Genetic and environmental control of dormancy in white-grained wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Grain dormancy in wheat is an important component of resistance to preharvest sprouting and hence an important trait for wheat breeders. The significant influence of environment on the dormancy phenotype makes this trait an obvious target for marker-assisted-selection. Closely related breeding lines, SUN325B and QT7475, containing a major dormancy QTL derived from AUS1408 located on chromosome 4A, but substantially different in dormancy phenotype, were compared with a non-dormant cultivar, Hartog, in a range of controlled environments. As temperature increased, dormancy at harvest-ripeness decreased particularly for QT7475. The dormancy phenotypes of reciprocal F₁ grains involving all possible combinations of Hartog, QT7475 and SUN325B were also compared in two environments with different temperatures. The results were consistent with the presence of QTL in addition to 4A in SUN325B, compared with QT7475, at least one of which was associated with the seed coat. Genetic analysis of a doubled haploid population derived from SUN325B × QT7475 identified a highly significant QTL located on chromosome 3BL, close to the expected position of the mutant allele of the red seed coat colour gene in white-grained wheat, R-B1a. When the lines in the population were grouped according to the parental alleles at marker loci flanking the 3B QTL, the dormancy phenotype frequency distribution for the SUN325B group was shifted towards greater dormancy compared with the QT7475 group. However, significant variation for dormancy phenotype remained within each group. Lines representing the extremes of the range of phenotypes within each group maintained their relative ranking across seven environments consistent with the presence of another unidentified QTL contributing to dormancy in SUN325B.     

Grain dormancy in fixed lines of white-grained wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) grown under controlled environmental conditions

Pre-harvest sprouting (PHS) in wheat (Triticum aestivum L.) can be a significant problem, causing deleterious effects on grain quality. However, the adverse impacts of PHS can be reduced by introgressing genes controlling grain dormancy into white-grained bread wheat. Screening for grain dormancy typically involves germination testing of harvest-ripe grain grown in a glasshouse or field. However, the more uniform environmental conditions provided by temperature controlled glasshouses (i.e. controlled environmental conditions—CEC) may provide significant benefits for the assessment of grain dormancy. In this study, the dormancy phenotype of grain grown under CEC incorporating an extended photoperiod, was compared with 2 years of data from field grown material. Four dormant double haploid lines (derived from SW95-50213 and AUS1408) and two locally adapted non-dormant cultivars EGA Gregory and EGA Wills were compared in three replicated experiments grown under CEC (22 ± 3°C and 24 h photoperiod). The germination response of harvest-ripe grain was examined to assess the expression of grain dormancy. Two measures of germination, the predicted time to 50% germination (G ₅₀) and a weighted germination index, both clearly differentiated dormant and non-dormant lines grown under CEC. In addition, levels of grain dormancy were similar to field-grown plants. These results demonstrated that CEC with an extended photoperiod can be used for rapid and reliable characterisation of grain dormancy in fixed lines of bread wheat.     

Identification of genomic regions associated with seed dormancy in white-grained wheat

Pre-harvest sprouting (PHS) in developing wheat (Triticum aestivum L.) spikes is stimulated by cool and wet weather and leads to a decline in grain quality. A low level of harvest-time seed dormancy is a major factor for PHS, which generally is a larger problem in white-grained as compared to red-grained wheat. We have in this study analyzed seed dormancy levels at the 92nd Zadok growth stage of spike development in a doubled-haploid (DH) white wheat population and associated variation for the trait with regions on the wheat genome. The phenotypic data was generated by growing the parent lines Argent (non-dormant) and W98616 (dormant) and 151 lines of the DH population in the field during 2002 and 2003, at two locations each year, followed by assessment of harvest-time seed dormancy by germination tests. A genetic map of 2681 cM was constructed for the population upon genotyping 90 DH lines using 361 SSR, 292 AFLP, 252 DArT and 10 EST markers. Single marker analysis of the 90 genotyped lines associated regions on chromosomes 1A, 2B, 3A, 4A, 5B, 6B, and 7A with seed dormancy in at least two out of the four trials. All seven putative quantitative trait loci (QTLs) were contributed by alleles of the dormant parent, W98616. The strongest QTLs positioned on chromosomes 1A, 3A, 4A and 7A were confirmed by interval mapping and markers at these loci have potential use in marker-assisted selection of PHS resistant white-grained wheat.     

