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Soil and Crop Sciences Dep., Colorado State Univ., Fort Collins, CO 80523. Research supported through funding from Colorado Agric. Exp. Stn. Projects 795 and 646, the Colorado Wheat Administrative Committee, and the Colorado Wheat Research Foundation
* Corresponding author (scott.haley{at}colostate.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: GA, gibberellic acid • PCR, polymerase chain reaction • RIL, recombinant inbred line
Soil and Crop Sciences Dep., Colorado State Univ., Fort Collins, CO 80523. Research supported through funding from Colorado Agric. Exp. Stn. Projects 795 and 646, the Colorado Wheat Administrative Committee, and the Colorado Wheat Research Foundation
* Corresponding author (scott.haley{at}colostate.edu).
Microsatellite marker locus Xgwm 261 has been associated with reduced plant height attributed to the linked gibberellic acid (GA) sensitive Rht8c allele in wheat (Triticum aestivum L.). Our objective was to determine effects of allelic variation at the Xgwm 261 and Rht-B1 loci in two winter wheat recombinant inbred line (RIL) populations. ‘Longhorn’/‘Akron’ and Longhorn/‘Yuma’ RIL populations were evaluated in controlled environments for GA sensitivity and coleoptile length and were grown in Colorado at five locations for 2 yr for agronomic evaluation. Microsatellite markers were used to identify alleles at the Xgwm 261 and Rht-B1 loci. In both populations, the Rht-B1b class showed reduced plant height (8.9% average), GA sensitivity (average 73.5%), coleoptile length (average 11.3%), and test weight (average 1.9%) relative to the Rht-B1a class. In the Longhorn/Yuma population, Rht-B1b and Xgwm 261 165-bp classes were shorter than the Xgwm 261 210-bp and Rht-B1a classes. Increased GA sensitivity and coleoptile length of the Xgwm 261 165-bp class were observed, although only in the Rht-B1a background. Plant height of the Xgwm 261 165-bp + Rht-B1a combination was intermediate to the shorter Xgwm 261 165-bp + Rht-B1b and the taller Xgwm 261-210-bp + Rht-B1a combinations. No negative associations attributed to the Xgwm 261 165-bp allele were observed. Our results suggest that the Xgwm 261 165-bp allele may provide reduced plant height, increased coleoptile length, and increased test weight when combined in a background with the Rht-B1a allele.
Abbreviations: GA, gibberellic acid • PCR, polymerase chain reaction • RIL, recombinant inbred line
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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To alleviate these problems, other sources of semidwarfing genes have been explored. One of these sources is the semidwarfing allele Rht8c that was introduced from Japanese wheat into European wheat germplasm in the 1930s (Gale and Youssefian, 1985). Rht8c is widely distributed among wheat cultivars in southern and eastern Europe, China, Japan, and several Russian Federation countries (Worland and Law, 1986; Borojevic and Borojevic, 2005; Worland et al., 1998; Rebetzke and Richards, 2000). Rht8c-carrying genotypes do not exhibit the same reduction in coleoptile length as genotypes carrying Rht-B1b or Rht-D1b and thus may provide greater yield stability in environments where emergence may be problematic because of dry soil conditions at planting (Worland et al., 1988; Kertész et al., 1991).
Incorporation of Rht8c into breeding programs has been hampered by the inability to distinguish Rht8c-carrying genotypes from those carrying Rht-B1b or Rht-D1b on the basis of plant height measurements. While coleoptile length or gibberellic acid (GA) sensitivity assays may be used along with plant height information to identify Rht8c-carrying genotypes, such measurements are somewhat labor intensive and not particularly amenable to large-scale or early-generation screening in breeding programs. Furthermore, coleoptile length or GA screening does not allow separation of Rht8c-carriers from tall (Rht-B1a/Rht-D1a) wheat genotypes that similarly tend to show GA sensitivity and long coleoptiles. Korzun et al. (1998) identified a linked microsatellite marker that may be useful in marker-assisted selection for Rht8c. In their study, three major allelic variants (Xgwm 261 165, 174, and 192 bp) were identified among the parents of two mapping populations. In one population with the Yugoslavian cultivar Mara as a parent, plant height of individuals with the 192-bp allele showed a 7- to 8-cm height reduction compared with individuals with the 174-bp allele. Mapping within this population showed that the Xgwm 261 192-bp allele was tightly linked (0.6 cM) with the height-reducing gene Rht8c on chromosome 2D. Subsequent surveys of a large collection of genotypes from throughout the world (Worland et al., 1998, 2001) showed that the Xgwm 261 165-, 174-, and 192-bp alleles are present in more than 90% of the entries tested, although at least 11 other allelic variants were identified at lower frequencies.
