Comparative mapping of a gibberellic acid-insensitive dwarfing gene (Dwf2) on chromosome 4HS in barley

 We report the genetic mapping of Dwf2, a dominant gibberellic acid (GA₃)-insensitive dwarfing gene which has been previously described to cause a very short growth habit in barley (Hordeum vulgare) mutant ‘93/B694’. Using RFLP and microsatellite markers we performed segregation analysis in an F₂ population comprising 86 individuals developed from a cross of ‘93/B694’ (Dwf2) with ‘Bonus M2’ (dwf2). Dwf2 was mapped on the short arm of barley chromosome 4H proximal to microsatellite marker XhvOle (5.7 cM) and distal to RFLP marker Xmwg2299 (18.3 cM). The genetic localization of the Dwf2 gene at a homoeologous position to the multiallelic Rht-B1 and Rht-D1 loci in wheat suggests synteny of GA-insensitive dwarfing genes within the Triticeae. Moreover, the extremely prostrate growth habit exhibited in barley ‘93/B694’ (Dwf2) resembles that of wheat plants carrying the genes Rht-B1c (Rht3) or Rht-D1c (Rht10). Received: 1 July 1998 / Accepted: 17 September 1998     

矮秆基因对小麦部分农艺性状的效应

以中国主要麦区的124份小麦品种为材料,利用分子标记和系谱分析相结合,对其按照所含的矮秆基因Rht-B1b、Rht-D1b和Rht8进行分类,结合田间株高、旗叶长、小穗数和穗粒数以及室内苗期根系长度等农艺形状的调查,分析不同矮秆基因对小麦农艺性状的效应.结果显示:(1)参试的124份小麦品种(系)中23份含有Rht-B1b,7份含有Rht-D1b,22份含有Rht8基因,34份同时含有Rht-B1b和Rht8,16份同时含有Rht-D1b和Rht8,可分为6组.(2)Rht-B1b和Rht-D1b在降低株高的同时也缩短了旗叶的长度和苗期叶长,Rht8对株高的影响较弱,对旗叶和苗期叶长的影响也较小;3个矮秆基因对苗期根系长度、小穗数没有显著影响;Rht-D1b和Rht8显著增加穗粒数.研究表明,矮秆基因Rht8对小麦株高以及其他农艺性状的影响均较小,但能够显著增加穗粒数,是小麦矮化育种中比较理想的矮秆基因.     

"Perfect" markers for the Rht-B1b and Rht-D1b dwarfing genes in wheat

PCR-based markers were developed to detect the point mutations responsible for the two major semi-dwarfing genes Rht-B1b ( Rht1) and Rht-D1b ( Rht2) in wheat. These markers were validated by testing 19 wheat varieties of known Rht genotype. They included Rht-B1b and Rht-D1b dwarfs, double-mutant varieties and tall wheats. These were correctly genotyped with the Rht-B1b and Rht-D1b-specific primers, as well as markers specific for the tall alleles Rht-B1a and Rht-D1a. Using a family of doubled-haploid lines segregating for Rht-B1b and Rht-D1b, the markers were mapped to the expected homoeologous regions of chromosomes 4B and 4D, respectively. Both markers were strongly correlated with a reduction in height, accounting for 23% ( Rht-B1b) and 44% ( Rht-D1b) of the phenotypic variance in the population. These markers will have utility in marker-assisted selection of the Rht-B1b and Rht-D1b genes in wheat breeding programs.     

Molecular mapping of two dwarfing genes differing in their GA response on chromosome 2H of barley

The two recessive dwarfing mutants gai (GA-ins) and gal (GA-less), differing in their response to exogenously applied gibberellic acid (GA₃), were mapped in the centromere region and on the long arm, respectively, of the barley chromosome 2H. The gene gai, which determines reduced plant height and GA insensitivity pleiotropically, was found to co-segregate with the two RFLP markers Xmwg2058 and Xmwg2287. Both markers are known to map close to the centromere. The GA-sensitive dwarfing gene gal was found to be linked to the three co-segregating RFLP markers Xmwg581, Xmwg882 and Xmwg2212 (proximal) and XksuG5 (distal) by 3.6 and 9.5. cM, respectively. The distance between the two mutant loci was estimated to be about 55 cM. Homoeologous relationships between the dwarfing genes within the Triticeae are discussed. Received: 11 December 1998 / Accepted: 11 February 1999     