Screening for grain dormancy in segregating generations of dormant × non-dormant crosses in white-grained wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Pre-harvest sprouting (PHS) in wheat (Triticum aestivum L.) is a significant problem. Introgression of genes controlling grain dormancy into white-grained bread wheat is one means of improving resistance to PHS. In this study seven dormant (containing the SW95-50213 and AUS1408 sources) × non-dormant crosses were produced to investigate the effectiveness of selection for grain dormancy in early segregating generations. Each generation (F₁–F₄) was grown in a temperature controlled glasshouse with an extended photoperiod (i.e. continuous light). F₂ and F₃ generations were subject to selection. Five hundred harvest-ripe grains were tested for germination over a 14 day period, and the 100 most dormant grains were retained and grown-on to produce the next generation within each cross. The response to selection was assessed through analysis of the time to 50% germination (G₅₀) in the F₂, F₃ and F₄ generations. In addition, changes in marker class frequencies for two SSR markers (barc170 and gpw2279) flanking a known quantitative trait locus (QTL) for grain dormancy on chromosome 4A were assessed in DNA from F₂ plants selected from early germinating (non-dormant) and late germinating (dormant) phenotypic extremes within each cross. Selection for grain dormancy in the F₂ and F₃ generations effectively recovered the dormant phenotype in all seven crosses, i.e. the F₄ generation was not significantly different from the dormant parent. Further, selection based on individual F₂ grains changed marker class frequencies for the 4A dormancy QTL; in most cases eliminating the marker class homozygous for the non-dormant alleles. Application of this screening method will enable breeders to better select for grain dormancy and may lead to development of new cultivars offering effective resistance to PHS in the near future.     

Increased grain dormancy in white-grained wheat by introgression of preharvest sprouting tolerance QTLs

White-grained wheat cultivars have long been recognized to be less resistant to preharvest sprouting (PHS) than the red-grained ones. Previously two QTLs for grain dormancy, QPhs.ocs-3A.1 (QPhs-3AS) and QPhs.ocs-4A.1 (QPhs-4AL) were identified in a highly dormant Japanese red wheat, Zenkoujikomugi (Zen). Aiming at improvement of PHS tolerance in white-grained wheat, the introgression effect of these two QTLs in a white-grained population consisting of 40 recombinant inbred lines (RILs) developed from a cross between Zen and white-grained Spica was examined here. Random 20 RILs with red grains were also developed from the same cross and used as a control population. The RILs were grown in the field and in the glasshouse to evaluate the grain dormancy by germination test. Several SSR markers closely linked to the QPhs-3AS and QPhs-4AL were used to estimate the alleles at the QTLs. Dormancy variation in the RILs was significantly associated with the differences for grain color and the alleles at QPhs-3AS over several years. Although allelic variation was detected in a SSR marker closely linked to QPhs-4AL there was no difference in germination data between the Zen-allele and the Spica-allele groups. As expected, the red-grained RILs with the Zen allele at QPhs-3AS were the most dormant. Some white-grained RILs with the Zen allele at QPhs-3AS showed higher dormancy compared to the red-grained RILs with the alternative allele. These results demonstrated that introgression of the QPhs-3AS gene could contribute to the increased grain dormancy in white-grained wheat.     

Dormancy breaking procedures and the breeding of white-grained wheat with resistance to pre-harvest sprouting

Summary Gibberellic acid, cold (4°C) and a combination of these two treatments were tested for use in breaking dormancy in 27 lines of white-grained wheat with varying levels of resistance to pre-harvest sprouting. Germination increased in all lines treated with gibberellic acid. Dormancy could be broken with 1 M gibberellic acid. Response to cold varied. A combination of gibberellic acid and cold treatment was the most effective. This technique has been found useful in treating seed in a breeding program aimed at producing wheats with resistance to pre-harvest sprouting.     

