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DOES CONSTITUTIVE PR GENE EXPRESSION IN SOLANUM SPP. CONTRIBUTE TO NON-SPECIFIC RESISTANCE TO PHYTOPHTHORA INFESTANS?

VGAA VLEESHOUWERS1,3, W VAN DOOIJEWEERT1, F GOVERS2,3, S KAMOUN2,3 and LT COLON1

1DLO - Centre for Plant Breeding and Reproduction Research (CPRO-DLO), PO Box 16, 6700 AA Wageningen, The Netherlands; 2Department of Phytopathology, and 3Graduate School Experimental Plant Sciences, Wageningen Agricultural University, Binnenhaven 9, 6709 PD Wageningen, The Netherlands

Background and objectives
Several wild Solanum spp. express varying levels of partial resistance to the potato late blight fungus Phytophthora infestans. In potato cultivar Robijn, P. infestans resistance is shown to be non-specific, based on multiple genes, and durable. A similar mechanism is thought to be present in wild Solanum species [1]. The Arabidopsis mutant cpr has a high endogenous level of systemic acquired resistance (SAR) and is highly resistant to the oomycete Peronospora parasitica [2]. We hypothesize that high Phytophthora resistance levels in Solanum spp. result from high endogenous levels of SAR. The pathogenesis-related (PR) proteins PR1, PR2 and PR5 are known to accumulate during SAR development, and are considered as markers for SAR. Several Solanum spp. were monitored for basal SAR level by analysing PR1, PR2 and PR5 expression in healthy non-infected leaves.

Materials and methods
Five potato cultivars and a collection of wild Solanum spp. (S. berthaultii, S. arnezii x hondelmannii, S. circaeifolium, S. microdontum, S. sucrense, S. vernei, S. nigrum and ABPT hybrid no. 44) were used in this study. Genomic DNA and RNA were isolated from healthy leaves from climate chamber-grown plants. Southern and Northern blots were prepared and hybridized with radiolabelled probes from PR1 (potato), PR2 (tomato), PR5 (tobacco) and tubulin (potato, for RNA-loading corrections). The constitutive expression levels of PR1, PR2 and PR5 were compared to resistance data based on inoculation experiments with five different P. infestans isolates.

Results and conclusions
Genomic Southern blot analysis revealed that the heterologous tobacco (PR5) and tomato (PR2) probes cross-hybridize with DNA from the various Solanum spp. Also, the probes derived from S. tuberosum cross-hybridize with DNA from all species tested. Potato as well as the wild Solanum spp. contain multiple copies of the PR1, PR2 and PR5 genes in their genome. Northern blot analysis revealed that Robijn, a highly resistant potato cultivar, displayed the highest levels of PR1, PR2 and PR5 mRNA within the S. tuberosum cultivars. Susceptible cultivar Bintje exhibited very low expression levels of the three tested PR genes. The cultivars Ehud, Estima and Premiere displayed intermediate levels of PR gene expression, corresponding to their individual resistance levels. Therefore, within the S. tuberosum group, there appears to be a correlation between the level of resistance and PR gene expression. High levels of PR1, PR2 and PR5 mRNA were found in the two tested genotypes of both S. arnezii x hondelmannii and S. sucrense. These genotypes possess considerable levels of non-specific resistance to P. infestans. S. berthaultii, S. circaeifolium, S. microdontum and S. vernei displayed moderate levels of PR gene expression, whereas ABPT-44 and S. nigrum exhibited only very low levels. Between different Solanum spp., no correlation between PR gene expression and resistance was found, which is possibly due to different genetic backgrounds of the various Solanum spp.

From the wild Solanum spp., only one to three genotypes were tested within each species. This is too limited to determine a correlation between levels of resistance and constitutive SAR. However, from S. tuberosum, five cultivars were tested and the results suggested a correlation between late blight resistance and expression of SAR marker genes. Considering this correlation within S. tuberosum, the partial resistance levels noted in other wild Solanum spp. might be due to enhanced constitutive expression of SAR.

References
1. Colon LT, Budding DJ, 1988. Euphytica (suppl.), 77-86.
2. Bowling SA, Guo AL, Cao H et al., 1994. Plant Cell 6, 1845-1857.