Molecular mapping of gibberellin-responsive dwarfing genes in bread wheat

Opportunities exist for replacing reduced height (Rht) genes Rht-B1b and Rht-D1b with alternative dwarfing genes for bread wheat improvement. In this study, the chromosomal locations of several height-reducing genes were determined by screening populations of recombinant inbred lines or doubled haploid lines varying for plant height with microsatellite markers. Linked markers were found for Rht5 (on chromosome 3BS), Rht12 (5AL) and Rht13 (7BS), which accounted for most of the phenotypic variance in height in the respective populations. Large height differences between genotypes (up to 43 cm) indicated linkage to major height-reducing genes. Rht4 was associated with molecular markers on chromosome 2BL, accounting for up to 30% of the variance in height. Confirming previous studies, Rht8 was linked to markers on chromosome 2DS, whereas a population varying for Rht9 revealed a region with a small but significant height effect on chromosome 5AL. The height-reducing effect of these dwarfing genes was repeatable across a range of environments. The molecular markers developed in this study will be useful for marker-assisted selection of alternative height-reducing genes, and to better understand the effects of different Rht genes on wheat growth and agronomic performance.     

青海育成小麦5个主效矮秆基因的分子检测

为系统了解青海小麦矮秆基因的分布特点,并进一步为青海高原小麦的株高育种提供优异种质资源。本研究利用5个矮秆基因的特异性分子标记对82份青海小麦品种资源中的矮秆基因进行了检测,并对不同矮秆基因的降秆效应进行了分析。结果表明:82份青海育成小麦品种中有49份材料至少含有一个矮秆基因,其中Rht-B1b的分布频率最高,约占参试材料的28.0%,其次是分布频率为23.2%的Rht8基因,而矮秆基因Rht-D1b、Rht5以及Rht12的分布频率分别为9.8%、13.4%、9.8%。在49份含有不同种类矮秆基因的材料中,其中16份材料同时含有2种及以上的矮秆基因,即RhtB1b和Rht8、Rht-D1b和Rht8、Rht-B1b和Rht5、Rht-D1b和Rht5、Rht8和Rht5、Rht-B1b和Rht12、Rht5和Rht12,并未发现同时含有矮秆基因Rht-B1b和Rht-D1b的品种;2份材料分别含有3种矮秆基因,即Rht-B1b、Rht8、Rht12和Rht-B1b、Rht5、Rht8;其余31份材料仅含有1种矮秆基因。82份青海育成小麦材料中仅含有Rht-B1b的材料11份,平均株高为86.2 cm,其降秆效应为5.7%;只含有Rht-D1b的材料有5份,平均株高为84.9 cm,其降秆效应为7.1%;仅含有Rht8的材料有9份,平均株高为88.6 cm,其降秆效应为3.1%。因此,在青海育成小麦品种中,矮秆基因的降秆效应为Rht-D1bRht-B1bRht8。     

Whole Genome Association Mapping of Plant Height in Winter Wheat (Triticum aestivum L.)