In subsequent germplasm surveys, Ahmad and Sorrells (2002) and Bai et al. (2004) documented the distribution of the Xgwm 261 165-, 174-, and 192-bp alleles among collections of U.S. wheat genotypes. Ahmad and Sorrells (2002) showed that the 174-bp allele was common among soft red winter wheats from the eastern United States, while Bai et al. (2004) showed that the 210-bp allele was present among hard winter wheats from the Great Plains region at a much greater frequency than reported for wheat genotypes from other areas of the world (Worland et al., 1998, 2001). Additionally, Bai et al. (2004) questioned the ability of the Xgwm 261 192-bp allele to identify genotypes with long coleoptiles. In a recent survey of historical and current U.S. hard winter wheat genotypes (unpublished data), we identified two cultivars (‘Longhorn’ and ‘Thunderbird’) that carried the Xgwm 261 210-bp allele while showing characteristics reportedly common to Rht8c (e.g., reduced plant height, increased GA sensitivity, long coleoptiles).
Plant height and coleoptile length are affected by alleles at the Rht-B1 and Rht-D1 loci, but until recently, distinguishing between these alleles had not been possible without testcrossing to known genetic stocks. Ellis et al. (2002) developed polymerase chain reaction (PCR)-based markers that differentiate between alleles at the Rht-B1 and Rht-D1 loci. Butler et al. (2005) used these markers to identify alleles at the Rht-B1 and Rht-D1 loci in a recombinant inbred line (RIL) spring wheat population and showed that individuals with tall alleles (Rht-B1a and Rht-D1a) were often higher yielding than individuals with one or both semidwarfing allele (Rht-B1b or Rht-D1b).
Several reports have suggested patterns of association between various allelic variants at the Xgwm 261 locus and agronomic characteristics in wheat (Korzun et al., 1998; Worland et al., 1998, 2001; Ahmad and Sorrells, 2002). In most cases, with the exception of Korzun et al. (1998), these studies have been done in the context of cultivar surveys that may be confounded due to inadequate sample sizes and genetic relatedness of test entries. Furthermore, no report is yet available documenting the effect of allelic variation at the Xgwm 261 and Rht-B1 loci in the same segregating population. The objective of this study was therefore to determine the effects of allelic variation at the Xgwm 261 and Rht-B1 loci on GA sensitivity, coleoptile length, and agronomic traits of two winter wheat RIL populations.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Field Trials
Field trials were conducted in 2003–2004 (2004) and 2004–2005 (2005) at five locations in eastern Colorado: Akron, Burlington, Julesburg, Walsh, and Fort Collins. In 2004 the RILs were at the F4:6 generation, while in 2005 the RILS were at the F4:7 generation. All sites were nonirrigated except for Fort Collins, where irrigation was applied with a linear overhead sprinkler system at approximately weekly intervals to achieve maximum yields. The trials were designed as 15 x 10, 
-0,1 rectangular lattices with two replicates (Patterson et al., 1978). Entries included RILs of both populations, one occurrence of each parent, and ‘Jagger’ (PI 593688) and ‘Prowers 99’ (PI 612420) as check cultivars. Plots at all locations except Walsh were planted 3.7 m long, six rows wide, with 23-cm spacing between rows; all six rows were harvested (effective plot size, 5.1 m2). At Walsh, plots were planted 3.7 m long and six rows wide with 30.5-cm spacing between rows; only the central four rows were harvested (effective plot area, 4.5 m2). In 2004 only the yield trials at Akron, Fort Collins, and Julesburg were harvested; the Burlington location was lost due to a spring freeze event, and the Walsh location was lost to a hail event before harvest. Plots at all locations were harvested in 2005.
