1Biotechnology Group, DIAS, KVL, Thorvaidsensvej 40, 1871 Frederiksberg C, Denmark; 2USDA-ARS, Horticultural Crops Research Laboratory, 3420 NW Orchard Ave, OSU, Corvallis 97330, USA

Background and objectives
Pea seedborne mosaic potyvirus (PSBMV) infects leguminous crops such as pea, faba bean and lentil. In pea, natural resistance due to recessive genes has been described in PI lines and cultivars. Based on the infection profiles of PSBMV isolates on resistant pea lines, four resistance genes and three pathotypes of PSBMV have been defined. Resistance gene sbm-1 confers resistance to pathotype P-1, sbm-2 and sbm-3 both confer resistance to P-2, and sbm-4 to P-4 [1]. Resistance genes sbm-1, -3, and -4 were found to be closely linked on chromosome 6 in a cluster including two other potyvirus resistance genes cyv-2 and wlv. These confer resistance to the clover yellow vein virus (CYVV) and white lupin mosaic virus, respectively. Gene sbm-2 was found to map in a cluster of potyvirus resistance genes on chromosome 2. It includes the genes cyv-1, pmv and bcm which confer resistances to CYVV, pea mosaic virus and bean common mosaic virus isolate-NL8, respectively; and mo that confers resistance to both bean yellow mosaic virus and watermelon mosaic virus 2. It has been speculated that sbm-2 and sbm-3 as well as cyv-1 and cyv-2 could be duplicate genes, because pea lines homozygous for either of these gene pairs are resistant to the same isolates of PSBMV and CYVV, respectively [2].

Our interest is to study the recessive resistance mechanisms of the sbm genes to the different pathotypes of PSBMV.

Results and conclusions
We initially generated chimaeras of DPD1 (a representative of pathotype P-1) and NY (a non-P-1 pathotype) and tested these on pea line PI 269818 which is homozygous for the sbm-1 gene, i.e. resistant to DPD1 and susceptible to NY. This enabled us to define the viral VPg protein with a putative function in virus replication as the avirulence determinant with respect to pathotype P-1 [3]. The determinant region was further narrowed down to 13 amino acids in the central part of VPg. To test if VPg is the general avirulence determinant with respect to recessive resistance, we replaced VPg in both pathotype DPD1 and NY with VPg from the pathotype P-2 isolate Ll. These chimaeras were tested on PI 269818 and on the pea cultivar Bonneville, which are both resistant to Ll due to homozygosity of the sbm-3 and sbm-2 genes, respectively. Interestingly, both chimaeras infected Bonneville but were unable to break the resistance in PI 269818. This result demonstrates that the sbm-2 resistance in Bonneville against Ll is not related to the VPg protein, suggesting that the resistance conferred by the sbm-2 gene may function by a mechanism different from sbm-1 and sbm-3 resistances, and that sbm-2 and sbm-3 are not duplicate genes.

At the moment we are generating additional NY-LI chimaeras in order to study the mechanism of the sbm-2 resistance to pathotype P-2. We are also attempting to purify VPg protein from pathotype P-1 to make biochemical studies of the sbm-1-mediated resistance.

1. Hampton RO, Provvidenti R, 1992. Archives of Virology Suppl. 5, 183-187.
2. Provvidenti R, Hampton RO, 1992. Archives of Virology Suppl. 5, 189-211.
3. Keller KE, Johansen IE, Martin RR, Hampton RO, 1998. Molecular Plant-Microbe Interactions 11, 124-130.