The genetic architecture of plant height was investigated in a set of 358 recent European winter wheat varieties plus 14 spring wheat varieties based on field data in eight environments. Genotyping of diagnostic markers revealed the Rht-D1b mutant allele in 58% of the investigated varieties, while the Rht-B1b mutant was only present in 7% of the varieties. Rht-D1 was significantly associated with plant height by using a mixed linear model and employing a kinship matrix to correct for population stratification. Further genotyping data included 732 microsatellite markers, resulting in 770 loci, of which 635 markers were placed on the ITMI map plus a set of 7769 mapped SNP markers genotyped with the 90 k iSELECT chip. When Bonferroni correction was applied, a total of 153 significant marker-trait associations (MTAs) were observed for plant height and the SSR markers (−log₁₀ (P-value) ≥4.82) and 280 (−log₁₀ (P-value) ≥5.89) for the SNPs. Linear regression between the most effective markers and the BLUEs for plant height indicated additive effects for the MTAs of different chromosomal regions. Analysis of syntenic regions in the rice genome revealed closely linked rice genes related to gibberellin acid (GA) metabolism and perception, i.e. GA20 and GA2 oxidases orthologous to wheat chromosomes 1A, 2A, 3A, 3B, 5B, 5D and 7B, ent-kaurenoic acid oxidase orthologous to wheat chromosome 7A, ent-kaurene synthase on wheat chromosome 2B, as well as GA-receptors like DELLA genes orthologous to wheat chromosomes 4B, 4D and 7A and genes of the GID family orthologous to chromosomes 2B and 5B. The data indicated that besides the widely used GA-insensitive dwarfing genes Rht-B1 and Rht-D1 there is a wide spectrum of loci available that could be used for modulating plant height in variety development.     

Mapping gibberellin-sensitive dwarfing locus <Emphasis Type="Italic">Rht18</Emphasis> in durum wheat and development of SSR and SNP markers for selection in breeding

Gibberellin-sensitive dwarfing gene Rht18 was mapped in two durum wheat recombinant inbred lines (RIL) populations developed from crosses, Bijaga Yellow/Icaro and HI 8498/Icaro. Rht18 was mapped within genetic interval of 1.8 cM on chromosome 6A. Simple sequence repeat (SSR) markers S470865SSR4, barc37 and TdGA2ox-A9 specific marker showed co-segregation with Rht18 in Bijaga Yellow/Icaro population consisting 256 RILs. Effect of Rht18 on plant height was validated in HI 8498/Icaro RIL population which segregated for Rht18 and Rht-B1b. Rht-B1b from HI 8498 showed pleiotropic effect on plant height and coleoptile length, on the other hand, Rht18 did not show effect on coleoptile length. The SSR and SNP markers linked to Rht18 were also validated by assessing their allelic frequency in 89 diverse durum and bread wheat accessions. It was observed that 204 bp allele of S470865SSR4 could differentiate Icaro from rest of the wheat accessions except HI 8498, suggesting its utility for selection of Rht18 in wheat improvement programs. Rht18 associated alleles of TdGA2ox-A9, IAW4371 and IAW7940 were absent in most of the tall Indian local durum wheat and bread wheat, hence could be used to transfer Rht18 to bread wheat and local durum wheat. SSR marker barc3 showed high recombination frequency with Rht18, though it showed allele unique to Icaro. Since semidwarf wheat with GA-sensitive dwarfing genes are useful in dry environments owing to their longer coleoptile, better emergence and seedling vigor, Rht18 may provide a useful alternative to widely used GA-insensitive dwarfing genes under dry environments.     

Effects of dwarfing genes on the genetic background of wheat varieties in southern Ukraine

The effects of the Rht8c, Rht-B1b, Rht-B1e, and Rht-D1b genes on wheat height have been investigated. Variations in these effects are significantly modified by the genetic background and year conditions. A combination of the Rht8c, Rht-B1a, Rht-D1b, and Ppd-D1a alleles is the most advantageous for the conditions of southern Ukraine, since it is associated with optimal plant height under contrasting conditions within different years. The genotypes of some varieties were shown to include gene(s) that were unidentifiable by the molecular markers and significantly decreased plant height.     

Influence of the gibberellin-sensitive Rht8 dwarfing gene on leaf epidermal cell dimensions and early vigour in wheat (Triticum aestivum L.)

