Abstract
Restriction of long distance movement of several potyviruses in Arabidopsis thaliana is controlled by at least three dominant restricted TEV movement (RTM) genes, named RTM1, RTM2 and RTM3 and acts as a non-conventional resistance. RTM1 encodes a protein belonging to the jacalin family and RTM2 encodes a protein which has similarities to small heat shock proteins. The recent cloning of RTM3 which encodes a protein belonging to an unknown protein family of 29 members that has a meprin and TRAF homology (MATH) domain in its N-terminal region and a coiled-coil (CC) domain at its C-terminal end is an important breakthrough for a better understanding of this resistance process. Not only the third gene involved in this resistance has been identified and has allowed revealing a new gene family in plant but the discovery that the RTM3 protein interacts directly with RTM1 strongly suggests that the RTM proteins form a multimeric complex. However, these data also highlight striking similarities of the RTM resistance with the well known R-gene mediated resistance.
Key words: plant virus, potyvirus, resistance, long distance movement, RTM genes, Arabidopsis thaliana
The RTM Resistance is not a Conventional Resistance
The resistances which control the systemic invasion of plants by viruses, even if they are widely used in crops, are poorly understood. The RTM [for Restricted Tobacco etch virus (TEV) Movement] resistance which restricts the long distance movement of some viruses belonging to the Potyvirus genus (the most important plant virus genus) in Arabidopsis thaliana (Columbia accession), is the best studied.
This resistance does not correspond to any of the known resistance mechanisms as the R gene-mediated resistance1 or the RNA interference.2 Indeed, viral replication and cell-to-cell movement in inoculated leaves appear unaffected, HR and systemic acquired resistance are not triggered, salicylic acid is not involved.3–5 We also showed that this resistance mechanism is independent of the RNA interference as the single, double or triple RNAseIII dicer-like knockout mutants in which the RNA silencing-based antiviral response is suppressed6 are still resistant to potyviruses (Revers, unpublished). In addition to be active against TEV,7 this resistance was also showed to be active against two other unrelated potyviruses, Lettuce mosaic virus (LMV) and Plum pox virus (PPV),5 showing that the RTM genes seem to be associated to a general resistance mechanism against potyviruses. At least three RTM genes, named RTM1, RTM2 and RTM3, are involved in this resistance process4,7 and seem to act in an interdependent way as a mutation in anyone of these three genes is sufficient to completely abolish the restriction of long distance movement.7 RTM1 and RTM2 were cloned at the end of the nineties and encode a protein belonging to the jacalin family8 and a protein which contains a transmembrane domain and a small heat shock protein domain,9 respectively. Both genes are specifically expressed in phloem-associated tissues.10 Recently from the analysis of some LMV or PPV isolates able and unable to overcome the RTM resistance, it was showed that the N-terminal domain of the coat protein (CP) is the RTM resistance-breaking determinant,11 which is not surprising as this domain is involved in the potyvirus long distance movement process.12
RTM3 Belongs to an Undescribed Gene Family
The recent cloning of RTM3 has revealed the involvement of a new gene family in the RTM resistance mechanism characterized by the presence in the corresponding protein of a meprin and TRAF homology (MATH) domain in the N-terminal part and a coiled coil domain in the C-terminal part.13 Interestingly, twenty-eight genes were found to contain in the corresponding protein these two domains which share for most of them more than 50% of identity with the RTM3 domains.13 Sixteen of these genes are located in a cluster with RTM3 in which five genes for which the corresponding protein contain only a coiled coil domain are also identified. Other smaller clusters of genes associating RTM3 homologous genes with or without a MATH domain in the corresponding protein were also found in the Arabidopsis genome suggesting the role of duplication events in shaping the evolutionary history of this gene family, including the possibility of deletion or duplication of one or the other domain. The role of RTM3 in a resistance mechanism is the first biological function identified for this gene family in plant. This is also the first function identified among the 71 Arabidopsis genes encoding a MATH protein.