控制普通野生稻种子休眠性QTL的定位

水稻种子休眠性是关系到稻米品质和稻种质量的一个重要农艺性状。研究水稻种子休眠性遗传及分子机制对培育具有适度休眠性的优良水稻品种具有重要意义。本研究以籼稻品种9311为受体、普通野生稻为供体的染色体片段置换系群体为材料,在后熟不同时间检测群体种子休眠性,对控制种子休眠性的QTL进行定位分析,共定位到14个QTL,分布在第3、第4、第5、第6、第7、第10、第11、第12染色体上。筛选休眠性显著强于背景亲本9311的家系,分析这些家系携带的QTL数目,表明携带的位点越多,休眠性越强。进一步利用家系Q14与9311的F2群体验证了第7染色体标记RM180和RM21323之间存在一个效应较大的QTL qSD-7-2,该位点LOD值为18.49,可解释的表型变异率为33.53%,表明该位点是一个控制普通野生稻种子休眠性的主效QTL,且能稳定遗传。本研究为野生稻种子休眠基因的精细定位及克隆奠定了基础,且为培育强休眠性籼稻品种提供了育种材料。     

Novel resistance to pre-harvest sprouting in Australian wheat from the wild relative Triticum tauschii

The diploid D-genome progenitor of hexaploid wheat, Triticum tauschii (Coss.) Schmahl., was screened to identify mechanisms for resistance to pre-harvest sprouting. A number of promising mechanisms were identified, and transferred to hexaploid wheat via wide-hybridisation. One identified mechanism, an inhibitory phenolic compound present in the bracts surrounding the grain, has been shown to function effectively in synthetic hexaploid wheats. A number of seed-borne dormancy mechanisms were also identified. Expression of embryo dormancy in synthetic hexaploid wheats was demonstrated when compared with non-dormant hexaploid wheat. Effects of the seed coat on dormancy were also studied, with the seed coat of synthetic hexaploids accelerating rather than inhibiting germination. Embryo dormancy was also demonstrated in two `direct-cross' hybrids. The results suggest that a combination of the described mechanisms may produce white wheats with resistance to pre-harvest sprouting adequate for most Australian climatic conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mapping and validation of PCR-based markers associated with a major QTL for seed dormancy in wheat

Seed dormancy is one of the important factors controlling pre-harvest sprouting (PHS) resistance in wheat. We identified a major quantitative trait locus (QTL) for seed dormancy on the long arm of wheat chromosome 4A (4AL) via simple sequence repeat (SSR)-based genetic mapping using doubled haploid lines from a cross between Japanese PHS resistant variety ‘Kitamoe’ and the Alpine non-resistant variety “Münstertaler” (K/M). The QTL explained 43.3% of total phenotypic variation for seed dormancy under greenhouse conditions. SSR markers flanking the QTL were assigned to the chromosome long arm fraction length 0.59–0.66 on the basis of chromosome deletion analysis, suggesting that the gene(s) controlling seed dormancy are probably located within this region. Under greenhouse conditions, the QTL explained 28.5 and 39.0% of total phenotypic variation for seed dormancy in Haruyutaka/Leader (HT/L) and OS21-5/Haruyokoi (O/HK) populations, respectively. However, in field conditions, the effect was relatively low or not significant in both the K/M and HT/L populations. These markers were considered to be widely useful in common with various genetic backgrounds for improvement of seed dormancy through the use of marker-assisted selection. Further detailed research using near isogenic lines will be needed to define how this major QTL interacts with environmental conditions in our area.     

Integration of marker-assisted selection for resistance to pre-harvest sprouting with selection for grain-filling rate in breeding of white-kernelled wheat for the Chinese environment

Few Chinese high yielding white-grained wheat cultivars possess sufficient dormancy to avoid pre-harvest sprouting (PHS). Because the field evaluation of PHS is difficult, the identification of informative molecular markers is a priority for improving the level of dormancy. In this report, the effectiveness of phenotypic and genotypic selection was compared. Four microsatellite loci Xbarc57, Xbarc294, Xbarc310 and Xbarc321, mapped on the short arm of chromosome 3A, were used for selection in white-grained wheat F₅ lines which were also selected on the basis of their grain filling rate (GFR). One of these (later designated cv. Zhongmai911) was further selected on the basis of its allelic constitution at the four SSR loci. This cultivar combines a high level of PHS resistance with high grain yield. The results suggested that rapid GFR and PHS resistance can be bred simultaneously.     

