| Utilization of late blight resistant potato varieties is the most cost-effective and environmentally friendly method to control late blight, a highly destructive disease caused by the oomycete pathogen Phytophthora infestans. Currently, all commercial potato cultivars lack sufficient late blight resistance. A resistance gene to potato late blight was previously cloned from a wild diploid potato species, Solanum bulbocastanum, and named RB. Incorporation of the RB gene into potato cultivars would give potato growers much needed protection against P. infestans. In order to achieve this goal faster, in this thesis I performed marker-assisted selection on multiple breeding generations in order to identify those harboring the RB gene. The RB gene was identified in ∼45% of the breeding families in the first and second year of analysis. This decreased the number of potatoes needing screening for agronomic traits by about half. Previously, RB was characterized molecularly as a familiar disease resistance protein. However, the disease response to late blight in potatoes containing the RB gene is not typical. Transgenic potatoes containing RB show a suppression of P. infestans growth, not total elimination of the pathogen; the suppression is effective against all known strains of P. infestans. Thus, the RB-mediated resistance was termed rate-reducing and race-non-specific. The transgenic potato lines containing the RB gene provide a unique opportunity and ideal material to investigate how a single resistance gene can provide strong resistance against various strains of P. infestans. In this thesis, I investigated the RB-transgenic lines further, determining that the transgenic plants with higher copy numbers of RB also had higher basal levels of the RB-transcript. The amount of RB-transcript also correlated with the level of RB-mediated late blight resistance. The resistance level was associated with a combination of rapid RB transcript induction immediately after pathogen infection, followed by the steady production of RB transcript. A microarray approach was then used to determine gene expression changes in the susceptible potato background after the addition of RB. We identified genes that had previously been shown to play a role in late blight resistance. Additionally, a few previously uncharacterized kinases, transcription factors and the Patatin gene family appear to be involved in the early RB-mediated resistance response. |
