| Plants have evolved a variety of mechanisms to defend themselves against microbial attack. Recently, great strides have been made in elucidating the molecular mechanisms involved in a specific resistance response referred to as the “gene-for-gene” response. In this response, the plant host possesses a dominant gene whose product recognizes a specific protein expressed by a pathogen. Due to the specificity of the interaction between the plant “R gene” and the pathogen “avr gene”, it is hypothesized that the R protein acts as a receptor for the pathogen Avr protein-ligand. This interaction elicits a signal transduction cascade responsible for local and systemic resistance responses. The N gene, which confers resistance to tobacco mosaic virus (TMV), has been cloned from tobacco and its Avr partner has been identified as the replicase protein of TMV. Even though these two partners have been identified, little is known about the down-stream components involved after the N protein elicits a resistance response upon TMV infection.; The N gene from tobacco confers resistance to TMV; even though N was cloned from tobacco, N also confers TMV resistance in transgenic tomato. Presented in this thesis is the development and use of a unique seedling screen based on N-mediated HR to isolate mutants in the N-dependent signal transduction pathway. The positive-selection screen was used to screen both an EMS mutagenized tomato::NN M2 population and a fast-neutron mutagenized tomato:: NN M2 population for N-signal transduction mutants. Two mutants were isolated. The EMS induced mutant, psn1 (p&barbelow;artial s&barbelow;uppressor of N&barbelow;), is partially suppressed in the N-dependent signal transduction pathway. Characterization of psn1 is presented in chapter two. The fast-neutron induced mutant son1 (s&barbelow;uppressor o&barbelow;f N&barbelow;) is completely suppressed in the N-dependent TMV resistance. Chapter three describes the mapping of son1 using a backcross population created by crossing son1 to Lycopersicon pennellii, and then crossing the resultant F1 hybrid back to son1. son1 mapped to chromosome 6 and is genetically linked to RFLP marker TG292.; In addition to isolating components of the N-signal transduction pathway, work in Chapter four is presented in which steps have been taken to clone a functional ortholog of N from a wild Nicotiana species. N was originally introgressed into Nicotiana tabacum (domesticated tobacco) by crossing tobacco to the wild species Nicotiana glutinosa. Several other Nicotiana species bear TMV resistance genes, and isolating these genes could enhance a study of N evolution within the Nicotiana species. Evidence is presented which demonstrates the Nicotiana repanda TMV resistance gene, Nrn (N&barbelow;. r&barbelow;epanda N&barbelow; ortholog), is a functional ortholog of the N gene from N. glutinosa. |