Inheritance of resistance to wheat midge, Sitodiplosis mosellana, in spring wheat

Inheritance of resistance to a wheat midge, Sitodiplosis mosellana (Géhin), was investigated in spring wheats derived from nine resistant winter wheat cultivars. F₁ hybrids were obtained from crosses between resistant winter wheats and susceptible spring wheats, and used to generate doubled haploid populations. These populations segregated in a ratio of 1:1 resistant to susceptible, indicating that a single gene confers the resistance. The F₂ progeny from an intercross among spring wheats derived from the nine resistance sources did not segregate for resistance. Therefore, the same gene confers resistance in all nine sources of resistance, although other genes probably affect expression because the level of resistance varied among lines. Heterozygous plants from five crosses between diverse susceptible and resistant spring wheat parents all showed intermediate levels of response, indicating that resistance is partly dominant. Susceptible plants were reliably discriminated from heterozygous or homozygous resistant ones in laboratory tests, based on the survival and development of wheat midge larvae on one or two spikes. This powerful resistance gene, designated Sm1, is simply inherited and can be incorporated readily into breeding programmes for spring or winter wheat. However, the use of this gene by itself may lead to the evolution of a virulent population, once a resistant cultivar is widely grown.     

短叶13号萝卜种子休眠的初步探讨

通过对入库的几种萝卜种子发芽率的测定,发现短叶13号萝卜种子有休眠现象存在。影响种子休眠的因素很多,休眠期的长短也不一样,短叶13号萝卜种子的休眠属自发休眠,打破休眠的途径可以从低温等条件着手。     

Seedling emergence,coleoptile length,and plant height relationships in crosses of dwarf and standard-height wheats

Summary Seedling emergence was closely correlated with coleoptile length and plant height among parents, F₂ and F₃, populations of crosses involving dwarf wheats Olesen Dwarf (CI 14497), Norin 10 derivative D6301, Tom Thumb derivative D6899, and the standard-height varieties Ramona 50 and Nainari 60. Genetic mechanisms that governed plant height also influenced coleoptile length, but the relative effects of genes showing dominant or epistatic effects appeared to be different. With respect to the two parents involved in each of 15 crosses, mean F₂ coleoptile lengths were consistently closer to the low parent value than were corresponding mean F₂ plant heights. A slight curvilinear relationship was also found between coleoptile length and plant height of F₃ lines. The results suggest that selection of semidwarf wheats with long coleoptiles and improved emergence properties from crosses involving the dwarf wheats of this study would be unlikely.     

Wheat ABA-insensitive mutants result in reduced grain dormancy

This paper describes the isolation of wheat mutants in the hard red spring Scarlet resulting in reduced sensitivity to the plant hormone abscisic acid (ABA) during seed germination. ABA induces seed dormancy during embryo maturation and inhibits the germination of mature seeds. Wheat sensitivity to ABA gradually decreases with dry after-ripening. Scarlet grain normally fails to germinate when fully dormant, shows ABA sensitive germination when partially after-ripened, and becomes ABA insensitive when after-ripened for 8?C12?months. Scarlet ABA-insensitive (ScABI) mutants were isolated based on the ability to germinate on 5???M ABA after only 3?weeks of after-ripening, a condition under which Scarlet would fail to germinate. Six independent seed-specific mutants were recovered. ScABI1, ScABI2, ScABI3 and ScABI4 are able to germinate more efficiently than Scarlet at up to 25???M ABA. The two strongest ABA insensitive lines, ScABI3 and ScABI4, both proved to be partly dominant suggesting that they result from gain-of-function mutations. The ScABI1, ScABI2, ScABI3, ScABI4, and ScABI5 mutants after-ripen more rapidly than Scarlet. Thus, ABA insensitivity is associated with decreased grain dormancy in Scarlet wheat. This suggests that ABA sensitivity is an important factor controlling grain dormancy in wheat, a trait that impacts seedling emergence and pre-harvest sprouting resistance.     

