Evolutionary Pathways to Break Down the Resistance of Allelic Versions of the PVY Resistance Gene va.

Emergence of viral genotypes can make control strategies based on resistance genes ineffective. A few years after the deployment of tobacco genotypes carrying alleles of the Potato virus Y (PVY) recessive resistance gene va, virulent PVY isolates have been reported, suggesting the low durability of va. To have a broader view of the evolutionary processes involved in PVY adaptation to va, we studied mutational pathways leading to the emergence of PVY resistance-breaking populations. The viral genome-linked protein (VPg) has been described to be potentially involved in va adaptation. Analyses of the VPg sequence of PVY isolates sampled from susceptible and resistant tobacco allowed us to identify mutations in the central part of the VPg. Analysis of the virulence of wild-type isolates with known VPg sequences and of mutated versions of PVY infectious clones allowed us to (i) validate VPg as the PVY virulence factor corresponding to va, (ii) highlight the fact that virulence gain in PVY occurs rapidly and preferentially by substitution at position AA105 in the VPg, and (iii) show that the 101G substitution in the VPg of a PVYC isolate is responsible for cross-virulence toward two resistance sources. Moreover, it appears that the evolutionary pathway of PVY adaptation to va depends on both virus and host genetic backgrounds.

Development of SCAR markers linked to three disease resistances based on AFLP within Nicotiana tabacum L.

Amplified fragment length polymorphism (AFLP) was conducted on a set of 92 Nicotiana tabacum L. accessions from diverse types (flue-cured, dark air-cured, burley, oriental, and cigar wrapper) and breeding origins to identify markers associated with disease resistances. Eleven primer combinations were required to identify 33 polymorphic fragments. This allowed the identification of 92% of these accessions, and yielded sufficient information for building a neighbor joining tree. Clusters of accessions with common traits or breeding origins were observed. An important part of this polymorphism could be related to interspecific introgressions from other Nicotiana species, performed during the breeding history of N. tabacum to confer resistance to pathogens. Seven fragments were associated with three different resistances: two for the blue-mold (Peronospora tabacina Adam) resistance derived from Nicotiana debneyi Domin, two for the Va gene (Potato Virus Y susceptibility), and three for the black root rot (Chalara elegans) resistance of N. debneyi origin. Some of these markers were converted into sequence characterized amplified region markers, and validated on recombinant inbred lines or doubled-haploid lines.

Pathogen-induced elicitin production in transgenic tobacco generates a hypersensitive response and nonspecific disease resistance.

The rapid and effective activation of disease resistance responses is essential for plant defense against pathogen attack. These responses are initiated when pathogen-derived molecules (elicitors) are recognized by the host. We have developed a strategy for creating novel disease resistance traits whereby transgenic plants respond to infection by a virulent pathogen with the production of an elicitor. To this end, we generated transgenic tobacco plants harboring a fusion between the pathogen-inducible tobacco hsr 203J gene promoter and a Phytophthora cryptogea gene encoding the highly active elicitor cryptogein. Under noninduced conditions, the transgene was silent, and no cryptogein could be detected in the transgenic plants. In contrast, infection by the virulent fungus P. parasitica var nicotianae stimulated cryptogein production that coincided with the fast induction of several defense genes at and around the infection sites. Induced elicitor production resulted in a localized necrosis that resembled a P. cryptogea-induced hypersensitive response and that restricted further growth of the pathogen. The transgenic plants displayed enhanced resistance to fungal pathogens that were unrelated to Phytophthora species, such as Thielaviopsis basicola, Erysiphe cichoracearum, and Botrytis cinerea. Thus, broad-spectrum disease resistance of a plant can be generated without the constitutive synthesis of a transgene product.