DIFFERENTIAL INVOLVEMENT OF INDOLE-3-ACETIC-ACID BIOSYNTHETIC PATHWAYS IN PATHOGENICITY AND EPIPHYTIC FITNESS OF ERWINIA-HERBICOLA PV, GYPSOPHILAE

Erwinia herbicola pv, gypsophilae (Ehg), which induces galls on Gypsop hila paniculata, harbors two major pathways for indole-3-acetic acid ( IAA) synthesis, the indole-3-acetamide (IAM) and indole-3-pyruvate (IP yA) routes, as well as cytokinin biosynthetic genes. Mutants were gene rated in which the various biosynthetic routes were disrupted separate ly or jointly in order to assess the contribution of IAA of various or igins and cytokinins to pathogenicity and epiphytic fitness. Inactivat ion of the IAM pathway or cytokinin biosynthesis caused the largest re duction in gall size. Inactivation of the IPyA pathway caused a minor, nonsignificant decrease in pathogenicity, No further reduction in gal l size was observed by the simultaneous inactivation of both IAA pathw ays only or in combination with that of cytokinin production, However, inactivation of the IPyA pathway caused a 14-fold reduction in the po pulation of Ehg on bean plants. Inactivation of the IAM pathway or cyt okinin production did not affect epiphytic fitness, While the apparent transcriptional activity of iaaM-inaZ fusion increased slightly in ce lls of Ehg on bean and gypsophila leaves, compared with that in cultur e, very high levels of induction were observed in cells injected into gypsophila stems. In contrast, moderate levels of induction of ipdC-in aZ in Ehg were observed on leaves of these plants and in gypsophila st ems, when compared with that in culture, These results suggest that th e IAM pathway is involved primarily in gall formation and support the main contribution of the IpyA pathway to the epiphytic fitness of this bacterial species.