The RTM resistance to potyviruses in Arabidopsis thaliana: natural variation of the RTM genes and evidence for the implication of additional genes

Abstract

Background: The non conventional RTM (Restricted Tobacco etch virus Movement) resistance which restricts long distance movement of some plant viruses in Arabidopsis thaliana is still poorly understood. Though at least three RTM genes have been identified, their precise role(s) in the process as well as whether other genes are involved needs to be elucidated.

Methodology/principal findings: In this study, the natural variation of the RTM genes was analysed at the amino acid level in relation with their functionality to restrict the long distance movement of Lettuce mosaic potyvirus (LMV). We identified non-functional RTM alleles in LMV-susceptible Arabidopsis accessions as well as some of the mutations leading to the non-functionality of the RTM proteins. Our data also indicate that more than 40% of the resistant accessions to LMV are controlled by the RTM genes. In addition, two new RTM loci were genetically identified.

Conclusions/significance: Our results show that the RTM resistance seems to be a complex biological process which would involves at least five different proteins. The next challenges will be to understand how the different RTM protein domains are involved in the resistance mechanism and to characterise the new RTM genes for a better understanding of the blocking of the long distance transport of plant viruses.

Figure 1. Amino acid changes in the different allelic forms of the three RTM proteins.

Numbers in the first line correspond to the position of the amino acid changes in each RTM protein according to the Col-0 sequence which corresponds to the allele number 1. The different protein domains are delimited by arrows above the table. (A) Amino acid changes in RTM1; (B) Amino acid changes in RTM2; (C) Amino acid changes in RTM3.

Figure 2. Q-RT-PCR analysis of the expression level of the three RTM genes in different Arabidopsis accessions.

(A) RTM1 expression; (B) RTM2 expression; (C) RTM3 expression. Fold change is determined relative to the value of Col-0 which is set arbitrarily at 1. The qPCR results are normalized to an ubiquitine-conjugating enzyme family gene (At2g36060). The graph represents the average values from three independent experiments involving 3 plants each. Bars represent SD of Ct values calculated using the Roche software. * : P<0.05; indicates that scoring values differ significantly from Col-0. Nd: not determined.