ROLE OF INDOLEACETIC ACID PRODUCTION IN PATHOGENS IN THE PLANT-MICROBE INTERACTION
T YAMADA, A YUNUS, Y MURAKAMI, T NISHINO, Y ICHINOSE, T SHIRAISHI and K TOYODA
Laboratory of Plant Pathology and Genetic Engineering, Faculty of Agriculture, Okayama University, Tsushima Okayama, 700, Japan
Background and objectives
A wide variety of microorganisms that live in close association with plants either as parasites or as symbionts have developed the capacity to alter host physiological processes to their own advantage. One example is that microbial pathogens use phytohormone(s) to alter physiological conditions in the host, such as the production of indoleacetic acid (IAA) and cytokinin that cause plant neoplastic diseases. Examples are Pseudomonas syringae pv. savastanoi, Agrobacterium tumefaciens, Agrobacterium rhizogenes and Erwinia herbicola. Although convincing evidence is not yet available, hypertrophy is thought to be caused by high concentrations of auxin and/or cytokinin, produced by these microorganisms, which perturb the regulated hormone balance in the host cells. The production of IAA itself is widespread in the plant kingdom, including bacteria and fungi, although the amount of IAA production and its biosynthetic pathway vary from one organism to the other. Most microorganisms carry the IAA biosynthetic pathway from tryptophan (Trp) to indole-3-pyruvic acid (IPA) and indole-3-acetaldehyde (IAAID) as intermediates. One intriguing feature in neoplastic disease caused by the microorganisms examined so far is their common IAA biosynthetic pathway from Trp to indole-3-acetamide (IAM) as an intermediate and the fact that genes involved in this pathway reside on the virulence plasmid. P. ; syringae pv. syringae and P. ;glycinea, which do not cause hypertrophic disease, carry genes highly homologous to iaaM and iaaH of P. ;savastanoi, although biosynthetic capacity of IAA may not be the same.
Results and conclusions
To understand the role of these genes in plant-associated bacteria, we investigated the effect of expression of prokaryotic-type iaaM and iaaH genes on A. ;tumefaciens virulence in plant tumorigenicity. To direct the expression of P. ;savastanoi iaa genes in Agrobacterium, a DNA fragment spanning the iaa promoter region, iaaM and iaaH, was subcloned into the broad host-range vector pRK415 (pTOY1). A. ;tumefaciens A208 carrying pTOY1 suppressed the activity of PAL, which catalyses the first committed step in the synthesis of antimicrobial phytoalexins and lignin (comparing with the wild type A. ;tumefaciens A208).
We speculated that IAA production via IAM as an intermediate in these plant-associated microorganisms might facilitate the interaction with the host plant. To develop this concept in fungi, an endophyte, Acremonium spp., was transformed with an Aspergillus trp promoter-P. ; savastanoi iaam chimeric gene to explore the enlargement of the host range The results will be discussed in the session.