Many diseases of viral etiology have been identified affecting tomato production worldwide, causing important qualitative and quantitative losses. Among the main viral diseases, there are the ones caused by the genus Tospovirus and the...
moreMany diseases of viral etiology have been identified affecting tomato
production worldwide, causing important qualitative and quantitative losses. Among the main viral diseases, there are the ones caused by the genus Tospovirus and the genus Potyvirus. The Tospovirus (family Bunyaviridae) are responsible for the disease known as “vira-cabeça”, causing severe annual losses, mainly in horticultural crops. Species of this genus have a worldwide distribution and a great number of species infecting a vast array of hosts. The main resistance factor employed in tomato breeding for broad spectrum resistance to different tospovirus species is the Sw-5 gene. Plants that harbor this gene express hypersensitivity reactions that restrict the viral systemic infection. However, the ‘breaking’ of this resistance gene has already been reported in different parts of the world. Apart from tospoviruses, species of the genus Potyvirus represent a constant threat to tomato culture. A new species of this genus, Pepper yellow mosaic virus (PepYMV), was initially identified in pepper but it is also capable of infecting tomatoes, causing up to 100% losses. The search for resistance sources for PepYMV on tomato have already started, but the explored germplasm still represents a reduced number of accessions. This thesis shows the evaluation of germplasm collections seeking to identify new resistance sources to PepYMV and to the
neotropical species of tospoviruses. Beyond that, a co-dominant marker for the Sw-5 gene was developed, capable of identifying resistant individuals through a simple PCR. A preliminary analysis of the avirulence component of the Sw-5/Tomato spotted wilt virus (TSWV) interaction was also done, using constructions containing independent TSWV genes, isolates BR-01 (avirulent for the Sw-5 gene) and GRAU (capable of breaking the Sw-5 gene resistance), using a platform based on transgenic Nicotiana benthamiana expressing the Sw-5 gene. In relation to the identification of the new resistance sources (Chapters 2 and 3, respectively), accessions of S. habrochaites showed to be promising sources of resistance to both PepYMV
and Potato virus Y (PVY). On the other hand, accessions of S. peruvianum confirmed this species as the main source of resistance factors to tospoviruses. An accession of S. chilense and a selection of an accession of 4 S. lycopersicum, showed good resistance/tolerance levels to the tospovirus species tested. Both analyses where compared to screenings that where previously done. The Sw-5 specific marker (Chapter 4) was evaluated with a broad array of accessions, showing a profile capable of identifying resistant and susceptible accessions in all situations. Therefore, this marker represents an efficient new tool for assisted selection systems and its future potential use for isolation or detection of alleles or analogous genes in tomato and other solanaceous species. The avirulence component analysis showed that none of the viral genes that where evaluated (Nsm, the glycoprotein precursors, N and Nss) seems to be related in an independent manner with the induction process of the Sw-5 gene recognition mechanism. None of the genes tested was able to induce typical hypersensitive reaction observed when tomato genotypes containing this resistance gene are challenged by TSWV. The strategy used in this study also demonstrated that the biological model represented by transgenic N. benthamiana expressing an active copy of the Sw-5 gene is suitable for analyzing the mechanisms involved in tospovirus resistance. In addition, this system can also be used to investigate Sw-5-resistance breaking isolates.
