the leaves of the wild - type NN - tomato control .   This suggests the mutation in B201 does not affect the cell death pathway leading to HR, but does affect the pathway(s) responsible for N - mediated viral inhibition . Partial resistant phenotype in line B201 is due to a single, recessive mutation:     Line B201 was crossed to wild - type VF36:: NN   tomato, as well as crossed to the near - isogenic TMV sensitive line VF36 .   F1’s from these crosses were inoculated with TMV, and all responded with a wild - type HR and were able to inhibit virus spread, suggesting the mutation is recessive and not a mutation in N .    F2 seed generated from these crosses were screened for their response to TMV .    The phenotype is segregating as a single, recessive mutation . Mapping of the Partial Resistant Mutation in B201 is in progress:   The F2 progeny created by crossing line B201 to L . pennellii has been generated .   The mutant phenotype is scorable in the F2 mapping population, and we are currently in the process of mapping the gene . Fast Neutron - Induced N - Suppressor Mutant:   Approximately 40,000 fast - neutron mutagenized M2 NN tomato seed have been screened to date .   One fully - susceptible mutant has been isolated so far .   This mutant develops normal mosaic symptoms (Fig 2b) .   Preliminary southern blot analysis confirms all N transgenes are intact .   This mutant has been crossed to VF36, and the F1 from this cross have been inoculated with TMV .   All F1 progeny are resistant to TMV, suggesting the mutation responsible for the fully susceptible phenotype is recessive .   Analyses of the F2 progeny are underway . We are continuing to screen our fast - neutron mutagenized population for new mutant classes/loci .   With enough mutants isolated, we hope to identify most of the components necessary for N - mediated TMV resistance .   Characterization of these mutants and study of the relationship between the components will help us to better understand the molecular mechanism(s) behind the disease resistance response .   This study will also provide us an opportunity to look for an alternative way to manipulate the pathway to generate a broad spectrum and durable resistance to improve the health and productivity of the plants . Literature cited: Eshed, Y . and Zamir, D . (1994) .   A genomic library of Lycopersicon pennellii in L . esculentum :   A tool for fine mapping of genes .   Euphytica 79 , 175 - 179 . Whitham, S . , Dinesh - Kumar S . P . , Choi, D . , Hehl, R, Corr, C . , Baker, B . ; (1994) .   The product of the tobacco mosaic virus resistance gene N :   Similarity to Toll and the Interleukin - 1 Receptor .   Cell 78, 1101 - 1115 . Whitham, S . , McCormick, S . , Baker, B . (1996) .   The N gene of tobacco confers resistance to tobacco mosaic virus in transgenic tomato .   Proc . Natl . Acad . Sci . USA 93 , 8776 - 8781 .

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