Kimberly Garland Campbell

Identification and mapping QTL for high-temperature adult-plant resistance to stripe rust in winter wheat (Triticum aestivum L.) cultivar ‘Stephens’

High-temperature adult-plant (HTAP) resistance from the winter wheat (Triticum aestivum) cultivar ‘Stephens’ has protected wheat crops from stripe rust caused by Puccinia striiformis f. sp. tritici for 30 years. The objectives of this study were to identify quantitative trait loci (QTL) for HTAP resistance in Stephens through genetic linkage analysis and identify DNA markers linked to the QTL for use in marker-assisted breeding. Mapping populations consisted of 101 recombinant inbred lines (RILs) through single-seed descent from ‘Stephens’ (resistant) × ‘Michigan Amber’ (susceptible). F5, F6 and F7 RILs were evaluated for stripe rust resistance at Pullman, WA in 1996, 1997 and 1998, respectively, whereas F8 RILs were evaluated at Mt Vernon, WA, USA in 2005. The 101 F8 RILs were evaluated with 250 resistance gene analog polymorphism (RGAP), 245 simple sequence repeat (SSR) and 1 sequence tagged site (STS) markers for genetic linkage map construction. Two QTL, which explained 48–61% of the total phenotypic variation of the HTAP resistance in Stephens, were identified. QYrst.wgp-6BS.1 was within a 3.9-cM region flanked by Xbarc101 and Xbarc136. QYrst.wgp-6BS.2 was mapped in a 17.5-cM region flanked by Xgwm132 and Xgdm113. Both two QTL were physically mapped to the short arm of chromosome 6B, but in different bins. Validation and polymorphism tests of the flanking markers in 43 wheat genotypes indicated that the molecular markers associated with these QTL should be useful in marker-assisted breeding programs to efficiently incorporate HTAP resistance into new wheat cultivars.

Construction and characterization of a full-length cDNA library for the wheat stripe rust pathogen (Puccinia striiformis f. sp. tritici).

BackgroundPuccinia striiformis is a plant pathogenic fungus causing stripe rust, one of the most important diseases on cereal crops and grasses worldwide. However, little is know about its genome and genes involved in the biology and pathogenicity of the pathogen. We initiated the functional genomic research of the fungus by constructing a full-length cDNA and determined functions of the first group of genes by sequence comparison of cDNA clones to genes reported in other fungi.ResultsA full-length cDNA library, consisting of 42,240 clones with an average cDNA insert of 1.9 kb, was constructed using urediniospores of race PST-78 of P. striiformis f. sp. tritici. From 196 sequenced cDNA clones, we determined functions of 73 clones (37.2%). In addition, 36 clones (18.4%) had significant homology to hypothetical proteins, 37 clones (18.9%) had some homology to genes in other fungi, and the remaining 50 clones (25.5%) did not produce any hits. From the 73 clones with functions, we identified 51 different genes encoding protein products that are involved in amino acid metabolism, cell defense, cell cycle, cell signaling, cell structure and growth, energy cycle, lipid and nucleotide metabolism, protein modification, ribosomal protein complex, sugar metabolism, transcription factor, transport metabolism, and virulence/infection.ConclusionThe full-length cDNA library is useful in identifying functional genes of P. striiformis.

Evaluation of Adapted Wheat Cultivars for Tolerance to Pythium Root Rot

Genetic resistance in wheat (Triticum aestivum) against Pythium species would be an efficient means of control of this major root fungal pathogen, but so far no source has been identified. In addition, no long-term, sustainable options for controlling Pythium root rot are available; therefore, identifying and then incorporating genetic resistance into wheat cultivars would create an ideal method of control for this disease. The objective of this study was to examine the level of tolerance to Pythium root rot among a diverse set of wheat germ plasm collected from all major wheat production regions in the United States. Pythium debaryanum isolate 90136 and P. ultimum isolate 90038, previously identified as the most virulent Pythium isolates on wheat, were used to infest pasteurized soil, which was seeded with wheat genotypes and placed in a growth chamber maintained at a constant 16°C with a 12-h photoperiod and ambient humidity. Length of the first leaf and plant height measurements were recorded, and roots were digitally scanned to create computer files that were analyzed using WinRhizo software for length and number of tips. Significant (P < 0.05) differences in plant variables were detected among wheat genotypes in the presence of both Pythium species, and a significant (P < 0.0001) correlation between plant stunting and root loss was detected. Based on both shoot and root measurements, Caledonia, Chinese Spring, MN97695, and OR942504 appear to be highly susceptible to Pythium root rot, whereas genotypes KS93U161, OH708, and Sunco were the most tolerant to this disease.

The wheat ABA hypersensitive ERA8 mutant is associated with increased preharvest sprouting tolerance and altered hormone accumulation

Wheat preharvest sprouting (PHS) is the germination of mature grain on the mother plant when rain occurs before harvest. Higher abscisic acid (ABA) hormone levels and sensitivity are associated with higher seed dormancy and PHS tolerance. Consistent with this, the ABA hypersensitive ENHANCED RESPONSE TO ABA8 (ERA8) mutant resulted in increased dormancy and PHS tolerance in soft white spring wheat ‘Zak’. ERA8 seeds were initially less responsive to germination-rescue by the hormone gibberellin (GA). ERA8 gained GA and lost ABA sensitivity more slowly than wild-type during dormancy loss through after-ripening and cold imbibition. This study examined if increased ABA sensitivity in ERA8 likely resulted from increased ABA signaling or increased ABA hormone levels. Zak ERA8 had higher initial grain dormancy although endogenous embryo ABA levels were similar in Zak ERA8 and wild-type, suggesting that increased dormancy was due to increased ABA signaling rather than increased ABA accumulation. ABA levels declined with Zak ERA8 after-ripening, suggesting that ABA turnover is not defective. Elevated ERA8 dormancy was also associated with increased embryonic jasmonic acid-Ile and aleurone indole-3-acetic acid (IAA) levels. The possible implication that other plant hormones may influence wheat seed dormancy and germination are discussed.

Genome-Wide Association Mapping for Tolerance to Preharvest Sprouting and Low Falling Numbers in Wheat

Preharvest sprouting (PHS), the germination of grain on the mother plant under cool and wet conditions, is a recurring problem for wheat farmers worldwide. α-amylase enzyme produced during PHS degrades starch resulting in baked good with poor end-use quality. The Hagberg-Perten Falling Number (FN) test is used to measure this problem in the wheat industry, and determines how much a farmers wheat is discounted for PHS damage. PHS tolerance is associated with higher grain dormancy. Thus, breeding programs use germination-based assays such as the spike-wetting test to measure PHS susceptibility. Association mapping identified loci associated with PHS tolerance in U.S. Pacific Northwest germplasm based both on FN and on spike-wetting test data. The study was performed using a panel of 469 white winter wheat cultivars and elite breeding lines grown in six Washington state environments, and genotyped for 15,229 polymorphic markers using the 90k SNP Illumina iSelect array. Marker-trait associations were identified using the FarmCPU R package. Principal component analysis was directly and a kinship matrix was indirectly used to account for population structure. Nine loci were associated with FN and 34 loci associated with PHS based on sprouting scores. None of the QFN.wsu loci were detected in multiple environments, whereas six of the 34 QPHS.wsu loci were detected in two of the five environments. There was no overlap between the QTN detected based on FN and PHS, and there was little correlation between the two traits. However, both traits appear to be PHS-related since 19 of the 34 QPHS.wsu loci and four of the nine QFN.wsu loci co-localized with previously published dormancy and PHS QTL. Identification of these loci will lead to a better understanding of the genetic architecture of PHS and will help with the future development of genomic selection models.