively-regulated by AOX. However, RDR1, a factor in resistance to TMV , is also inducible by SA but not by antimycin A [20, 46]. Since RDR1 gene expression is not affected by AOX, it was suggested that this is why an increase in alternative respiratory pathway capacity inhibits antimycin A-induced resistance but does not completely inhibit SA-induced resistance to TMV [20, 46]. To further investigate the AOX-regulated mechanism of resistance to viruses we have used the experimental host plant N. benthamiana. This is highly susceptible to a wide range of viruses, which is explained to some extent by the fact that its RDR1 ortholog, NbRDR1m, encodes an inactive enzyme . Thus, N. benthamiana provides a natural mutant background for exploring the effect of AOX on plant responses to virus infection without the potentially confounding presence of active RDR1.
Construction and characterization of stably transformed N. benthamianaplants with modified alternative respiratory pathway capacity
Alternative respiratory pathway capacity in transgenic plant lines expressing AOX or AOX-E that were used in this study
Oxygen Consumption Rates (nmol O2 min-1/106 cell)2
Antimycin A + SHAM
1.86 ± 0.08
1.43 ± 0.01
0.04 ± 0.04
5.35 ± 0.36
6.08 ± 0.68
0.79 ± 0.13
3.38 ± 0.33
4.62 ± 0.54
0.54 ± 0.11
2.03 ± 0.28
0.77 ± 0.13
0.39 ± 0.06
1.68 ± 0.11
0.73 ± 0.11
0.18 ± 0.02
Empty vector control
2.18 ± 0.27
1.34 ± 0.11
0.29 ± 0.03
1.99 ± 0.06
2.47 ± 0.13
0.065 ± 0.003
2.08 ± 0.03
2.55 ± 0.01
0.105 ± 0.024
Altering alternative respiratory pathway capacity affected basal resistance to potato virus X
Salicylic acid-induced resistance to PVX is modified in plants with increased and decreased alternative respiratory pathway capacity
Following treatment with SA, PVX coat protein accumulation was monitored in transgenic plants from lines with increased or decreased alternative respiratory capacities at 28 days post-inoculation. Treatment with SA induced resistance to PVX in non-transgenic N. benthamiana but the treatment did not induce resistance to the virus in Aox-transgenic plants (Figure 3). However, as an inducer of resistance to PVX, SA was markedly more effective in these plants than in non-transgenic or Aox-transgenic plants, with virus accumulation decreased to barely detectable levels (Figure 3). These data show that in N. benthamiana plants, suppression of alternative respiratory pathway capacity combined with SA treatment led to greater PVX resistance and that SA-induced resistance to PVX was compromised by enhancement of alternative respiratory pathway capacity. Thus, for PVX, AOX-modulated defensive signalling is the predominant factor in the regulation of SA-induced resistance.
Altering alternative respiratory pathway capacity affects chemical induction of resistance to tobacco mosaic virus in N. benthamiana
Pre-treatment of non-transgenic N. benthamiana plants with SA inhibited TMV-induced symptom development in systemically infected tissues. However, modifying alternative respiratory pathway capacity in transgenic plants did not affect the timing or appearance of systemic disease symptoms induced by TMV regardless of whether or not the transgenic plants had been treated with SA (data not shown), similar to results in tobacco .
The effects of altering Aox and RDR1 gene expression on chemical induction of resistance to tobacco mosaic virus in N. benthamiana
In line with a model proposed by Singh and colleagues , modifying alternative respiration by constitutive expression of AOX or the dominant-negative mutant AOX-E affected the outcome of infection by viruses. For PVX, our data indicates that SA-induced resistance is regulated strongly by AOX and, unexpectedly, that this enzyme also plays a role in maintaining basal resistance to this virus. The interactions of these transgenic N. benthamiana plants with TMV are similar to those observed previously in tobacco plants transformed with Aox-derived transgenes [20, 50]. Thus, as in tobacco, SA-induced resistance to TMV was enhanced in N. benthamiana plants with diminished alternative respiratory pathway capacity but apparently unaffected in plants with an enhanced respiratory capacity. Antimycin A-induced resistance to TMV was, respectively, enhanced or inhibited in transgenic plants in which alternative respiratory pathway capacity had been diminished or increased. However, basal resistance to TMV in N. benthamiana was not affected by modification of alternative respiratory capacity.