It is quite surprising that any functional data are available for this gene family. As numerous MATH proteins are found in plants (Medicago, rice), in lower eukaryota (Trypanosoma, Plasmodium) and in lower metazoa such as the nematode C. elegans, we can assume they play important roles in cells. MATH domains seem to be important for protein-protein interaction and several studies on human and C. elegans MATH proteins seem to indicate they might have important functions in the regulation of protein processing.14
The Hypothesis of the RTM Protein Complex
Another major data we recently produced for the understanding of this resistance is the fact that RTM3 directly interacts with itself and with RTM1 in planta.13 This interaction suggests that the RTM proteins might act as a multiprotein complex to block the long distance movement of potyviruses. However, no direct interaction has been shown between RTM1 or R TM3 with RTM2 and between the RTM proteins and the CP of potyviruses. As the protein-protein interaction experiments used in this study do not allow to reveal interactions between more than two proteins, it can not rule out the possibility that the RTM complex formed by monomers of RTM1 and RTM3 may interact with virions or viral ribonucleoprotein complexes by the N-terminal domains of the CP monomers exposed at the surface of the virus particles.
As the RTM proteins are expressed in companion cells (CC), several mechanisms can be suggested from all these data. (1) The RTM protein complex could sequester or degrade virions or mobile viral RNPs which could not be translocated from CCs to sieve elements (SE) through plasmodesmata (PD). (2) The RTM complex could sequester one or several factors necessary for translocation of the virions/viral RNPs from CC to SE. (3) The RTM factors may induce or activate an antiviral response in the phloem tissue which recognizes and targets either the virus movement competent form through an interaction with the N-terminal end of the CP, or this N-terminal end of the CP itself from free CP monomers which might be considered as an effector. Overbreaking virus isolates which have mutation in the N-terminal domain of the CP and are therefore able to move systemically, do not interact with the RTM complex in the proposed mechanisms (1) and (3) or might be able to use other plant factors to move systemically in the case of mechanism (2).
Several arguments might be in favor of an induced plant defence response. Indeed, the identification of RTM3 reveals intriguing and striking similarities with the dominant R-gene response in plants: (1) clustered arrays of RTM3, a dominant resistance gene, and its homologs in the Arabidopsis genome showing evidence of gene duplication and deletion events; (2) RTM3 being a protein with a CC domain that is important for the resistance response; (3) the potyvirus CP being the necessary determinant (effector?) and the indication that the recognition of this viral factor might involve a RTM multiprotein complex; (4) involvement of a heat shock protein (RTM2). However, there is no HR, production of SA or induction of SAR. Nevertheless as R gene-effector interactions have to date only been studied in leaf mesophyll and epidermal cells and has not been investigated in phloem-associated tissues, signaling may be different as an HR in the phloem would likely prove fatal to the plant. The RTM resistance may be a novel form of immunity.
If the cloning of RTM3 and the characterisation of protein-protein interactions between the RTM proteins help to a better knowledge of the involved factors and their putative roles in this resistance process, it remains several important questions to solve in order to decipher this resistance mechanism. Among them, subcellular localization of the different RTM proteins will have to be elucidated as well as the subcellular blocking of the virions/viral RNPs. The identification of the plant factors interacting either with the RTM complex and/or with virions or viral RNPs will be also an important challenge for the next years.
