1.6.12
DISCOVERY OF PATHOGENICITY GENES IN PSEUDOMONAS SYRINGAE pv. PHASEOLICOLA

E ATHANASSOPOULOS, G TSIAMIS, C STEVENS and J MANSFIELD

Department of Biological Sciences, Wye College, University of London, Wye, Ashford, Kent TN25 5AH, UK

Background and objectives
The aim of this research is to identify genes encoding factors required for basic pathogenicity. All phytopathogenic bacteria carry a cluster of genes called hrp genes, which are essential for the bacteria to multiply within susceptible plants and cause the hypersensitive response (HR) in resistant plants. The hrp cluster encodes the type III secretion system. Here we describe the development of an assay using E. coli to identify pathogenicity genes from P.s. pv. phaseolicola (Psph) which interact with French bean, Phaseolus vulgaris.

Results and conclusions
E. coli fails to grow in the bean plant, but the introduction of cloned hrp genes allows the saprophyte to deliver Avr proteins and cause the HR in plants carrying the corresponding genes for resistance [1, 2]. When E. coli was mixed with Psph K was able to multiply in the plant. When an Hrp mutant strain of Psph was used in the mixture, E. coli failed to grow. Hrp genes therefore appear to be required for delivery of pathogenicity factors which probably suppress defence responses by the plant and release nutrients for bacterial growth. In order to search for pathogenicity factors which will enable E. coli to multiply in the plant, strains of E. coli MC4100 have been constructed which contain the hrp gene cluster on pPPY4301 (INCP plasmid). The regulator hrpL expressed from Bluescript (COLE1 plasmid) and cosmid clones of a genomic library of Psph in pHM1 (incq plasmid). The library has been screened for clones which enhance the multiplication of E. coli in the bean plant. Pathogenicity determinants of plasmid and chromosomal origin have been compared.

References
1. Mansfield JW, Jenner C, Hockenhull R et al., 1994. Molecular Plant-Microbe Interactions 7, 726-329.
2. Puri N, Jenner C , Bennett MA et al., 1997. Molecular Plant-Microbe Interactions 10, 247-256.