Previous studies showed that in tobacco, NtRDR1 activity contributes to the maintenance of basal resistance to PVX whilst in N. benthamiana it is NbRDR6, not NbRDR1, which is a major determinant of basal resistance against this virus [19, 22, 51]. Our data suggests that in N. benthamiana the defensive signal transduction pathway regulated by AOX also plays an important role in basal resistance to PVX. This result was unexpected since evidence from Gilliland and colleagues  had indicated that AOX-regulated defensive signaling plays no discernable role in basal resistance to TMV.
The result was further surprising since PVX, like TMV, is a positive-strand RNA virus . Thus, these viruses replicate in a similar fashion, although they have differing strategies for gene expression and cell-to-cell movement [reviewed by ]. It may be that TMV is more effective than PVX at subverting or inhibiting the antiviral mechanisms that are regulated by AOX. TMV, for example, appears to be able to interfere with a wide range of SA-mediated responses through interaction between its replicase proteins and a transcription factor that regulates basal defense . Alternatively, it may be that PVX is able to evade some of the SA-induced resistance mechanisms that inhibit TMV infection. For instance, although the accumulation of TMV and PVX was inhibited by NtRDR1, the RDR1 of Medicago truncatula inhibited TMV accumulation but PVX appears to be unaffected by this RDR1 ortholog [19, 22].
Another possibility is that PVX, but not TMV, elicits a reaction similar or analogous to PAMP-triggered immunity. PAMPs or pathogen-associated molecular patterns are chemical signatures produced by cellular pathogens (bacteria, fungi, etc) that are perceived by receptor-like kinases, resulting in the triggering of localized defense responses including the generation of ROS . These localized defense responses can underlie non-host resistance (in which plants are not susceptible to the pathogen) or provide some minimal level of resistance to infection (basal resistance) in susceptible plants. Typically, PAMPs are quite generic in their nature and for bacterial pathogens include fragments of flagellin or translation factors . Baurès and colleagues  have theorized that in PVX and other potexviruses a highly conserved amino acid sequence or folded structure in the coat protein might function as a PAMP. It is also known that the PVX coat protein interacts with a factor, the RanGAP2 protein, which may act as a target or decoy for the PVX coat protein and (in plants harbouring the Rx resistance gene) may participate in triggering resistance [58–60]. It is possible that interactions of the PVX coat protein with an unknown receptor, or with RanGAP2 in susceptible plants (such as the N. benthamiana used in this study), result in induction of basal resistance mechanisms via the production of ROS. Maxwell and colleagues  showed that AOX can affect ROS levels throughout the cell, so it could be that this signaling mechanism is affected in plants with modified alternative respiration. Possibly consistent with this idea are observations showing that potexvirus infections are associated with complex changes in ROS and in the activity of anti-oxidant enzymes .
The study revealed that for PVX, SA-induced resistance is much simpler in its execution than SA-induced resistance to TMV. For PVX, AOX-modulated defensive signaling is the predominant factor in controlling SA-induced resistance. In conclusion, the results of this study show that AOX-regulated signaling constitutes an important part of plant antiviral resistance. However, it is also clear that the phenomena of SA-induced resistance and basal resistance to viruses both result from the operation of multiple antiviral mechanisms that do not inhibit all viruses to an equal extent.
Plant Growth Conditions
Seeds of Nicotiana benthamiana (Domin.) non-transgenic and transgenic plant lines were, as appropriate, germinated on soil or under sterile conditions on 1% (w/v) agar containing Murashige and Skoog medium (Melford Ltd, Ipswich, UK). For germination of transgenic seed, agar media were supplemented with kanamycin (50 μg.ml-1), hygromycin B (29 μg.ml-1), or both antibiotics as appropriate. After transfer to soil, all plants were maintained in a growth room (Conviron Ltd., Winnipeg, Manitoba, Canada) under a 16 h photoperiod (200 μE.m-2.s-1 of photosynthetically active radiation) at 22°C and 60% relative humidity.