Data were recorded for days to heading, plant height, grain yield, test weight, kernel number, and kernel weight. Days to heading was recorded (at Julesburg and Fort Collins in 2004 and Akron and Fort Collins in 2005) as the number of days from 1 January to which 50% of the spikes in a plot were fully visible above the flag leaf collar. Plant height was recorded at harvest and measured from the soil surface to the tip of the heads excluding the awns. Grain yield was determined following harvest with a small plot combine. A subsample of grain from each plot was cleaned and used to determine test weight. At Akron and Burlington in 2005, test weight was not obtained due to inadequate sample size resulting from low grain yield. At two dryland (Akron 2004, Julesburg 2005) and two irrigated (Fort Collins 2004 and 2005) locations, a 1-m section of a single row from each plot was cut at the soil surface immediately before harvest. Five heads from this sample were randomly selected and threshed together; kernel number and kernel weight were determined from this sample.
Molecular Marker Analysis
DNA was isolated from bulked leaves of five to six greenhouse-grown seedlings of each RIL according to the procedures of Riede and Anderson (1996). DNA was further purified with addition of 1 unit RNAase followed by incubation at 37°C for 30 min. Microsatellite marker Xgwm 261 was amplified in a 20-µL PCR containing 80 ng of DNA, 1x PCR buffer, 120 µM of dNTP, 0.25 µM of each Xgwm 261 primer, and 0.8 units of Taq polymerase. The following touchdown thermal profile was used for DNA amplification: 3 min at 94°C, 1 min at 94°C, 1 min at 55°C, and 2 min at 72°C for five cycles, in which the annealing temperature was lowered by °C per cycle; and 31 cycles in which the annealing temperature remained constant at 50°C. Ten minutes at 72°C was used for the final extension. Amplified DNA was loaded into 4% high-resolution agarose gels (SFRA, Amresco, Solon, OH) and electrophoresed at 100 V for 3 h 30 min. Agarose gels were stained with ethidium bromide for 30 min, rinsed for 30 s in deionized water, and then digitally photographed under ultraviolet light using the Alphaimager documentation system (Alpha Innotech Corp., San Leandro, CA). Molecular sizes of the DNA fragments were determined in relation to the Amplisize Molecular Ruler (BioRad, Richmond, CA) and comparison with three entries: Yuma (Xgwm 261 165 bp), Longhorn (Xgwm 261 210 bp), and Mara (Xgwm 261 192 bp).
The BF-MR1 primer pair developed by Ellis et al. (2002) was used to identify Rht-B1 alleles for the RILs. The PCR conditions were the same as specified by Ellis et al. (2002) except that the annealing temperature for the BF-MR1 primer pair was increased to 62.4°C. Separation and visualization of PCR products was as described for Xgwm 261. Molecular sizes of the DNA fragments were determined in relation to the Amplisize Molecular Ruler (BioRad, Richmond, CA) and comparison with two entries: Yuma (Rht-B1b) and Longhorn (Rht-B1a).
Gibberellic Acid Sensitivity Measurement
Plastic 38.1- x 15.3- x 5.1-cm trays (Rubbermaid Co., Wooster, OH) were filled three-quarters full with coarse vermiculite (American Clay Works, Denver, CO). Two trays (one for GA treatment and one for control) were planted with 10 test entries and 2 check entries (Mara and Yuma) in each tray. Ten seeds of each entry were planted in a single row within the tray and covered with a layer of vermiculite. Trays were placed in the greenhouse and watered, either with distilled water or 25 mg kg–1 GA solution (Sigma-Aldrich Inc, St. Louis, MO), as necessary. After 21 d, plant height was measured from the seed to the tip of the tallest leaf. Gibberellic acid sensitivity was determined between the mean of the 10 seedlings treated with distilled water and the mean of the 10 seedlings treated with the GA solution. The experimental design was a randomized complete block with two replications. Replications were temporally blocked due to a 6-mo separation between replications.
