Changes in disease resistance phenotypes associated with plant physiological age are not caused by variation in R gene transcript abundance

Foliar late blight is one of the most important diseases of potato. Foliar blight resistance has been observed to peak at the time a plant flowers and slowly taper off as the plant ages. In other pathosystems, R gene transcript levels appear to be correlated to disease resistance. The cloning of the broad spectrum, foliar blight resistance gene RB provided the opportunity to explore how foliar blight resistance and R gene transcription varies with plant age. Plants of Solanum bulbocastanum PT29, the genotype from which RB was cloned, and plants of S. tuberosum cv. Dark Red Norland (untransformed and RB-transformed) representing three different developmental stages were screened for resistance to late blight and RB transcript levels. Pre-flowering plants of all genotypes tested exhibited the highest levels of resistance, followed by post-flowering and near-senescing plants. The RB transgene significantly affected resistance, enhancing resistance levels of all RB-containing lines, especially in younger plants. Transcript of the RB transgene was detected at all plant ages, though amounts were weakly correlated with disease resistance. Consistent transcript levels in plants of different physiological ages with variable levels of disease resistance suggest that developmentally-related factors, in addition to R genes, may be important in determining disease resistance. Our data demonstrate that changes in disease resistance phenotypes that correlate with plant age cannot be attributed to changes in R gene transcription.