Generation of transgenic tobacco and N. benthamiana constitutively expressing wild-type and mutant Aox1agene sequences
Adapting a method previously used , pDJSnAOX-E was generated by mutagenesis of the Aox1 cDNA insert of pDJSn  to replace the codon for the active site glutamate 221 with one for alanine using the oligonucleotides 5'-GAAGCTGAAAATGCCAGGATGCACCTCATGAC-3' and 5'-GTCATGAGGTGCATCCTGGCATTTTCAGCTTC-3', and the Stratagene Quikchange XL site-directed mutagenesis kit (http://www.stratagene.com/). Agrobacterium tumefaciens strain GV3101 cells were transformed with DNA for pDJSn or PDJSnAOX-E using the freeze-thaw method . The leaf disc method  was used for A. tumefaciens-mediated transformation of N. benthamiana and transgenic N. benthamiana harboring the 35S:MtRDR1 transgene . Conditions used for transformation of N. benthamiana were similar to those used for tobacco , except that shooting and rooting media were modified to contain 1 μM 1-naphthaleneacetic acid. Transformed lines were screened for transgene incorporation and expression using PCR and RT-PCR, respectively. Immunoblot analysis was used to detect constitutive AOX production and altered alternative respiratory pathway capacity was determined using established methods [20, 45, 40, 64]. Lines of doubly-transformed plants derived from MtRDR1-transgenic plants that had been super-transformed with pDJSn were also checked for expression of the MtRDR1 by RT-PCR and using in vitro RDR activity assays as described by Xie et al. .
Virus strains, detection of infection and chemical treatments
TMV (Genus, Tobamovirus; Species, Tobacco mosaic virus) strain U1  and PVX (Genus Potexvirus; Species Potato virus X) strain UK3  were used in this study. Immunoblot detection of TMV and PVX using appropriate anti-coat protein sera followed a previously described method . For quantification of PVX accumulation, leaf samples were homogenized as described by Murphy et al.  and used in double-antibody sandwich-ELISA  using reagents and antibodies and following instructions supplied by Bioreba (http://www.bioreba.ch/). Quantification was achieved with a standard curve using known amounts of purified virus. Data from transgenic plants were assessed for statistically-significant (p < 0.05) differences from non-transgenic controls using two-tailed t-tests [statistical software Genstat® 2010 Thirteenth Edition, © Lawes Agricultural Trust (Rothamsted Research), VSN International Ltd., Hemel Hempstead, UK.; ].
For whole-plant treatments with SA, five-to-six week old N. benthamiana plants were sprayed for four consecutive days with either a control solution [0.05% (w/v) ethanol] or 1 mM SA dissolved in 0.05% (w/v) ethanol before rub inoculation with PVX or TMV on one or two lower leaves as previously described for tobacco [11, 20]. Chemical treatment of leaf tissue by infiltration was carried out as described by Gilliland et al.  using SA or antimycin A concentrations described in Results and figure legends.
The authors thank Alex Murphy, Androulla Gilliland and Mat Lewsey for useful discussions and Zhixiang Chen for advice on performing in vitro RDR assays. We thank Rick Nelson for MtRDR1-expressing N. benthamiana and the transformed control line, Tom Elthon for anti-AOX monoclonal antibody, and Mike Wilson for anti-TMV CP polyclonal serum. FSF was funded by grants from the Cambridge Overseas Trust and the Ministry of Education of Taiwan, and WSL was funded by a studentship from the Biotechnology and Biological Sciences Research Council (BBSRC). JVL was funded by a USDA CSREES-NRI-2007-01530 Research Sabbatical Grant. Work in JPC's laboratory is supported by grants from The Leverhulme Trust (F/09 741/F), BBSRC (BB/D008204/1; BB/F014376/1), the EU and the Cambridge University Isaac Newton Trust.
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