The Effects of Dwarfing Genes Rht1 and Rht2 on Cellular Dimensions and Rate of Leaf Elongation in Wheat

The gibberellin insensitivity genes, Rht1 and Rht2, reducedepidermal cell lengths in leaves of isogenic lines of field-and laboratory-grown wheat (Triticum aestivum L.). Rht dosagesof zero (wild type), two (semi-dwarf) and four alleles (doubledwarf) had a linear negative effect on cell length in flag leavesof field-grown plants, and in the sheaths and blades of leafnumber 1 in laboratory grown plants. Decrease in cell length,rather than reduced cell number, accounted for most to all ofthe reduction in blade and sheath length. In sheaths, cell widthincreased with Rht dosage, but not sufficiently to compensatefor decreased length in determining average projected surfacearea. Rates of extension of leaf number 1 in laboratory-grownplants were negatively and linearly correlated with Rht dosage.Maximal growth rate was maintained longer in wild type thanin double dwarf, but the total duration of measurable extensionin leaf number 1 was not affected by Rht dosage. Cell size, elongation, Rht, wheat, Triticum aestivum L     

A modern Green Revolution gene for reduced height in wheat

Increases in the yield of wheat during the Green Revolution of the late 20^th century were achieved through the introduction of Reduced height (Rht) dwarfing genes. The Rht‐B1 and Rht‐D1 loci ensured short stature by limiting the response to the growth‐promoting hormone gibberellin, and are now widespread through international breeding programs. Despite this advantage, interference with the plant's response to gibberellin also triggers adverse effects for a range of important agronomic traits, and consequently modern Green Revolution genes are urgently required. In this study, we revisited the genetic control of wheat height using an association mapping approach and a large panel of 1110 worldwide winter wheat cultivars. This led to the identification of a major Rht locus on chromosome 6A, Rht24, which substantially reduces plant height alone as well as in combination with Rht‐1b alleles. Remarkably, behind Rht‐D1, Rht24 was the second most important locus for reduced height, explaining 15.0% of the genotypic variance and exerting an allele substitution effect of –8.8 cm. Unlike the two Rht‐1b alleles, plants carrying Rht24 remain sensitive to gibberellic acid treatment. Rht24 appears in breeding programs from all countries of origin investigated, with increased frequency over the last decades, indicating that wheat breeders have actively selected for this locus. Taken together, this study reveals Rht24 as an important Rht gene of commercial relevance in worldwide wheat breeding.     

Identification and characterization of <Emphasis Type="Italic">Rht25</Emphasis>, a locus on chromosome arm 6AS affecting wheat plant height,heading time,and spike development

Key message

This study identified Rht25, a new plant height locus on wheat chromosome arm 6AS, and characterized its pleiotropic effects on important agronomic traits.

Abstract

Understanding genes regulating wheat plant height is important to optimize harvest index and maximize grain yield. In modern wheat varieties grown under high-input conditions, the gibberellin-insensitive semi-dwarfing alleles Rht-B1b and Rht-D1b have been used extensively to confer lodging tolerance and improve harvest index. However, negative pleiotropic effects of these alleles (e.g., poor seedling emergence and reduced biomass) can cause yield losses in hot and dry environments. As part of current efforts to diversify the dwarfing alleles used in wheat breeding, we identified a quantitative trait locus (QHt.ucw-6AS) affecting plant height in the proximal region of chromosome arm 6AS (<?0.4 cM from the centromere). Using a large segregating population (~?2800 gametes) and extensive progeny tests (70–93 plants per recombinant family), we mapped QHt.ucw-6AS as a Mendelian locus to a 0.2 cM interval (144.0–148.3 Mb, IWGSC Ref Seq v1.0) and show that it is different from Rht18. QHt.ucw-6AS is officially designated as Rht25, with Rht25a representing the height-increasing allele and Rht25b the dwarfing allele. The average dwarfing effect of Rht25b was found to be approximately half of the effect observed for Rht-B1b and Rht-D1b, and the effect is greater in the presence of the height-increasing Rht-B1a and Rht-D1a alleles than in the presence of the dwarfing alleles. Rht25b is gibberellin-sensitive and shows significant pleiotropic effects on coleoptile length, heading date, spike length, spikelet number, spikelet density, and grain weight. Rht25 represents a new alternative dwarfing locus that should be evaluated for its potential to improve wheat yield in different environments.

