A key to understanding the molecular basis for bacterial pathogenicity in plants is the production and deployment of plant defense elicitors and host range determinants. We have previously shown that the hrp/hrm gene cluster isolated from Pseudomonas syringae pv. syringae 61 enables nonpathogenic bacteria to elicit plant defenses. The hrp/hrm gene cluster, therefore, appears to constitute a minimum genetic unit sufficient to produce plant defense elicitors. Fourteen apparent transnational units have been identified within the cluster that are organized as nine apparent transcriptional units. Several hrp genes act as positive regulatory elements. The overall goal of this project is to elucidate the role of specific hrp/hrm genes in determining the pathogenicity and host range of P. syringae strains. Our approach will be to: 1) obtain the nucleotide sequence for remaining portions of the cluster to facilitate later manipulation, comparative analyses with other bacteria and deduction of gene function; 2) characterize the regulatory cascade controlling expression of hrp genes so that the genes directly contributing to the production of plant defense elicitors can be identified and manipulated; and 3) perform gene replacement experiments to test the role of specific hrp genes in determining pathogenicity and host range. %%% The study of these genes should lead to techniques useful in stopping the damage to plants susceptible to bacterial infection.
