Disease resistance in plants is dependent on an active defense response. The negation of these pleiotropic responses results in susceptibility. Several genes of known function and some opf unknown function are activated in plant tissue soon after challenge by the pathogen. Several genes controlling functional enzymes such as beta-glucanase and of several disease resistance response genes (DRRG) have been cloned in this laboratory. The latter genes are highly activated in temporal association with the expression of resistance and suppressed when the plant becomes susceptible. Two recent observations, that the 3' and 5' regions of DRRG-49 possess eukaryotic topoisomerase II consensus sequences and that the fungal challenge of pea tissue sharply increases topoisomerase I activity, form the basis for this proposal. A substantial body of recent research has indicated that some increases in transcription are associated with the topological features of chromosomal loops. This proposal focuses on the plant topoisomerases, their cleavage sites, and the influence of their activity on plant defense genes. Because of the limited information on these enzymes in plants the objectives are designed to purify the enzymes and specifically define the DNA attachment sites. Eventually, it is planned to determine whether their accumulation, cellular localization or DNA helix- altering potential cause or can be a factor in the transcriptional changes associated with fungal- or fungal elicitor-induced disease resistance responses. The disease resistance of plants is associated with a response which is activated following contact of the pathogen with host plant tissue. Multiple plant genes are activated immediately following the transfer of a signal from pathogen to host. The proposed research will test the hypothesis that activation of the transcription of the multiple plant defense response genes is, at least in part, due to increases in the activity of topoisomerases, enzymes that affect gene expression by altering the three dimensional structure of coiled DNA in the nucleus.