     

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.     

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.     

Effects of Rht-dosage on the breaking strength of wheat seedling leaves

We report the effect of the reduced-height alleles Rht1 and Rht2 on the breaking strength and breaking stress of the first foliage leaves of wheat seedlings. Transverse size variables such as blade transverse area and blade width were positively related to the breaking strength and breaking stress of leaves, as was the total wall area of the principal fiber strands. A negative correlation was found for the Rht-dosage with respect to the breaking strength. Because Rht plants had greater transverse dimensions and equal amounts of principal fiber strands compared to wild type, we posit that Rht may affect the mechanical quality of leaf tissues per se. Rht may serve as an example of a simple genetic system that profoundly affects plant morphology, development, and biomechanics.     

Effects of NORIN 10 and Tom Thumb Dwarfing Genes on Morphology, Physiology and Abscisic Acid Production in Wheat

The pleiotropic effects of three genetically related dwarfinggenes were investigated in near-isogenic lines of wheat. TheNORIN 10 semi-dwarfing alleles, Rht 1 and Rht 2, and the TomThumb allele, Rht 3, were assessed for effects on some vegetativemorphological and physiological characters. The Rht allelesaffected leaf size with a resultant decrease in leaf area ofthe whole plant. Rht 3, which had the most marked effects, reducedleaf area in young plants by as much as 30 per cent. Althoughflag leaf dimensions and stomatal distributions of the flagleaf were altered, the gene had no effect on its area, stomatalconductance or net CO₂ exchange rate. Comparisons of Rht andtall plants revealed no differences in the abscisic acid (ABA)levels of either turgid or partially dehydrated leaves. Triticum aestivum L., wheat, dwarfing genes, leaf structure, abscisic acid, stomatal conductance, CO₂, exchange, relative growth rate     

The effects of ‘gibberellin-insensitive’ dwarfing alleles in wheat on grain weight and protein content

Summary Three series of near-isogenic wheat lines differing in dwarfing alleles, in the varietal backgrounds of Maris Huntsman, Maris Widgeon and Bersee, and the F₂ grain on intravarietal F₁ hybrids, produced with a chemical hybridising agent, were examined for grain size and protein content. Individual F₂ grains from Rht1/rht, Rht2/rht and Rht3/rht F₁ spikes were classified for Rht genotype by assaying embryo half grains in a gibberellic acid seedling response test, while the remaining half was used for protein determination. Mean grain weight and protein percentage were lower in all homozygous isogenic lines and the Rht/rht F₁ hybrids than in the respective tall lines, in an allele dose-dependent manner. In all the hybrids, the Rht genotype of individual F₂ grains, which segregated within the spikes of F₁ plants, had no significant effects on grain weight or protein. Consequently, the pleiotropic effects of the Rht alleles on these yield and quality components must be attributed to their presence in maternal plant tissues rather than in the endosperm or embryo tissues of individual grains.     

GA-Responsive Dwarfing Gene Rht12 Affects the Developmental and Agronomic Traits in Common Bread Wheat

Opportunities exist for replacing reduced height (Rht) genes Rht-B1b and Rht-D1b with alternative dwarfing genes, such as the gibberellin-responsive gene Rht12, for bread wheat improvement. However, a comprehensive understanding of the effects and mode of action of Rht12 is lacking. In the present study, the effects of Rht12 were characterized by analyzing its effects on seeding vigour, seedling roots, leaf and stem morphology, spike development and carbohydrate assimilation and distribution. This was carried out in the four genotypes of F_2:3 lines derived from a cross between Ningchun45 and Karcagi (12) in two experiments of autumn sowing and spring sowing. Rht12 significantly decreased stem length (43%∼48% for peduncle) and leaf length (25%∼30% for flag leaf) while the thickness of the internode walls and width of the leaves were increased. Though the final plant stature was shortened (40%) by Rht12, the seedling vigour, especially coleoptile length and root traits at the seedling stage, were not affected adversely. Rht12 elongated the duration of the spike development phase, improved the proportion of spike dry weight at anthesis and significantly increased floret fertility (14%) in the autumn sowing experiment. However, Rht12 delayed anthesis date by around 5 days and even the dominant Vrn-B1 allele could not compensate this negative effect. Additionally, grain size was reduced with the ability to support spike development after anthesis decreased in Rht12 lines. Finally, grain yield was similar between the dwarf and tall lines in the autumn sowing experiment. Thus, Rht12 could substantially reduce plant height without altering seeding vigour and significantly increase spikelet fertility in the favourable autumn sowing environment. The successful utilization of Rht12 in breeding programs will require careful selection since it might delay ear emergence. Nonetheless, the potential exists for wheat improvement by using Rht12.     

