Evaluation of resistance to tomato curly stunt virus in tomato
Date
2013-01-31
Authors
Dias, Katia
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Abstract
Solanum lycopersicon (the cultivated tomato) is a commodity of great economic importance in South
Africa (SA) as well as worldwide. A destructive viral disease known as Tomato curly stunt virus,
ToCSV-[ZA:Ond:98], belonging to the genus Begomovirus has negatively impacted on tomato
production in SA. This has brought about the need to develop resistant cultivars to ToCSV. Since all
cultivated tomato cultivars are susceptible to ToCSV, resistance genes against the virus found in wild
tomato plant species have been introgressed into the cultivated tomato by plant breeding
techniques. Wild relatives of tomato were adapted to many pathogens (including viruses) as well as
stresses from the surrounding environment. During breeding for improved fruit quality and
increased yield, the gene networks giving rise to many biotic and abiotic stress resistances have been
lost leaving the domesticated tomato extremely susceptible. Plant breeders have reconstituted
some of the gene networks into the cultivated tomato that provide tolerance to stresses including
viruses. They have achieved this by the help of marker-assisted selection (MAS), where the
associated marker is used as an indirect selection criterion. This is an important process in
commercial breeding programs as it allows for a speedy selection of selected traits in the
development of tomato hybrids. The defence response to abiotic stresses in plants includes the
expression of heat shock proteins (HSPs) that function as stress response proteins, molecular
chaperones and proteases which repair or degrade damaged proteins.
The objective of this study was to elucidate the type of resistance mechanism of a tomato inbred
line (TAM), to ToCSV. Since TYLCV-IL shows 77% nucleotide identity with ToCSV, molecular markers
already established for the detection of resistance genes for TYLCV-IL were used to screen TAM.
The inbred line, TAM, was screened for the absence of any of the known resistant genes to TYLCV-IL
using molecular markers already established for the screening of TYCLV-IL resistance genes. TAM
was crossed with susceptible cultivar, Rooikhaki, to produce F1 hybrids. These F1 hybrids were
selfed to produce an F2 population. Infection trials using ToCSV were conducted using TAM inbred
line, F1 hybrids and the F2 population. Since TAM did not have any of the known resistance genes to
TYLCV-IL, a possible novel resistance source to ToCSV was speculated. A clue to the resistant
mechanism against ToCSV resistance in TAM was indicated by the segregation patterns of the F2
population after inoculation with ToCSV. The results suggest that the resistance is under the control
of partially dominant resistant genes. The level of resistance of commercial South African tomato cultivars (Tyler and Tovi-star) against
TYLCV-IL was investigated. The heat shock protein (HSP) profiles of these two SA lines including
susceptible cultivar, Rooikhaki, were treated with abiotic stresses (salt and heat) and results were
compared with a similar study conducted with TYCLV-IL resistant and susceptible tomato cultivars.
Heat shock protein 70 accumulation patterns were similar in that HSP70 was more stable in the
resistant cultivars throughout the application when abiotic stresses were applied to the SA resistant
and susceptible tomato cultivars as compared to Israel resistant and susceptible breeding lines. A
relation between infection severity and the pattern of HSP expression was found. A higher level of
HSP 70 in resistant tomato plants could contribute to a lower symptom severity phenotype.