Role and In situ Regulation of Growth Regulators Produced in Plant-Microbe Interactions by Erwinia herbicola

The main objective of this work was to gain a better understanding of how some strains of Erwinia herbicola have evolved into serious plant pathogens while also commonly existing as epiphytes on the surface of healthy plants. The focus of our studies was to determine the nature of, and regulation, of virulence factors, including the phytohormones IAA and cytokinins, which are encoded on a large plasmid (pPATH) found in gall-forming strains of this species. In addition, the in situ regulation and contribution to epiphytic fitness of a second, chromosomal, IAA biosynthetic locus (ipdC) was determined to ascertain the relative contribution of the two redundant IAA-biosynthetic pathways to the biology of E. herbicola. Genes (pre-etz and etz) conferring production of cytokinins were clustered immediately 3' of the iaaM and iaaH genes conferring IAA boisynthesis on pPATH. A new insertion-like element, IS1327, was also found immediately 3' of etz on pPATH, suggesting that these virulence factors were all introduced onto pPATH from another pathogenic bacterium. Mutants of E. herbicola in which etz, iaaH, and iaaM, but not ipdC, were disrupted caused smaller galls to form on gypsophila plants. In contrast, ipdC but not iaaH or iaaM mutants of E. herbicola exhibited reduced ability to grow and survive on plant surfaces. Transcription of ipdC was induced when cells were on plants compared to in culture, suggesting that idpC may play a selective role in fitness on leaves.