Effect of Temperature on Gibberellin (GA) Responsiveness and on Endogenous GA(1) Content of Tall and Dwarf Wheat Genotypes

Near-isogenic wheat (Triticum aestivum L.) lines differing in height-reducing (Rht) alleles were used to investigate the effects of temperature on endogenous gibberellin (GA) levels and seedling growth response to applied GA₃. Sheath and lamina lengths of the first leaf were measured in GA treated and control seedlings, grown at 11, 18, and 25°C, of six Rht genotypes in each of two varietal backgrounds, cv Maris Huntsman and cv April Bearded. Endogenous GA₁ levels in the leaf extension zone of untreated seedlings were determined by gas chromatography-mass spectrometry with a deuterated internal standard in the six Maris Huntsman Rht lines grown at 10 and 25°C. Higher temperature increased leaf length considerably in the tall genotype, less so in the Rht1 and Rht2 genotypes, and had no consistent effect on the Rht1+2, Rht3 and Rht2+3 genotypes. In all genotypes, endogenous GA₁ was higher at 25°C than at 10°C. At 10°C the endogenous GA₁ was at a similar level in all the genotypes (except Rht2+3). At 25°C it increased 1.6-fold in the tall genotype, 3-fold in Rht1 and Rht2, 6-fold in Rht3, and 9-fold in Rht1+2. Likewise, the genotypic differences in leaf length were very conspicuous at 25°C, but were only slight and often unsignificant at 11°C. The response of leaf length to applied GA₃ in the Rht1, Rht2, and Rht1+2 genotypes increased significantly with lowering of temperature. These results suggest the possibility that the temperature effect on leaf elongation is mediated through its effect on the level of endogenous GA₁ and that leaf elongation response to endogenous or applied GAs is restricted by the upper limits set by the different Rht alleles.     

Use of a large multiparent wheat mapping population in genomic dissection of coleoptile and seedling growth

Identification of alleles towards the selection for improved seedling vigour is a key objective of many wheat breeding programmes. A multiparent advanced generation intercross (MAGIC) population developed from four commercial spring wheat cultivars (cvv. Baxter, Chara, Westonia and Yitpi) and containing ca. 1000 F₂‐derived, F_6:7 RILs was assessed at two contrasting soil temperatures (12 and 20 °C) for shoot length and coleoptile characteristics length and thickness. Narrow‐sense heritabilities were high for coleoptile and shoot length (h² = 0.68–0.70), indicating a strong genetic basis for the differences among progeny. Genotypic variation was large, and distributions of genotype means were approximately Gaussian with evidence for transgressive segregation for all traits. A number of significant QTL were identified for all early growth traits, and these were commonly repeatable across the different soil temperatures. The largest negative effects on coleoptile lengths were associated with Rht‐B1b (?8.2%) and Rht‐D1b (?10.9%) dwarfing genes varying in the population. Reduction in coleoptile length with either gene was particularly large at the warmer soil temperature. Other large QTL for coleoptile length were identified on chromosomes 1A, 2B, 4A, 5A and 6B, but these were relatively smaller than allelic effects at the Rht‐B1 and Rht‐D1 loci. A large coleoptile length effect allele (a = 5.3 mm at 12 °C) was identified on chromosome 1AS despite the relatively shorter coleoptile length of the donor Yitpi. Strong, positive genetic correlations for coleoptile and shoot lengths (r_g = 0.85–0.90) support the co‐location of QTL for these traits and suggest a common physiological basis for both. The multiparent population has enabled the identification of promising shoot and coleoptile QTL despite the potential for the confounding of large effect dwarfing gene alleles present in the commercial parents. The incidence of these alleles in commercial wheat breeding programmes should facilitate their ready implementation in selection of varieties with improved establishment and early growth.     

