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1.Genetic Analysis and Improvement of the Genes Confering on Colonization of Bacteria on Plants

Yuichi Takikawa, Takashi Kumakura, Yuikiko Imanishi, Yasuhiro Inoue and Alit Susanta(Faculty of Agriculture, Shizuoka University)

Functions of hrp-homologous genes were investigated in a strain of Pseudomonas fluorescens, PfG32R, a saprophytic, root-colonizing bio-control agent against bacterial wilt of tomato. A mutant derivative of PfG32R having disruption in the hrpS homologue was constructed with homologous recombination technique. The mutant was unable to produce antibiotic substance(s) and protease(s) which the parental strain can produce. The gacA and gacS genes known to confer these phenotypes were identified in PfG32R. The gacS homologue of PfG32R was cloned. Introduction of a plasmid containing gacS gene of P. fluorescens restored the ability to produce a protease. The hrp-homologues of PfG32R lack hrpA, Z, B genes that play key role in the pathogenesis of plant pathogenic Pseudomonas strains. To investigate the activity of other hrp-homologous genes in PfG32R, some clones containing hrpA, Z, B genes derived from P. syringae pathovars were introduced into PfG32R derivatives. Only single clone, pHIR11 which contains entire region of hrp gene cluster gave the ability to induce hypersensitive reactions (HR) on tobacco plants, and none of other clones did which lack various hrp genes on the left side of the cluster. Therefore, it was suggested that not only the lack of hrpA, Z, B genes, but also some other mechanisms are responsible for the inability of induction of HR in tobacco by PfG32R.

Keywords: Pseudomonas fluorescens, hrp genes, gac genes, hypersensitive reaction