Coleoptile Length Measurement
Forty seeds were placed embryo side down approximately 1 cm apart and 2.2 cm from the bottom on premoistened 39.6- x 13.2-cm blotter paper (Anchor Paper Co., St. Paul, MN). A premoistened blotter paper was placed over the seeds, and both sheets were loosely rolled and secured with a size no. 32 rubber band (S.P. Richards Co., Atlanta, GA). Samples were placed in a plastic 43.8- x 29.5- x 16.5-cm container (RubberMaid Co., Wooster, OH) and filled to a depth of 2 cm with distilled water. Containers were covered with foil and placed in a dark cold room at 4°C for four d. Samples were then incubated in a dark Percival growth chamber (Percival Scientific, Inc., Boone, IA) at 16°C for 16 d. Measurements were taken from the seed to the top of the coleoptile. The experimental design was a randomized complete block with two replications. Replications were temporally blocked due to a 1-mo separation between replications.
Data Analysis
Estimated means for plant height, grain yield, test weight, heading date, kernel number, and kernel weight were determined using an analysis of variance with SAS PROC Mixed (SAS Institute, 1999), in which the model adjusted for incomplete block effects based on the rectangular lattice design (Patterson et al., 1978). Incomplete blocks were considered to be random effects, while replications and RILs were considered as fixed effects. The significance and magnitude of the effects of alleles at the Xgwm 261 and Rht-B1 loci on measured variables was determined using SAS PROC GLM (SAS Institute, 1999). The normality of trait distribution for GA sensitivity and coleoptile length was determined using the Shapiro-Wilk test conducted with the SAS UNIVARIATE procedure (SAS Institute, 1999). A Chi-square analysis was performed to detect significant deviation from expected segregation ratios for the two markers. Expected ratios were 1:1 at the individual Xgwm 261 and Rht-B1 loci and 1:1:1:1 for the combinations of the four allelic classes.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In fall 2004, trials at each location were planted with good topsoil moisture that was sufficient for abundant plant growth through the winter months into April. From late April into May, a period of severe drought stress occurred, punctuated by high temperatures that adversely affected trials at Akron, Burlington, and Julesburg. At Akron and Burlington, the combination of drought and high temperature stress at heading caused sterility in the emerged spike, and in some cases the plants ceased growing, greatly reducing plant height, grain yield, and test weight. Heading date was slightly later at Julesburg than at Akron and Burlington, and thus the trial was not heading during the peak high temperature stress. Plant height was reduced at Julesburg, but grain yield, kernel weight, kernel number, and test weight were not severely affected. At Walsh, excellent moisture and moderate temperatures from mid to late May promoted severe stripe rust (caused by Puccinia striiformis Westend.) infection. However, segregation for resistance among the RILs was not observed, and measured variables were thus not differentially affected. Fort Collins had adequate soil moisture throughout the growing season, but yield was reduced compared with 2004 due to higher temperatures during grain filling and a severe stripe rust infection. All yield trials were harvested in 2005.
Means for plant height, grain yield, and test weight of the two populations differed (P 
0.05) among the eight environments (Table 1
). At Akron, drought and high temperature stress in 2005 resulted in reduced plant height and lower grain yield compared with 2004. The drought and high temperature stress at Burlington in 2005 resulted in reduced plant height and grain yield compared with the other dryland locations, with lower values only observed at Akron in 2005. In 2005, Fort Collins, an irrigated environment, did not experience the drought stress or the high temperature stress at heading experienced at the other eastern Colorado sites. Because of higher temperatures during the spring and later in grain filling, however, plant height and grain yield at Fort Collins were both reduced in 2005 compared with 2004. The Julesburg environments also differed between 2004 and 2005, with greater values for plant height and yield observed in 2005 compared with 2004; test weight, however, was lower in 2005 than 2004 due to high temperatures that occurred during grain filling. While Walsh in 2005 showed greater plant height than other dryland or irrigated environments, yields and test weights were reduced largely due to the severe level of stripe rust in that environment.