BACKGROUND AND AIMS: The gibberellin-insensitive Rht-B1b and Rht-D1b dwarfing genes are known to reduce the size of cells in culms, leaves and coleoptiles of wheat. Resulting leaf area development of gibberellin-insensitive wheats is poor compared to standard height (Rht-B1a and Rht-D1a) genotypes. Alternative dwarfing genes to Rht-B1b and Rht-D1b are available that reduce plant height, such as the gibberellin-responsive Rht8 gene. This study aims to investigate if Rht8 has a similar dwarfing effect on the size of leaf cells to reduce leaf area. METHODS: The effect of Rht8 on cell size and leaf area was assessed in four types of epidermal cells (interstomatal, long, sister and bulliform) measured on leaf 2 of standard height (rht8) and semi-dwarf (Rht8) doubled-haploid lines (DHLs). The DHLs were derived from a cross between very vigorous, standard height (rht8) ('Vigour18') and less vigorous, semi-dwarf (Rht8) ('Chuan-Mai 18') parents. KEY RESULTS: Large differences were observed in seedling vigour between the parents, where 'Vigour18' had a much greater plant leaf area than 'Chuan-Mai 18'. Accordingly, 'Vigour18' had on average longer, wider and more epidermal cells and cell files than 'Chuan-Mai 18'. Although there was correspondingly large genotypic variation among DHLs for these traits, the contrast between semi-dwarf Rht8 and tall rht8 DHLs revealed no difference in the size of leaf 2 or average cell characteristics. Hence, these traits were independent of plant height and therefore Rht8 in the DHLs. Correlations for leaf and average cell size across DHLs revealed a strong and positive relationship between leaf width and cell files, while the relationships between leaf and cell width, and leaf and cell length were not statistically different. The relative contribution of the four cell types (long, sister, interstomatal and bulliform) to leaf size in the parents, comparative controls and DHLs is discussed. CONCLUSIONS: Despite a large range in early vigour among the DHLs, none of the DHLs attained the leaf area or epidermal cell size and numbers of the vigorous rht8 parent. Nonetheless, the potential exists to increase the early vigour of semi-dwarf wheats by using GA-sensitive dwarfing genes such as Rht8.     

Exogenous GA3 Application Can Compensate the Morphogenetic Effects of the GA-Responsive Dwarfing Gene Rht12 in Bread Wheat

The most common dwarfing genes in wheat, Rht-B1b and Rht-D1b, classified as gibberellin-insensitive (GAI) dwarfing genes due to their reduced response to exogenous GA, have been verified as encoding negative regulators of gibberellin signaling. In contrast, the response of gibberellin-responsive (GAR) dwarfing genes, such as Rht12, to exogenous GA is still unclear and the role of them, if any, in GA biosynthesis or signaling is unknown. The responses of Rht12 to exogenous GA₃ were investigated on seedling vigour, spike phenological development, plant height and other agronomic traits, using F_2∶3 and F_3∶4 lines derived from a cross between Ningchun45 and Karcagi-12 in three experiments. The application of exogenous GA₃ significantly increased coleoptile length and seedling leaf 1 length and area. While there was no significant difference between the dwarf and the tall lines at the seedling stage in the responsiveness to GA₃, plant height was significantly increased, by 41 cm (53%) averaged across the three experiments, in the GA₃-treated Rht12 dwarf lines. Plant height of the tall lines was not affected significantly by GA₃ treatment (<10 cm increased). Plant biomass and seed size of the GA₃-treated dwarf lines was significantly increased compared with untreated dwarf plants while there was no such difference in the tall lines. GA₃-treated Rht12 dwarf plants with the dominant Vrn-B1 developed faster than untreated plants and reached double ridge stage 57 days, 11 days and 50 days earlier and finally flowered earlier by almost 7 days while the GA₃-treated tall lines flowering only 1–2 days earlier than the untreated tall lines. Thus, it is clear that exogenous GA₃ can break the masking effect of Rht12 on Vrn-B1 and also restore other characters of Rht12 to normal. It suggested that Rht12 mutants may be deficient in GA biosynthesis rather than in GA signal transduction like the GA-insensitive dwarfs.     

