Potato Tuber Blight Resistance Phenotypes Correlate with RB Transgene Transcript Levels in an Age-Dependent Manner.

Plants have evolved strategies and mechanisms to detect and respond to pathogen attack. Different organs of the same plant may be subjected to different environments (e.g., aboveground versus belowground) and pathogens with different lifestyles. Accordingly, plants commonly need to tailor defense strategies in an organ-specific manner. Phytophthora infestans, causal agent of potato late blight disease, infects both aboveground foliage and belowground tubers. We examined the efficacy of transgene RB (known for conferring foliar late blight resistance) in defending against tuber late blight disease. Our results indicate that the presence of the transgene has a positive yet only marginally significant effect on tuber disease resistance on average. However, a significant association between transgene transcript levels and tuber resistance was established for specific transformed lines in an age-dependent manner, with higher transcript levels indicating enhanced tuber resistance. Thus, RB has potential to function in both foliage and tuber to impart late blight resistance. Our data suggest that organ-specific resistance might result directly from transcriptional regulation of the resistance gene itself.

Insights into organ-specific pathogen defense responses in plants: RNA-seq analysis of potato tuber-Phytophthora infestans interactions.

BACKGROUND: The late blight pathogen Phytophthora infestans can attack both potato foliage and tubers. Although interaction transcriptome dynamics between potato foliage and various pathogens have been reported, no transcriptome study has focused specifically upon how potato tubers respond to pathogen infection. When inoculated with P. infestans, tubers of nontransformed 'Russet Burbank' (WT) potato develop late blight disease while those of transgenic 'Russet Burbank' line SP2211 (+RB), which expresses the potato late blight resistance gene RB (Rpi-blb1), do not. We compared transcriptome responses to P. infestans inoculation in tubers of these two lines. RESULTS: We demonstrated the practicality of RNA-seq to study tetraploid potato and present the first RNA-seq study of potato tuber diseases. A total of 483 million paired end Illumina RNA-seq reads were generated, representing the transcription of around 30,000 potato genes. Differentially expressed genes, gene groups and ontology bins that exhibited differences between the WT and +RB lines were identified. P. infestans transcripts, including those of known effectors, were also identified. CONCLUSION: Faster and stronger activation of defense related genes, gene groups and ontology bins correlate with successful tuber resistance against P. infestans. Our results suggest that the hypersensitive response is likely a general form of resistance against the hemibiotrophic P. infestans-even in potato tubers, organs that develop below ground.

Haplotype diversity and population structure in cultivated and wild barley evaluated for Fusarium head blight responses.

Fusarium head blight (FHB) is a threat to barley (Hordeum vulgare L.) production in many parts of the world. A number of barley accessions with partial resistance have been reported and used in mapping experiments to identify quantitative trait loci (QTL) associated with FHB resistance. Here, we present a set of barley germplasm that exhibits FHB resistance identified through screening a global collection of 23,255 wild (Hordeum vulgare ssp. spontaneum) and cultivated (Hordeum vulgare ssp. vulgare) accessions. Seventy-eight accessions were classified as resistant or moderately resistant. The collection of FHB resistant accessions consists of 5, 27, 46 of winter, wild and spring barley, respectively. The population structure and genetic relationships of the germplasm were investigated with 1,727 Diversity Array Technology (DArT) markers. Multiple clustering analyses suggest the presence of four subpopulations. Within cultivated barley, substructure is largely centered on spike morphology and growth habit. Analysis of molecular variance indicated highly significant genetic variance among clusters and within clusters, suggesting that the FHB resistant sources have broad genetic diversity. The haplotype diversity was characterized with DArT markers associated with the four FHB QTLs on chromosome 2H bin8, 10 and 13 and 6H bin7. In general, the wild barley accessions had distinct haplotypes from those of cultivated barley. The haplotype of the resistant source Chevron was the most prevalent in all four QTL regions, followed by those of the resistant sources Fredrickson and CIho4196. These resistant QTL haplotypes were rare in the susceptible cultivars and accessions grown in the upper Midwest USA. Some two- and six-rowed accessions were identified with high FHB resistance, but contained distinct haplotypes at FHB QTLs from known resistance sources. These germplasm warrant further genetic studies and possible incorporation into barley breeding programs.

Quantitative trait loci conferring resistance to Fusarium head blight in barley respond differentially to Fusarium graminearum infection.

Fusarium head blight (FHB), primarily caused by Fusarium graminearum, reduces grain yield and quality in barley. Resistance to FHB is partial and quantitatively inherited. Previously, major FHB resistant QTL were detected on barley chromosome 2H Bin 8 and 2H Bin 10, and another QTL for reduced deoxynivalenol (DON) accumulation was identified on chromosome 3H Bin 6. To develop an understanding of the molecular responses controlled by these loci, we examined DON and fungal biomass levels and the transcriptome differences in near-isogenic line (NIL) pairs carrying contrasting resistant and susceptible alleles at these QTL during F. graminearum infection. No overlap was found among the differentially accumulated transcripts of the three NIL pairs, indicating that the response to infection controlled by the resistance alleles at each QTL may be distinct. Transcripts showing differential accumulation between resistant and susceptible NILs were compared to results from previous wheat/barley-F. graminearum studies and integrated into a wheat/barley-F. graminearum interaction model.

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 shown to change as a plant ages. In other pathosystems, resistance (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 transcript levels vary with plant age. Plants of Solanum bulbocastanum PT29, from which RB, including the native promoter and other flanking regions, was cloned, and S. tuberosum cv. Dark Red Norland (nontransformed and RB-transformed) representing three different developmental stages were screened for resistance to late blight and RB transcript levels. Preflowering plants of all genotypes exhibited the highest levels of resistance, followed by postflowering and near-senescing plants. The RB transgene significantly affected resistance, enhancing resistance levels of all RB-containing lines, especially in younger plants. RB transgene transcripts were detected at all plant ages, despite weak correlation with disease resistance. Consistent transcript levels in plants of different physiological ages with variable levels of disease resistance demonstrate that changes in disease-resistance phenotypes associated with plant age cannot be attributed to changes in R-gene transcript abundance.

Higher copy numbers of the potato RB transgene correspond to enhanced transcript and late blight resistance levels.

Late blight of potato ranks among the costliest of crop diseases worldwide. Host resistance offers the best means for controlling late blight, but previously deployed single resistance genes have been short-lived in their effectiveness. The foliar blight resistance gene RB, previously cloned from the wild potato Solanum bulbocastanum, has proven effective in greenhouse tests of transgenic cultivated potato. In this study, we examined the effects of the RB transgene on foliar late blight resistance in transgenic cultivated potato under field production conditions. In a two-year replicated trial, the RB transgene, under the control of its endogenous promoter, provided effective disease resistance in various genetic backgrounds, including commercially prominent potato cultivars, without fungicides. RB copy numbers and transcript levels were estimated with transgene-specific assays. Disease resistance was enhanced as copy numbers and transcript levels increased. The RB gene, like many other disease resistance genes, is constitutively transcribed at low levels. Transgenic potato lines with an estimated 15 copies of the RB transgene maintain high RB transcript levels and were ranked among the most resistant of 57 lines tested. We conclude that even in these ultra-high copy number lines, innate RNA silencing mechanisms have not been fully activated. Our findings suggest resistance-gene transcript levels may have to surpass a threshold before triggering RNA silencing. Strategies for the deployment of RB are discussed in light of the current research.