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Among the Longhorn/Akron RILs, significant (P 0.05) differences were observed between the Rht-B1b and Rht-B1a classes for both GA sensitivity and coleoptile length (Table 2
). For GA sensitivity, frequency distributions of the Longhorn/Akron RILs were non-normal (P < 0.05) as RILs appeared to segregate according to the Rht-B1 allele they were carrying from Akron (Rht-B1b) and Longhorn (Rht-B1a). Check entries and RILs carrying the Rht-B1b allele were less sensitive to GA than those carrying the Rht-B1a allele. In contrast to GA sensitivity, RILs exhibited a normal frequency distribution for coleoptile length (P > 0.05). As observed with GA sensitivity, check entries and RILs carrying the Rht-B1b allele had shorter coleoptiles than those carrying the Rht-B1a allele.
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Evaluation of yield components (kernel weight and number) revealed little useful information regarding observed yield responses of the Rht-B1a and Rht-B1b allelic classes (Table 3). While kernel weight differences were observed between the Rht-B1b and Rht-B1a allelic classes, significant differences were only observed in environments where grain yield of the two classes was the same. In two of these environments (Akron and Fort Collins 2004), the Rht-B1a class had higher kernel weight than the Rht-B1b class, while in one environment (Julesburg 2005) this trend was reversed. Possible compensatory reduction in kernel number was observed in only one environment (Fort Collins 2004) where the Rht-B1a class showed lower kernel number than the Rht-B1b class (Table 3). While yield and yield component estimates revealed no clear trends, differences in test weight between the two allelic classes were observed in five of six environments where test weight was estimated (Table 3). In each instance, the Rht-B1a allelic class had higher test weight than the Rht-B1b allelic class. Differences ranged from 0.8% at Akron in 2004 to 2.9% at Julesburg in 2005, with an average difference of 1.6%. Our results are consistent with Butler et al. (2005), who observed higher test weight of Rht-B1a spring wheat RILs than Rht-B1b RILs at each of four environments. Additionally, Guttieri et al. (2001) evaluated 16 spring wheat cultivars under drought stress and observed test weights of tall cultivars to be greater than that of semidwarf cultivars. Our results, along with those of Butler et al. (2005) and Guttieri et al. (2001), suggest that wheat genotypes that carry the Rht-B1a allele have a better capacity to fill the grain under postanthesis stress than genotypes that carry the Rht-B1b allele.
Longhorn/Yuma Recombinant Inbred Line Population
Molecular marker analysis showed that Longhorn carries the Xgwm 261 210-bp and Rht-B1a alleles and that Yuma carries the Xgwm 261 165-bp and Rht-B1b alleles. Thus, the Longhorn/Yuma RIL population segregated for alleles at both the Xgwm 261 and Rht-B1 loci. Twenty-eight RILs carried the Xgwm 261 165-bp allele, 35 carried the Xgwm 261 210-bp allele, and 7 were heterogeneous for the two marker fragments. The 7 heterogeneous RILs were eliminated from the analysis; ratios of the remaining RILs were examined with Chi-square and were in agreement with the 1:1 expectation (P > 0.05). The Rht-B1b allele was found in 38 RILs, while the Rht-B1a allele was found in 27 RILs, the ratio being in agreement (P > 0.05) with the 1:1 expectation using Chi-square analysis. The ratios of the four allelic classes, Xgwm 261 165 + Rht-B1b (18 RILs), Xgwm 261 165 + Rht-B1a (10 RILs), Xgwm 261 210 + Rht-B1b (22 RILs), and Xgwm 261 210 + Rht-B1a (13 RILs), also agreed (P > 0.05) with a 1:1:1:1 segregation ratio using Chi-square analysis.