<Emphasis Type="Italic">Rht24</Emphasis> reduces height in the winter wheat population ‘Solitär × Bussard’ without adverse effects on Fusarium head blight infection

Key message

The dwarfing gene Rht24 on chromosome 6A acts in the wheat population ‘Solitär × Bussard’, considerably reducing plant height without increasing Fusarium head blight severity and delaying heading stage.

Abstract

The introduction of the Reduced height (Rht)-B1 and Rht-D1 semi-dwarfing genes led to remarkable increases in wheat yields during the Green Revolution. However, their utilization also brings about some unwanted characteristics, including the increased susceptibility to Fusarium head blight. Thus, Rht loci that hold the potential to reduce plant height in wheat without concomitantly increasing Fusarium head blight (FHB) susceptibility are urgently required. The biparental population ‘Solitär × Bussard’ fixed for the Rht-1 wild-type alleles, but segregating for the recently described gibberellic acid (GA)-sensitive Rht24 gene, was analyzed to identify quantitative trait loci (QTL) for FHB severity, plant height, and heading date and to evaluate the effect of the Rht24 locus on these traits. The most prominent QTL was Rht24 on chromosome 6A explaining 51% of genotypic variation for plant height and exerting an additive effect of ? 4.80 cm. For FHB severity three QTL were detected, whereas five and six QTL were found for plant height and heading date, respectively. No FHB resistance QTL was co-localized with QTL for plant height. Unlike the Rht-1 semi-dwarfing alleles, Rht24b did not significantly affect FHB severity. This demonstrates that the choice of semi-dwarfing genes used in plant breeding programs is of utmost consideration where resistance to FHB is an important breeding target.

     

Genotypic variation in the responsiveness to GA3 within tall (rht1) and semi-dwarf (Rht1) spring wheat

The effect of GA₃ on coleoptile-and first leaf elongation of tall (rht1) and semi-dwarf (Rht1) nearly-isogenic genotypes, within each of 25 random F₉ wheat families, was determined on seedlings grown in a growth room at 18 °C. Conspicuous and very significant inter-family variation in the response of the first leaf to GA₃ application was found in both the rht1 and Rht1 genotypes. The magnitudes of the response of the different families within genotypes to GA₃ were not related to the leaf length of their untreated seedlings. It is suggested that, under given environmental conditions, background genotypic effects, inducing inter-family variation in responsiveness to GA₃, regulate the elongation growth up to the limits set by the Rht alleles.     

Allele Distribution at Locus WMS261Marking the Dwarfing Gene Rht8in Common Wheat Cultivars of Southern Ukraine

The use of codominant microsatellite molecular markers allows one to study the inheritance and distribution of alleles linked to important agronomic characters. A microsatellite locus WMS261tightly linked to a dwarfing geneRht8was analyzed in wheat cultivars and selection material of the Institute of Plant Breeding and Genetics. PCR screening of common wheat cultivars produced in the southern Ukraine showed the prevalence of a 192-bp allele at locus WMS261that indicates adaptive significance of a corresponding allele of the Rht8gene in the southern regions.     

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.     

