Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: October 19, 2004
Publication Date: February 7, 2005
Citation: Kong, H.N., Patterson, C.D., Lydon, J. 2005. A mutation in a TonB membrane transport system disrupts tagetitoxin production by Pseudomonas syringae pv. tagetis. Weed Science Society of America Meeting Abstracts. 45:64.
Pseudomonas syringae pv. tagetis EB037 is a tagetitoxin-producing bacterium isolated from common ragweed (Ambrosia artemisifolia L.). To identify genes required for tagetitoxin production, we conducted transposon Tn5 mutagenesis studies on this strain. One mutant (Tox-9) obtained was capable of producing only trace amounts of tagetitoxin when grown under conditions that resulted in the production of very high amounts of toxin by the wild-type strain. A DNA translation BLAST analysis (http://www.ncbi.nlm.nih.gov) revealed that the transpositional insertion of Tn5 occurred in an open reading frame designated as exbD2, which was upstream from a related open reading frame, exbB2. The ExbB and ExbD proteins are auxiliary proteins to TonB in the TonB/ExbB/ExbD complex, a transport system common among Gram-negative bacteria that interacts with outer membrane receptors involved in iron uptake. Homologs of these genes have not previously been described for P. syringae pv. tagetis . The growth of the exbD2 mutant in minimal media (MM) without ferric iron or in MM with ethylenediamine-di(o-hydroxyphenyacetic acid) and without ferric iron was inhibited as compared to growth in MM with ferric iron. However, the reductions in growth were similar to that of the wild-type strain grown under the same conditions. Siderophore production under iron-limiting conditions was the same for the wild type and exbD2 mutant. In addition, the exbD2 mutant induced a hypersensitive response in Nicotiana tabacum L. leaves that was of the same intensity as that induced by the wild-type strain. The results demonstrate that the mutation in exbD2 disrupted normal toxin accumulation while not altering ferric iron uptake. While other TonB systems present in P. syringae pv. tagetis may fully compensate for the role of exbD2 in ferric iron uptake, such compensation does not correct for the compromised extracelluar accumulation of tagetitoxin. Whether the disruption in the transport function of the TonB2 system affects the uptake of metabolites required for tagetitoxin production or the export of the toxin from the cell remains to be determined.