Effects of Rht-B1 alleles on GA sensitivity and coleoptile length were very similar to observations in the Longhorn/Akron RIL population. Frequency distributions for the RILs carrying the Rht-B1b and Rht-B1a allele were non-normal (P < 0.05) for GA sensitivity and normal (P > 0.05) for coleoptile length. The Rht-B1b class showed reduced GA sensitivity and coleoptile length compared with the Rht-B1a class (Table 2), with the percentage reduction (72% for GA sensitivity and 15% for coleoptile length) similar to observations in the Longhorn/Akron population. For the Xgwm 261 marker locus, frequency distributions for the RILs carrying the Xgwm 261 165-bp and 210-bp alleles were normal (P > 0.05) for both GA sensitivity and coleoptile length. No differences in GA sensitivity or coleoptile length were observed between the two Xgwm 261 allelic classes (Table 2).
In the field studies, differences were observed between allelic classes at both the Rht-B1 and Xgwm 261 marker loci (Table 4 ). For plant height, the Rht-B1b class was shorter than the Rht-B1a class in each environment except Fort Collins in 2005. The average height reduction observed was 8.3%, less than was observed in the Longhorn/Akron population or as reported in previous studies (Butler et al., 2005; Flintham et al., 1997; Richards, 1992). Similarly, the Rht-B1b allele accounted for only 23% (across eight environments) of the phenotypic variance for plant height, less than observed with the Longhorn/Akron RIL population (34%) and in the spring wheat RIL population (59%) evaluated by Butler et al. (2005). The disparity of explained phenotypic variance among these populations is likely due to differences in modifying genes for plant height among the parents of the respective RIL populations. Other observed effects due to alleles at the Rht-B1 locus included higher grain yield of the Rht-B1b class at one irrigated environment (Fort Collins 2004), higher kernel number of the Rht-B1b class at the other irrigated environment (Fort Collins 2005), and lower test weight of the Rht-B1b class in three of four environments (Fort Collins 2004, Julesburg 2004, and Walsh 2005) where test weight was estimated (Table 4).
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Simultaneous segregation of alleles at the Xgwm 261 and Rht-B1 loci allowed estimation of the combined effects of alleles at the two loci in the Longhorn/Yuma RIL population. Both GA sensitivity and coleoptile length were significantly reduced for the two Rht-B1b classes relative to the two Rht-B1a classes, regardless of which Xgwm 261 allele was present (Table 2). These observations demonstrate that the inhibitory effects of the Rht-B1b semidwarfing allele on GA response, and thus coleoptile elongation, are preserved regardless of the presence of alleles at the Rht8 locus that confer GA sensitivity. Because the Xgwm 261 -192-bp allele was not segregating in this population, we could not determine if this pattern would persist when this allele is present.
In the field studies, differences in plant height were observed at each environment among RILs with different allelic combinations at the Xgwm 261 and Rht-B1 loci (Table 4). In general, RILs with the Xgwm 261 210-bp + Rht-B1a combination were the tallest among the four combinations. In six of eight environments, no differences were observed between RILs carrying the Xgwm 261 165-bp + Rht-B1b combination and RILs carrying the Xgwm 261 210-bp + Rht-B1b combination, suggesting that allelic variation at the Xgwm 261 locus has little effect on plant height in an Rht-B1b background. In five of eight environments, RILs with the Xgwm 261 165-bp + Rht-B1a combination were shorter than those with the Xgwm 261 210-bp + Rht-B1a combination, the average height reduction being 8.5%. These results confirm the results previously described for this population that showed that the Xgwm 261 165-bp allele was associated with a height-reducing effect (Table 4), contrary to results reported by Korzun et al. (1998).
Of considerable interest is the comparison between the Rht-B1b and Rht-B1a alleles in the Xgwm 261 165-bp background. While differences were only statistically significant in four of eight environments, a general trend was apparent whereby the Xgwm 261 165-bp + Rht-B1a combination was intermediate in height to the shorter Xgwm 261 165-bp + Rht-B1b and the taller Xgwm 261 210-bp + Rht-B1a combinations (Table 4). Because the Xgwm 261 165-bp + Rht-B1a combination also had longer coleoptiles than either of the two Rht-B1b combinations (Table 2), these data suggest that incorporating the Xgwm 261 165-bp allele into certain Rht-B1a backgrounds (Longhorn in this case) may provide the means by which increased coleoptile length and reduced plant height relative to standard height wheat cultivars could be achieved. This combination of characteristics may be advantageous in stressful environments where Rht-B1b semidwarfs have been shown to exhibit reduced test weight, coleoptile length, extreme dwarfism in drier years, and yield instability. Validation of this association in other genetic backgrounds is needed.