Molecular genetic mapping of dwarfing genes in oat

 Restriction fragment length polymorphism (RFLP) analysis provides a valuable tool for characterizing and understanding relationships among genes for useful traits in crop species, particularly in ones with complex genomes such as the hexaploid cultivated oat Avena sativa L. (2n=6x=42). Using Bulked Segregant Analysis (BSA) and F₂ RFLP linkage data, we mapped three dominant oat dwarfing loci to different regions of the oat genome. Dw6, in oat line OT207, is 3.3±1.3 cM from the Xumn145B locus, which has not been placed on the hexaploid oat linkage map. Dw7, in line NC2469-3, is 4.3±2.3 cM from Xcdo1437B and 33±4.1 cM from Xcdo708B. This places Dw7 to linkage group 22. Dw8, in the Japanese lines AV17/3/10 and AV18/2/4, mapped 4.9±2.2 cM from Xcdo1319A in an AV17/3/10בKanota’ F₂ population and 6.6±2.6 cM from it in an AV18/2/4בKanota’ population. This places Dw8 to linkage group 3. Aneuploid analysis of markers linked to the dwarfing genes located Dw6 on the smallest oat chromosome (chromosome 18) and Dw7 on the longest satellited chromosome (chromosome 19). The RFLP markers closely linked to the three dwarfing genes identify distinct regions of the oat genome that contribute to plant height and they should be useful in characterizing new genetic sources of dwarfness in oat. Received: 8 May 1997 / Accepted: 20 May 1997     

Pleiotropic effects of gibberellin-sensitive and giberrellin-insensitive dwarfing genes in bread wheat of the southern step region of the Black Sea

Investigation of the pleiotropic effects of GA-sensitive (Rht8) and GA-insensitive (Rht-B1 and Rht-D1) winter bread wheat dwarfing genes and the gene that determines the response of plants to photoperiod—Ppd-D1—were carried out for 3 years in the southern step region of the Black Sea bank on five different genetic backgrounds. It is shown that, in addition to direct effects on plant height, GA-sensitive and GA-insensitive dwarfing genes have pleiotropic effects on all studied traits except the number of fertile spikelets. Presence of the dwarfing genes in the genotype of tall forms led to the decrease of stem and ear length, and, at the same time, to the increase of ear density. The number of spikelets per spike decreased due to sterile spikelets, whereas the number of fertile spikelets did not change. There was a significant increase in the number of grains per ear as a result of increasing of spikelets in ears. The number and weight of grains did not decrease, even though the plants were characterized by a smaller number of productive tillers. The presence of Rht8x allele on genetic background of variety Stepnyak resulted in a significant decrease of plants productivity. However, in combination with Ppd-D1a allele, plants with Rht8x increased the potential productivity and surpassed the parental form (Rht8x Ppd-D1a). The presence of Rht-Ble allele resulted in reduction of weight of kernels from the main ear and 1000-kernels weight, increase of l/h, and left the number of seeds per spikelet stable in comparison with Rht8x.     

Mapping of three self-fertility mutations in rye (Secale cereale L.) using RFLP, isozyme and morphological markers

 Three mutations determining self-fertility at the S, Z and S5 self-incompatibility loci on chromosomes 1R, 2R and 5R of rye, respectively, were mapped using three different F₂ populations. There was a close linkage of one isozyme and four RFLP markers, and no recombinant plants were detected. These markers are Prx7, Xiag249 and Xpsr634 for the S locus (1R), Xbcd266 for the Z locus (2R) and Xpsr100 for the S5 locus (5R). Linkage data for markers associated to the self-fertility mutations at the S, Z and S5 loci were calculated and compared with genetic maps computed by MAPMAKER multipoint analysis. Received: 8 October 1997 / Acepted: 26 November 1997     

Semi-dwarfing Rht-B1 and Rht-D1 loci of wheat differ significantly in their influence on resistance to Fusarium head blight

Fusarium head blight (FHB) is an important disease of wheat worldwide. Soissons is one of the most resistant varieties grown in UK. The current study was undertaken to identify QTL for FHB resistance in Soissons and to determine whether the semi-dwarfing alleles Rht-B1b and Rht-D1b have a similar influence on susceptibility to FHB. A Soissons (Rht-B1b; Rht-D1a) × Orvantis (Rht-B1a; Rht-D1b) doubled haploid (DH) population was assessed for FHB resistance in three trials. Soissons contributed a single, stable major FHB QTL linked to the Rht-D1 locus. In contrast, the Rht-B1b allele (contributed by Soissons) conferred no negative effect on FHB resistance, even conferring a very minor positive effect in one trial. The influence of the Rht-B1b and Rht-D1b alleles on FHB resistance was further investigated using both Mercia and Maris Huntsman near-isogenic lines. Under high disease pressure both Rht-B1b and Rht-D1b significantly decreased Type 1 resistance (resistance to initial infection). However, whilst Rht-D1b has no effect on Type 2 resistance (resistance to spread of the fungus within the spike), Rht-B1b significantly increased Type 2 resistance. Our study demonstrates that the choice of semi-dwarfing gene used in plant breeding programmes may be a significant consideration where resistance to FHB is an important breeding target.     