Footnotes
Previously published online: www.landesbioscience.com/journals/psb/article/13244
References
- 1.Soosaar JLM, Burch-Smith TM, Dinesh-Kumar SP. Mechanisms of plant resistance to viruses. Nat Rev Micro. 2005;3:789–798. doi: 10.1038/nrmicro1239. [DOI] [PubMed] [Google Scholar]
- 2.Voinnet O. Induction and suppression of RNA silencing: insights from viral infections. Nat Rev Genet. 2005;6:206–220. doi: 10.1038/nrg1555. [DOI] [PubMed] [Google Scholar]
- 3.Revers F, Guiraud T, Houvenaghel MC, Mauduit T, Le Gall O, Candresse T. Multiple resistance phenotypes to lettuce mosaic virus among Arabidopsis thaliana accessions. Mol Plant-Microbe Interact. 2003;16:608–616. doi: 10.1094/MPMI.2003.16.7.608. [DOI] [PubMed] [Google Scholar]
- 4.Mahajan SK, Chisholm ST, Whitham SA, Carrington JC. Identification and characterization of a locus (RTM1) that restricts long-distance movement of tobacco etch virus in Arabidopsis thaliana. Plant J. 1998;14:177–186. doi: 10.1046/j.1365-313x.1998.00105.x. [DOI] [PubMed] [Google Scholar]
- 5.Decroocq V, Sicard O, Alamillo JM, Lansac M, Eyquard JP, Garcia JA, et al. Multiple resistance traits control Plum pox virus infection in Arabidopsis thaliana. Mol Plant Microbe Interact. 2006;19:541–549. doi: 10.1094/MPMI-19-0541. [DOI] [PubMed] [Google Scholar]
- 6.Deleris A, Gallego-Bartolome J, Bao J, Kasschau KD, Carrington JC, Voinnet O. Hierarchical action and inhibition of plant Dicer-like proteins in antiviral defense. Science. 2006;313:68–71. doi: 10.1126/science.1128214. [DOI] [PubMed] [Google Scholar]
- 7.Whitham SA, Yamamoto ML, Carrington JC. Selectable viruses and altered susceptibility mutants in Arabidopsis thaliana. Proc Natl Acad Sci USA. 1999;96:772–777. doi: 10.1073/pnas.96.2.772. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Chisholm ST, Mahajan SK, Whitham SA, Yamamoto ML, Carrington JC. Cloning of the Arabidopsis RTM1 gene, which controls restriction of long-distance movement of tobacco etch virus. Proc Natl Acad Sci USA. 2000;97:489–494. doi: 10.1073/pnas.97.1.489. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Whitham SA, Anderberg RJ, Chisholm ST, Carrington JC. Arabidopsis RTM2 gene is necessary for specific restriction of tobacco etch virus and encodes an unusual small heat shock-like protein. Plant Cell. 2000;12:569–582. doi: 10.1105/tpc.12.4.569. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Chisholm J, Parra MA, Anderberg RJ, Carrington JC. Arabidopsis RTM1 and RTM2 genes function in phloem to restrict long-distance movement of tobacco etch virus. Plant Physiol. 2001;127:1667–1675. [PMC free article] [PubMed] [Google Scholar]
- 11.Decroocq V, Salvador B, Sicard O, Glasa M, Cosson P, Svanella-Dumas L, et al. The determinant of potyvirus ability to overcome the RTM resistance of Arabidopsis thaliana maps to the N-terminal region of the coat protein. Mol Plant Microbe Interact. 2009;22:1302–1311. doi: 10.1094/MPMI-22-10-1302. [DOI] [PubMed] [Google Scholar]
- 12.Revers F, Le Gall O, Candresse T, Maule AJ. New advances in understanding the molecular biology of plant/potyvirus interactions. Mol Plant Microbe Interact. 1999;12:367–376. [Google Scholar]
- 13.Cosson P, Sofer L, Le H, Leger V, Schurdi-Levraud V, Whitham SA, et al. RTM3 which controls long distance movement of potyviruses is a member of a new plant gene family encoding a meprin and TRAF homology (MATH) domain-containing protein. Plant Physiol. 2010;154:222–232. doi: 10.1104/pp.110.155754. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Zapata JM, Martinez-Garcia V, Lefebvre S. Phylogeny of the TRAF/MATH domain. Adv Exp Med Biol. 2007;597:1–24. doi: 10.1007/978-0-387-70630-6_1. [DOI] [PubMed] [Google Scholar]