Identification and characterization of quantitative trait loci for root elongation by using introgression lines with genetic background of Indica-type rice variety IR64

The root is the sole organ taking up water and nutrients from soils. