Dirk B. Hays

Competitive Inhibition of Abscisic Acid-Regulated Gene Expression by Stereoisomeric Acetylenic Analogs of Abscisic Acid.

The properties of two enantiomeric synthetic acetylenic abscisic acid (ABA) analogs (PBI-51 and PBI-63) in relation to ABA-sensitive gene expression are reported. Using microspore-derived embryos of Brassica napus as the biological material and their responsiveness to ABA in the expression of genes encoding storage proteins as a quantitative bioassay, we measured the biological activity of PBI-51 and PBI-63. Assays to evaluate agonistic activity of either compound applied individually showed a dose-dependent increase in napin gene expression on application of PBI-63. Maximal activity of about 40 [mu]M indicated that PBI-63 was an agonist, although somewhat weaker than ABA. PBI-63 has a similar stereochemistry to natural ABA at the junction of the ring and side chain. In contrast, PBI-51 showed no agonistic effects until applied at 40 to 50 [mu]M. Even then, the response was fairly weak. PBI-51 has the opposite stereochemistry to natural ABA at the junction of the ring and side chain. When applied concurrently with ABA, PBI-63 and PBI-51 had distinctly different properties. PBI-63 (40 [mu]M) and ABA (5 [mu]M) combined gave results similar to the application of either compound separately with high levels of induction of napin expression. PBI-51 displayed a reversible antagonistic effect with ABA, shifting the typical ABA dose-response curve by a factor of 4 to 5. This antagonism was noted for the expression of two ABA-sensitive genes, napin and oleosin. To test whether this antagonism was at the level of ABA recognition or uptake, ABA uptake was monitored in the presence of PBI-51 or PBI-63. Neither compound decreased ABA uptake. Treatments with either PBI-51 or PBI-63 showed an effect on endogenous ABA pools by permitting increases of 5- to 7-fold. It is hypothesized that this increase occurs because of competition for ABA catabolic enzymes by both compounds. The fact that ABA pools did not decrease in the presence of PBI-51 suggests that PBI-51 must exert its antagonistic properties through direct competition with ABA at a hormone-recognition site.

Genetic loci linking improved heat tolerance in wheat (Triticum aestivum L.) to lower leaf and spike temperatures under controlled conditions

Heat stress adversely affects wheat production in many regions of the world and is particularly detrimental during reproductive development. The objective of this study was to identify novel quantitative trait loci (QTL) associated with improved heat tolerance in wheat (Triticum aestivum L.) and to confirm previous QTL results. To accomplish this, a recombinant inbred line (RIL) population was subjected to a three-day 38°C daytime heat stress treatment during early grain-filling. At maturity, a heat susceptibility index (HSI) was calculated from the reduction of three main spike yield components; kernel number, total kernel weight, and single kernel weight. The HSI, as well as temperature depression (TD) of the main spike and main flag leaf during heat stress were used as phenotypic measures of heat tolerance. QTL analysis identified 14 QTL for HSI, with individual QTL explaining from 4.5 to 19.3% of the phenotypic variance. Seven of these QTL co-localized for both TD and HSI. At all seven loci, the allele for a cooler flag leaf or spike temperature (up to 0.81°C) was associated with greater heat tolerance, indicated by a lower HSI. In a comparison to previous QTL results in a RIL population utilizing the same source of heat tolerance, seven genome regions for heat tolerance were consistently detected across populations. The genetic effect of combining three of these QTL, located on chromosomes 1B, 5A, and 6D, demonstrate the potential benefit of selecting for multiple heat tolerance alleles simultaneously. The genome regions identified in this study serve as potential target regions for fine-mapping and development of molecular markers for more rapid development of heat tolerant germplasm.

Ethanol Fermentation Performance of Grain Sorghums (Sorghum bicolor) with Modified Endosperm Matrices

We tested 13 sorghum entries (lines and hybrids) with different endosperm matrices for ethanol production using a laboratory dry grind process. Waxy and heterowaxy samples had the highest efficiencies. Free amino nitrogen (FAN) contents in sorghum samples were positively related to the fermentation rate during fermentation (R2=0.8618). Dried distillers grain with solubles (DDGS) from different sorghums had significantly different crude protein and crude fat contents. Residual starch content in DDGS ranged from 0.60% for the most efficient sample to 2.66% for the least efficient sample. This study showed that the HD lines (TX1, TX3, TX5, TX7, and TX9) with modified endosperm protein matrix have several attributes desirable for ethanol production: easily pasted starch granules, significantly higher FAN content in finished mashes, 30-45% faster ethanol fermentation rate during early stages, and 50-60% higher lysine content in DDGS.

QTL Mapping of a High Protein Digestibility Trait in Sorghum bicolor

Compared with other cereal grains, Sorghum bicolor shows lower protein digestibility. The low digestibility is thought to result from disulfide cross linking in the β- and γ-kafirins. In contrast, the single recessive high digestibility/high lysine content (HD) mutation which confers greater grain digestibility exists in sorghum that is thought to result from reduced accumulation of γ-kafirin that allows greater access to the high digestible α-kafarin fraction. In an effort to both clearly define the molecular basis for the HD trait and develop tools to improve the introgression of this difficult-to-screen trait, this study focuses on mapping the QTLs linked to this trait. While the HD trait has been defined as a single recessive gene, our results uncovered that two major QTLs on chromosome 1 are associated with protein digestibility—one QTL (locus 1 from the HD parent) unfavorably affects digestibility and one QTL (locus 2 from the HD parent) only 20 cM away favorably affects digestibility. A contrast analysis between genotypic groups at these two loci shows that a higher level of protein digestibility may be obtained when this linkage in repulsion is broken and favorable alleles are allowed to recombine.

Vegetative axillary bud dormancy induced by shade and defoliation signals in the grasses.

Vegetative axillary bud dormancy and outgrowth is regulated by several hormonal and environmental signals. In perennials, the dormancy induced by hormonal and environmental signals has been categorized as eco-, endo- or para-dormancy. Over the past several decades para-dormancy has primarily been investigated in eudicot annuals. Recently, we initiated a study using the monoculm phyB mutant (phyB-1) and the freely branching near isogenic wild type (WT) sorghum (Sorghum bicolor) to identify molecular mechanisms and signaling pathways regulating dormancy and outgrowth of axillary buds in the grasses. In a paper published in the January 2010 issue of Plant Cell and Environment, we reported the role of branching genes in the inhibition of bud outgrowth by phyB, shade and defoliation signals. Here we present a model that depicts the molecular mechanisms and pathways regulating axillary bud dormancy induced by shade and defoliation signals in the grasses.

Family-based QTL mapping of heat stress tolerance in primitive tetraploid wheat (Triticum turgidum L.)

Identification of quantitative trait loci (QTL) and markers associated with heat and drought tolerance is warranted for marker-assisted selection in wheat (Triticum aestivum L.) breeding programs in areas prone to these abiotic stresses. Our study used a family-based mapping approach in which 19 families consisting of 384 individuals were developed by three-way crosses involving the heat tolerant, tetraploid cultivated emmer (Triticum turgidum L. var dicoccum) genotype IG45069 and ten heat susceptible tetraploid genotypes, IG44999, IG44961, IG45413, IG83047, IG45441, IG127682, IG45448, IG110572, IG88723 and IG54073, in order to detect QTL and markers associated with heat tolerance. The 384 individuals were phenotyped for physiological traits associated with heat tolerance and genotyped by SSR markers. The QTL associated with heat stress tolerance, as measured by chlorophyll content, flag leaf temperature depression (FLTD) and individual kernel weight (IKW) were mapped on chromosomes 1B (QChlc.tamu-1B), 2B (QFlt.tamu-2B), and 5A (QIkw.tamu-5A), respectively, using linkage analysis. Alleles from IG45069 possessed the highest associations with the phenotypic data for the studied traits. This study demonstrates that a family-based mapping approach can be utilized in rapid detection of QTL associated with heat tolerance in wheat based on linkage and association analyses.

Wild Tetraploid Wheat (Triticum turgidum L.) Response to Heat Stress

Identifying reliable screening tools and characterizing tolerant germplasm sources are essential for developing wheat (Triticum aestivum L.) varieties suited for the hot areas of the world. Our objective was to evaluate heat tolerance of promising wild tetraploid wheat (Triticum turgidum L.) accessions that could be used as sources of heat tolerance in common- and durum-wheat (Triticum durum) breeding programs. We screened 16 wild tetraploid wheat accessions and two common wheat checks for their response to heat stress by measuring damage to the thylakoid membranes, flag leaf temperature depression (FLTD), and spike temperature depression (STD) during exposure to heat stress for 16 days post-anthesis (DPA). Measurements were taken on the day of anthesis then 4, 8, 12, 16 DPA under controlled optimum and heat-stress conditions. Individual kernel weight (IKW) and heat susceptibility index (HSI) measurements were also obtained. Prolonged exposure to heat stress was associated with increased damage to thylakoid membranes, as indicated by the high ratio of constant fluorescence (O) to peak variable fluorescence (P). Some wild tetraploid wheat accessions exhibited a better HSI than the common heat-tolerant wheat cultivar ‘Kauz.’ A positive and significant correlation was found between O/P ratio and each of FLTD and STD under heat-stress conditions. A negative and significant correlation was found between FLTD and HSI and between STD and HSI based on the second and third measurements (4 and 8 DPA). Correlations obtained after the third measurement were not significant because of heat-induced, accelerated maturity and a lack of green leaf tissue. This study identified potential heat-tolerant wild tetraploid wheat germplasm that can be incorporated into wheat breeding programs to improve heat tolerance in cultivated common and durum wheat.

Effect of High-Molecular-Weight Glutenin Subunit Allelic Composition on Wheat Flour Tortilla Quality

ABSTRACT Wheat cultivars possessing quality attributes needed to produce optimum quality tortillas have not been identified. This study investigated the effect of variations in high-molecular-weight glutenin subunits encoded at the Glu-1 loci (Glu-A1, Glu-B1, and Glu-D1) on dough properties and tortilla quality. Flour protein profiles, dough texture, and tortilla physical quality attributes were evaluated. Deletion at Glu-D1 resulted in reduced insoluble polymeric protein content of flour, reduced dough compression force, and large dough extensibility. These properties produced very large tortillas (181 mm diameter) compared with a control made with commercial tortilla wheat flour (161 mm). Presence of a 7 + 9 allelic pair at Glu-B1 increased dough strength (largest compression force, reduced extensibility, and small-diameter tortillas). Deletion at Glu-A1 produced large tortillas (173 mm) but with unacceptable flexibility during storage (score <3.0 at day 16). In general, presence of 2* at Glu-A1, in com...

Functionality of Gliadin Proteins in Wheat Flour Tortillas

Gliadins are monomeric proteins that are encoded by the genes at the loci Gli 1 and Gli 2 present on the short arm of homologous wheat chromosomes 1 and 6, respectively. Studies have suggested that gliadins may play an important role in determining the functional properties of wheat flour. The main objective of this study was to understand the functionality of gliadins with respect to tortilla quality. The important tortilla quality attributes are diameter, opacity, and shelf stability, designated here as rollability or the ability to roll or fold the tortilla without cracking. In this study gliadin functionality in tortilla quality was studied using near-isogenic wheat lines that have deletions in either Gli A1, Gli D1, Gli A2, or Gli D2 gliadin loci. The deletion lines are designated by the same abbreviations. Dough and tortillas were prepared from the parent line used to derive these deletion lines, each individual deletion line, and a control commercial tortilla flour. Quantitative and qualitative evaluations were performed on the dough and tortillas derived from the flour from each of these lines. None of the deletions in the gliadin loci altered the shelf stability versus that found for the parent to the deletion lines or control tortilla flour. However, deletions in the Gli 2 loci, in particular Gli A2 reduced the relative proportion of alpha- and beta-gliadins with a greater cysteine amino acid content and gluten cross-link function versus the chain-terminating omega-gliadins in Gli 1, which were still present. As such, the dough and gluten matrix appeared to have greater extensibility, which improved the diameter and overall quality of the tortillas while not altering the rollability. Deletions in the Gli 1 loci had the opposite result with increased cross-linking of alpha- and beta-gliadins, polymeric protein content, and a stronger dough that decreased the diameter and overall quality of the tortillas. The data suggest that altering certain Gli 2 loci through null alleles could be a viable strategy to develop cultivars improved for the specific functionality requirements needed for the rapidly growing tortilla market.

Effect of Grain Sorghum Protein Digestibility on Starch Gelatinization and Enzymatic Conversion to Glucose

The objective of this study is to select the best high digestible grain sorghum (HDGS) lines that breeders have developed for low energy input liquefaction, saccharification and fermentation for bio-ethanol production. The parental lines and offspring generated by crosses of two HD lines (P850029 and P851171) with three wild type (WT) sorghum lines (B.Tx635, R.Tx436 and 96GCPOB124) were used in this study. After the identification of recombinant inbred lines (RILs) as high, medium and normal protein digestible grain sorghum using the protease turbidity assay, the flour pasting properties of the sorghum samples were examined using rapid visco-analyzer (RVA). Results showed that the HDGS lines with modified endosperm matrices has reduced temperature and duration needed to solubilize the grain starch for hydrolytic enzyme access and conversion to fermentable sugars.