For other traits evaluated, few consistent differences were observed among the four allelic combinations examined. While differences in grain yield were observed in only one environment (Fort Collins 2004) and differences in kernel weight in only one environment (Akron 2004), differences in kernel number among the four allelic combinations were observed in three out of four environments where kernel number was evaluated (Table 4). No general pattern was apparent, however, with respect to an optimum allele combination. Worland et al. (1988) observed increased spikelet fertility resulting from incorporation of the Rht8c semidwarfing allele from Mara. A consistent increase in kernel number attributed to either Xgwm 261 allele was not observed in our study. For example, increased kernel number attributed to the Xgwm 261 165-bp allele was observed in the Rht-B1b background in only one of four environments (10% enhancement in Fort Collins 2004) and in the Rht-B1a background in only one of four environments (15% higher in Julesburg 2005).
Differences among the four allelic combinations for test weight and heading date were similarly inconsistent. For test weight, in an Rht-B1b background, differences were only observed in one of six environments (Julesburg 2005) where the Xgwm 261 165-bp allele enhanced test weight 2% relative to the 210-bp allele (Table 4). In an Rht-B1a background, differences were observed in two of six environments and the allele responsible for the increase differed between these environments (9% higher with Xgwm 261 210-bp in Akron 2004, 1% higher with Xgwm 261 165-bp in Fort Collins 2004). For heading date, a general trend toward delayed maturity for the Xgwm 261 210-bp allele was observed, as with the single locus comparison, but the differences were clearly expressed in only two of four environments (Fort Collins and Julesburg 2004; Table 4). In one of the remaining environments (Fort Collins 2005), delayed heading date attributed to the Xgwm 261 210-bp allele relative to the 165-bp allele was observed in the Rht-B1a class (2.1 d) but not the Rht-B1b class.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In both populations evaluated, height reduction due to the Rht-B1b allele relative to the Rht-B1a allele was observed, although the extent of reduction was less than observed in previous studies. This likely was the result of height-promoting alleles in the background of the Longhorn parent. Advantages attributed to the Rht-B1a allele were observed in both populations for test weight and in one population (Longhorn/Akron) for grain yield at two locations with severe high temperature and drought stress.
In the population segregating for alleles at the Xgwm 261 locus (Longhorn/Yuma), no differences in GA sensitivity or coleoptile length were observed between the Xgwm 261 165-bp and Xgwm 261 210-bp allelic classes, although greater GA sensitivity and longer coleoptiles were observed in an Rht-B1a background. Plant height of the Rht-B1b and Xgwm 261 165-bp classes was similar and generally less than both the Xgwm 261 210-bp and Rht-B1a allelic classes, in contrast with previous research that suggested a height-promoting effect attributed to the Xgwm 261 165-bp allele (Korzun et al., 1998). This was also observed in comparisons involving allele combinations as the Xgwm 261 165-bp + Rht-B1a allelic class was significantly shorter than the Xgwm 261 210-bp + Rht-B1a class. A general trend was observed where the Xgwm 261 165-bp + Rht-B1a combination was intermediate in height to the shorter Xgwm 261 165-bp + Rht-B1b and the taller Xgwm 261 210-bp + Rht-B1a combinations. Coleoptile length of the Xgwm 261 165-bp + Rht-B1a combination was greater than either of the two Rht-B1b combinations, suggesting that incorporation of the Xgwm 261 165-bp allele into some Rht-B1a backgrounds could provide reduced plant height and increased coleoptile length relative to standard height wheat cultivars. No negative agronomic trait associations were observed resulting from the presence of the Xgwm 261 165-bp allele, although a significant delay in heading date was observed in RILs carrying the Xgwm 261 210-bp allele.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication December 23, 2006.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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