Molecular mapping of the photoperiod response gene ea7 in barley

 The gene ea ₇ determining photoperiod insensitivity under short day length was mapped on the short arm of chromosome 6H near the centromere. The gene was linked to the two flanking markers Xmwg2264 and Xmwg916 by 6.7 and 13.0 cM, respectively. Compared to Ppd-H1 (chromosome 2H) and Ppd-H2 (chromosome 1H), ea ₇ determines the strongest effect on flowering time with 55 and 18 days difference compared to photoperiod sensitive genotypes grown under short and long photoperiods, respectively. Allelic and homoeologous relationships to major genes and quantitative trait loci controlling flowering time in barley and wheat are discussed. Received: 10 March 1998 / Accepted: 7 April 1998     

Genetic analysis of the dwarfing gene Rht8 in wheat. Part II. The distribution and adaptive significance of allelic variants at the Rht8 locus of wheat as revealed by microsatellite screening

 Wheat microsatellite WMS 261 whose 192-bp allele has been shown to be diagnostic for the commercially important dwarfing gene Rht8 was used to screen over 100 wheat varieties to determine the worldwide spread of Rht8. The results showed Rht8 to be widespread in southern European wheats and to be present in many central European wheats including the Russian varieties ‘Avrora’, ‘Bezostaya’ and ‘Kavkaz’. Rht8 appears to be of importance to South European wheats as alternative giberellic acid (GA)-insensitive dwarfing genes do not appear to be adapted to this environment. The very successful semi-dwarf varieties bred by CIMMYT, Mexico, for distribution worldwide have been thought to carry Rht8 combined with GA-insensitive dwarfing genes. Additional height reduction would have been obtained from pleiotropic effects of the photoperiod-response gene Ppd1 that is essential to the adaptability of varieties bred for growing under short-winter days in tropical and sub-tropical areas. The microsatellite analysis showed that CIMMYT wheats lack Rht8 and carry a WMS 261 allelic variant of 165 bp that has been associated with promoting height. This presumably has adaptive significance in partly counteracting the effects of other dwarfing genes and preventing the plants being too short. Most UK, German and French wheats carry an allelic variant at the WMS 261 locus with 174 bp. This could be selected because of linkage with the recessive photoperiod-sensitive ppd1 allele that is thought to offer adaptive significance northern European wheats. Received: 17 October 1997 / Accepted: 12 November 1997     

Novel Genetic Variants of GA-Insensitive Rht-1 Genes in Hexaploid Wheat and Their Potential Agronomic Value

This study has found numerous novel genetic variants of GA-insensitive dwarfing genes with potential agricultural value for crop improvement. The cultivar, Spica is a tall genotype and possesses the wild-type genes of Rht-A1a, Rht-B1a and Rht-D1a. The cultivar Quarrion possesses a null mutant in the DELLA motif in each of the 3 genomes. This is a first report of a null mutant of Rht-A1. In addition, novel null mutants which differ from reported null alleles of Rht-B1b, Rht-B1e and Rht-D1b have been found in Quarrion, Carnamah and Whistler. The accession, Aus1408 has an allele of Rht-B1 with a mutation in the conserved ‘TVHYNP’ N-terminal signal binding domain with possible implications on its sensitivity to GA. Mutations in the conserved C-terminal GRAS domain of Rht-A1 alleles with possible effects on expression have been found in WW1842, Quarrion and Drysdale. Genetic variants with putative spliceosomal introns in the GRAS domain have been found in all accessions except Spica. Genome-specific cis-sequences about 124 bp upstream of the start codon of the Rht-1 gene have been identified for each of the three genomes.