1Horticulture Research International, Wellesbourne, Warwick, CV35 9EF, UK; 2The John ]nnes Centre, Coiney Lane, Norwich, NR4 7UH, UK

Background and objectives
Bacterial blight of pea is a disease caused by Pseudomonas syringae pathovar pisi (P.s. pv. pisi) Previous investigations showed that resistance to the 7 naturally occurring races of P.s. pv. pisi could be explained in terms of a gene-for-gene model, involving up to six resistance/avirulence gene pairs [1]. To further our understanding of the interaction of resistance and aviruience genes, molecular characterization of the plant resistance genes is required. Towards this objective, the present study describes the identification of markers and the mapping of specific resistance genes in Pisum.

Material and methods
Parental plant accessions and F2 populations were scored for resistance/susceptibility to races of P.s. pv. pisi, the presence/absence of 5 morphological characters and 24 isoenzyme loci. Genomic DNA was extracted from individual plants of F2 crosses, and pooled DNA samples representing the dominant and recessive alleles of each resistance locus were constructed with DNA from 10 plants per pool according to the bulked segregant method [2]. PCR amplification of each pool with a range of Operon Technologies primers (OPA-01 to OPA-20) was used to generate RAPD polymorphisms in the bulked pools which were then scored in DNA samples from each individual F2 plant. Further mapping of resistance genes was made in two recombinant inbred (RI) pea populations from the John lnnes Centre.

Results and conclusions
Analysis of the RI lines, mapped resistance gene Ppi2 to chromosomal linkage group Vil, between the loci for peptidase pep-3 and ribosomal DNA gene Rrn2 and thus relatively close to pgd-p. This location was reinforced by observed linkages between Ppi2 and isoenzyme markers aido and sod, between Ppi2 and oh and between sod and pgd-p. Linkage was also detected between Ppi2, and the RAPD marker OPA-052.30. The linkage predicted from previous work between Ppi3 and Ppi4 [1] was confirmed and supported by linkage between both genes and the RAPD marker OPA-200.71. Linkages were also detected between Ppi2 and both Ppi3 and Ppi4, between Ppi4 and pgd-p, between Ppi3 and oh, and between both Ppi3 and Ppi4 and both sod and the RAPD marker OPA-052.30, providing some evidence that all three resistance genes are linked. There was, however, some apparently conflicting evidence of a linkage which would place Ppi3 and Ppi4 on the chromosomal linkage group carrying a, whilst there was no evidence of any association between Ppi2 and any loci mapped to this linkage group.

Ppi1, which was only detectable in a single RI population, mapped between gs-p (the gene for the plastid form of Glutamine synthetase) and P1 (the black hilum locus) on chromosomal linkage group V). There was also a possibility that some component of resistance to P.s. pv. pisi race 1 may be conferred by a locus on chromosomal linkage group IV showing weak linkage with the Rrn1 locus, although this possibility was not well supported by the available data.

1. Bevan JR, Taylor JD, Crute IR, Hunter PJ, Vivian A, 1995. Plant Pathology 44, 98-108.
2. Michelmore RW, Paran 1, Kesseli RV 1991. Proceedings of the National Academy of Sciences, USA 88, 9828-9832.