小麦胚休眠中ABA信号转导的蛋白质组分析

利用ABA处理休眠和不休眠小麦品种的胚，并通过双向电泳－质谱技术，比较其蛋白质表达情况。结果发现, 共有18个ABA反应型蛋白点的表达存在显著差异。经MALDI-TOF-MS检测及肽指纹图谱（PMF）分析，其中16个蛋白点在有关数据库中得到了归属鉴定。进一步对这16个蛋白及其特性进行综合分析，认为其涉及不同的反应途径，如胁迫反应（冷调蛋白、热激蛋白和醛脱氢酶）、信号交互反应（生长素反应蛋白、乙烯感应因子、钙依赖的蛋白激酶CP4和乙烯反应蛋白），以及种子的基本发育过程（LEA蛋白、Em蛋白、bZIP转录因子、锌指蛋白、myb家系转录因子、淀粉合成酶和纤维素酶等）。另外还有2个是功能未知的蛋白，其中之一在已测序的水稻中具有全长的cDNA；另一个经ESI-MS/MS检测，认为是ABA信号转导系统中的一个新组分。对上述2个蛋白分别从籽粒发育不同阶段、不同浓度ABA处理，以及休眠中和打破休眠后这2种状态下不同温育时间等方面来验证其表达与休眠性状之间的关系，结果发现它们在籽粒发育中后期大量合成，与胚休眠性获得的时间是一致的。用同样浓度的ABA处理，这2个蛋白在休眠胚中更易于表达，而在打破休眠的胚中需要5倍的ABA浓度才能得到相同的表达效果；在用无菌水浸润期间，休眠胚中该蛋白表达水平下降的速率迟于后熟胚中，且随着休眠的打破，该蛋白也消失了。推测其表达可能与休眠性的获得与解除有关。对控制该蛋白表达的基因进行克隆与功能鉴定可能为小麦抗穗发芽育种提供候选位点。     

Chromosomes responsible for sensitivity of embryo to abscisic acid and dormancy in wheat

The sensitivity of the embryo to abscisic acid (ABA) has been reported toplay an important role in seed dormancy. Using ditelocentric lines of wheatcv. Chinese Spring (CS, nondormant and ABA insensitive), F₂ seedsbetween monosomic lines of CS and a wheat line Kitakei-1354 (dormant,ABA sensitive) and deletion lines of CS chromosome 4A, germinability ofseeds and embryo-half seeds incubated in water and ABA were examined. The results indicated that the long arm of chromosome 4A carried majorgene(s) for the embryo sensitivity to ABA and dormancy. Chromosome2D might be also involved in the sensitivity to ABA.     

Combining ability of low phytic acid (<Emphasis Type="Italic">lpa1</Emphasis>-<Emphasis Type="Italic">1</Emphasis>) and quality protein maize (QPM) lines for seed germination and vigour under stress and non-stress conditions

Grain yield is more likely to be compromised by poor seed germination and vigour in low phytic acid (LPA) and quality protein maize (QPM) than normal maize (Nm), especially when grown under stressful tropical environmental conditions. The objectives of this study were to determine the effect of stress conditions on seed germination and vigour traits (percentage germination, seedling dry weight, average root and shoot length and vigour index) and to determine the GCA effects of the parental lines and SCA effects of the crosses. A ten parent half diallel (LPA, Nm, QPM lines) was subjected to the standard germination (non-stress) and accelerated aging (AA) (stress) tests. All seed lots were at the same physiological age and produced under the same season and conditions. Genotypic and group differences were investigated. General combining ability (GCA) and specific combining ability (SCA) effects were significant (P ≤ 0.001) for all traits under both stress and non-stress conditions indicating that both additive and non-additive gene effects were significant. Generally SCA effects were superior for all traits. The LPA lines displayed 61 % reduction in germination and 23–52 % reduction in vigour under stress conditions thereby underscoring challenges that are expected in deploying LPA maize in stress conditions. However, LPA line CM 31 exhibited large positive and mostly significant GCA effects, while two LPA × LPA crosses showed significant positive SCA effects. Results indicate breeding is required to improve both germination and vigour of the LPA lines to adapt them to tropical conditions that are generally stress-prone.     