The Net Orientation of Wall Microfibrils in the Outer Periclinal Epidermal Walls of Seedling Leaves of Wheat

Use of the dichroic stain chlor-zinc-iodine revealed that thenet orientation of cellulose wall microfibrils in the outerparadermal wall of the epidermis of seedling wheat leaves isprincipally transverse in the extension zone. The net orientationof microfibrils changes abruptly to principally longitudinalat the end of cell elongation. The net angle of orientationof microfibrils in the extension zone was not a function ofRht-dosage (number of dwarfing alleles), and neither leaf extensionrate nor estimated maximum relative elemental rate of elongationwere functions of microfibril orientation. The results indicatethat elongation rates are not regulated by the net angle oforientation of microfibrils and support the concept that leafextension rate is regulated by the length of the extension zone.Copyright1995, 1999 Academic Press Cellulose wall microfibrils, extension zone, elongation, Rht, wheat, Triticum aestivum L     

Gibberellic Acid Sensitivity Determines the Length of the Extension Zone in Wheat Leaves

To test the hypothesis that gibberellic acid (GA) sensitivityaffects the length of the extension zone (LEZ) of leaf No. 1of wheat seedlings, we performed a gene dosage experiment usingRht dwarfing genes that condition GA insensitivity. We utilizednearly isogenic lines, at Rht-dosage levels of 0, 2 and 4 alleles.Anatomical markers (distances between successive stomates) wereused to infer the distribution of growth along the axis of theleaf. Interstomatal distance (ISD) and LEZ were inverse linearfunctions of Rht-dosage. The number of stomates matured perhour was independent of Rht-dosage. The relationship betweenISD and distance along the axis within the extension zone (EZ)was indistinguishable from linear. Rht-dosage did not affectthe slope of the regression of ISD against distance along theEZ. A-REST (AR; ancymidol, a potent GA synthesis inhibitor)reduced LEZ. Wild type was more sensitive to AR than doubledwarf. AR affected growth of leaf No. 1 more than length ofthe coleoptile, regardless of Rht-dosage. AR-dosage affectedcell division, whereas Rht-dosage did not. Extension zone, elongation, gibberellic acid, Rht, wheat, Triticum aesiivum L.     

Biochemical evaluation of the morphogenetic potential of wheat callus cells in vitro

Morphogenetic processes in immature embryos of wheat (Triticum aestivum L.) were investigated using a molecular marker for meristematic cells of cereals (proliferative antigen of meristem initial cells, PAIC). The used genetic model comprised a series of almost isogenic lines of wheat alternative by the alleles of dwarfing genes (RhtB1c, RhtB1b, and Rht14) and the original cv. Saratovskaya 29. The Rht genetic system was found to affect the level of PAIC in the wheat callus cells during callus formation and subsequent regeneration. We found differences in the level of PAIC at certain stages of culturing and a general trend of its content in the course of callus formation and secondary differentiation in the callus tissue. Possible role of PAIC as a marker of meristematic centers within the callus tissue and the prospect of its use in breeding as an additional criterion for estimation of morphogenetic ability of the newly produced lines of wheat are discussed.     

Effect of the Rht3 dwarfing gene on dynamics of cell extension in wheat leaves, and its modification by gibberellic acid and paclobutrazol

The second leaf of wheat was used as a model system to examinethe effects of the Rht3 dwarfing gene on leaf growth. Comparedto the rht3 wild type, the Rht3allele decreased final leaf length,surface area and dry mass by reducing the maximum growth rates,but without affecting growth duration. Gibberellic acid (GA₃)increased final leaf length and maximum growth rate in the rht3wild type, but was without effect on the Rht3 mutant, whichis generally regarded as being non-responsive to gibberellin(GA). Paclobutrazol, an inhibitor of GA biosynthesis, decreasedfinal leaf length and maximum growth rate in the rht3 wild typeto values similar to those in the untreated Rht3 mutant. NeitherGA₃ nor paclobutrazol affected the duration of leaf growth.The decrease in leaf length was produced by reduction of celllength rather than cell number. The maximum relative elementalgrowth rate (REGR) for cell extension was essentially the samein all treatments, as was the time between the cells leavingthe meristem and achieving maximum extension rate. The differencesbetween the genotypes and treatments were all almost entirelydue to differences in the time taken from the attainment ofmaximum REGR of cell extension to the cessation of extension.This was reflected in the length of the extension zone, whichwas approximately 6–8 per cent of final leaf length. Theeffects of the Rht3 allele, GA₃ and paclobutrazol all appearto be on the processes which promote the cessation of cell elongation. Key words: Cell extension, gibberellin, leaf growth, Rht3 gene, Triticum, wheat