Characterization of two N - suppressor mutants in tomato Ustach, C . V, Hu, G . , and Baker, B . J . Department of Plant Biology and Microbiology, Univeristy of Berkeley, and Plant Gene Expression Center, USDA, 800 Buchanan Street, Albany, CA   94710 . The N gene confers resistance to tobacco mosaic virus (TMV) in tobacco .   It is hypothesized that N constitutes an early component of a signal transduction pathway which results in the hypersensitive response (HR), systemic acquired resistant (SAR), and pathogen inhibition .   N confers a temperature sensitive hypersensitive response in tobacco and transgenic NN tomato   (Whitham et al, 1996) .   At temperatures below 28 ° C, N functions normally; HR lesions develop and TMV movement is restricted to sites of inoculation .   At temperatures above 28 ° C, the N - mediated HR is suppressed and TMV moves systemically in the plant .   HR is restored when TMV inoculated plants are shifted from high temperatures to temperatures below 28 ° C, resulting in massive cell death which kills the plant .   The ability to reconstruct the temperature sensitive - mediated resistance response in tomato demonstrates that all the components necessary for N - mediated resistance are conserved in tomato, making it an ideal genetic system to isolate and study components of the N signal transduction pathway .   We have identified two mutants that suppress the N gene function in tomato . Materials and Screen Used To Isolate N - Suppressor Mutants Materials:   Our NN transgenic tomato line contains three, linked copies of N , which reduces the possibility of isolating a mutation in N .   We have generated two different M2 mutagenized seed populations for our screen:   one pool is EMS mutagenized, and the other is fast neutron mutagenized . Screen:   N - mediated resistance to TMV in tobacco and transgenic NN tomato is reversibly inactivated at elevated temperatures .   We have exploited this temperature sensitive property of N to isolate mutants using a temperature shift assay developed in our lab (fig 1) .   In a screen using the temperature shift assay, plants bearing a mutation in the N - mediated resistance response will survive the screen, while plants able to mount a normal resistance response will die .   We have used this screen to isolate both EMS and fast neutron N - suppressor mutants . EMS Induced N - Suppressor Mutants :   250,00 M2 EMS mutagenized, NN tomato seed were screened using the temperature shift assay outlined in Figure 1 .   Sixty survivors were isolated; 21 were putative N - suppressor mutants .   All 21 lines share a similar “partial resistance” phenotype (Fig 2a) .   Partial resistance is characterized as simultaneous development of HR lesions and mosaic