The durable resistance gene Tm-2² from tomato confers resistance against ToMV in tobacco and preserves its viral specificity

Abstract

Recently we isolated the Tm-2² gene of tomato, which confers resistance to tobamoviruses. The gene encodes a polypeptide that belongs to the group of the CC-NBS-LRR resistance proteins. In this paper, we show that the tomato Tm-2² gene can be functionally transferred into tobacco. After introduction of the gene either under the control of the tomato Tm-2² promoter and polyadenylation signal or the CaMV 35S promoter and the NOS-polyadenylation signal, tobacco SR1 transgenic plants become resistant against infection with ToMV. Virus-specificity of the transgenic resistance remains as in the donor species tomato: of the six ToMV-isolates used, only the Tm-2² breaking strain was able to infect transgenic tobacco-lines.

In tomato the matching avirulence gene of the Tm-2² resistance is the Movement Protein (MP) of the virus. Transgenic tobacco plants expressing the MP were crossed to plants harbouring an expressing Tm-2² gene. This resulted in non-viable progeny seeds, implying that as in tomato also in tobacco the Tm-2² matching avirulence protein of ToMV is the MP and its sole presence is enough to trigger the defense response.

Introduction

Tobamoviruses can infect many vegetable and ornamental crops usually giving rise to characteristic mosaic symptoms and growth reduction. Consequently, infection of susceptible agronomically important species, like tomato and tobacco, results in considerable yield losses. Because the virus particles are extremely stable and highly infectious prevention against virus infection is difficult. The most effective defense against infections is, therefore, the introduction of genetic resistance from closely related resistant species. Examples of this strategy are the introduction of the N-resistance gene from Nicotiana glutinosa into N. tabacum [1] and the introgression of the Tm-1, Tm-2 and Tm-2² resistances from respectively, Lycopersicon hirsutum, L. peruvianum and L. pervuvianum into L. esculentum [2]. In both cases this led to a large decrease in yield losses that resulted from tobacco mosaic virus (TMV) and tomato mosaic virus (ToMV) infections.

In many plant species, however, no resistances against tobamoviruses are known or resistances are broken or under threat of being broken. The latter case is exemplified in the plant genus Capsicum (pepper). This plant genus contains a resistance locus, L, which has provided several tobamovirus resistance alleles [3]. However, the usefulness of these alleles appeared to be limited because the tobamoviruses evolved rapidly to circumvent these resistances and replaced the non-compatible strains in the pathogen population. Therefore, the transfer of resistances from one genus to another genus which can not be realized through sexual hybridization would be beneficial. New sources of resistance would become available and resistances, proven to be durable, could be applied in other geni, in which such genes are not available.

Recently, we cloned the Tm-2² resistance gene of L. esculentum [4]. The Tm-2² gene is already in use for almost four decades and, hence, this resistance against tobamoviruses has shown to be durable. The durable resistance of the Tm-2² allele is attributed to the fact that the amino acid residues in the movement protein that have to be changed [5], [6], [7], in order for the virus to circumvent the respective resistance, result in a reduction of virulence [8]. The isolation of this durable gene provides an additional tool against ToMV and TMV infections in crops, if the functionality of this gene in at least closely related species can be demonstrated. Here, we report on the functional introduction of the Tm-2² gene into Nicotiana tabacum SR1, a susceptible host to ToMV and TMV.