Seed development-related expression of ARGONAUTE4_9 class of genes in barley: possible role in seed dormancy

Resistance to pre-harvest sprouting is an important breeding objective for cereal crops like barley and wheat. Seed dormancy, which determines the resistance or susceptibility to pre-harvest sprouting (PHS), is a complex trait. It is largely controlled by the antagonistic action of the plant hormones abscisic acid and gibberellic acid, but also has a large component of genotype?×?environment interaction. Recent studies have revealed a role for epigenetic changes through histone modification in controlling seed dormancy. However, the role of DNA methylation in seed development and dormancy is not known. In this study, we explored the role of ARGONAUTE4_9 class genes of the DNA methylation pathway in seed development and dormancy in barley. Our results show that the two AGO4_9 class genes in barley, i.e. AGO1002 and AGO1003, are preferentially expressed in ovaries at meiosis and in embryos 25?days after pollination (DAP). The expression of AGO1003 is two to fivefold higher than that of AGO1002 in these tissues, demonstrating differential expression of these genes. We also analysed the expression of AGO1003 in embryos of PHS-resistant and -susceptible varieties at 25?DAP and found a significant variation in the expression of this gene in seeds of dormant and non-dormant lines. The observed expression pattern of AGO1002 and AGO1003 suggests a possible role in sporogenesis and post-fertilization seed development. Indirectly these results imply a potential role of DNA methylation in seed development and seed dormancy.     

人工合成六倍体小麦后代衍生群体Waxy蛋白亚基的分子标记

四倍体小麦与节节麦杂交培育的人工合成六倍体小麦已广泛应用于国内外小麦品种改良。以引自CIMMYT的Syn768、Syn769和Syn780人工合成六倍体小麦分别与中国四川成都平原主栽普通小麦品种杂交、回交的BC2F2:6后代群体中选育的113份优良高代系为材料,采用SSR特异引物的PAGE凝胶电泳对其Waxy蛋白亚基缺失类型进行了研究。结果表明,在所检测的121份材料中,8份材料缺失Waxy-B1型蛋白亚基,占全部材料的6.6%;没有检测到其他类型的缺失体。从每个人工合成六倍体小麦亲本材料所形成的后代衍生群体来看, Waxy-B1缺失体频率各不相同,说明Waxy蛋白亚基缺失类型在人工合成六倍体小麦后代衍生群体材料中的表现存在着随机性,与亲本的基因型状况关系极大。通过研究人工合成六倍体小麦与普通小麦杂交后代的Waxy蛋白亚基缺失类型,有助于提高分子标记育种效率。     

Divergent selection for sprouting resistance in spring wheat

The development of sprouting-resistant spring-wheat ( Triticum aestivum L.) cultivars is a major breeding objective in many wheat-producing regions. Sprouting resistance is thought to be associated with delayed maturity. The primary objective of this study was to measure the reciprocal effects of selection for sprouting resistance and maturity. Two experiments were conducted over a 3–4-year period in Saskatoon, Canada. In the first experiment, two populations of hard red spring wheat were subjected to divergent selection (k = 10%) for maturity. In the second experiment, six populations derived from crosses between two sprouting-resistant, late-maturing, white-grained cultivars ('AUS1293' and 'AUS1408') and three early maturing, red–grained cultivars ('Park', 'PT516' and 'Roblin'), were subjected to divergent selection (k = 10%) for sprouting resistance. Selection for earliness reduced sprouting resistance in one population but had no effect in the second. For both populations, earlier maturity was associated with higher test weight but lower grain yield. In the second experiment, selection for increased sprouting resistance was effective, with realized heritabilities averaging 0.74. Increased sprouting resistance was associated with a slight delay (1–2.5 days) in time to spike emergence in four out of six populations, but had little effect on time to maturity in most populations. There was a trend towards redder grain in the sprouting-resistant selections. The recovery of sprouting-resistant, early maturing segregants was relatively low, averaging less than 10% over the six populations. In conclusion, selection for increased sprouting resistance can result in delayed maturity, but the magnitude of that delay will vary among populations.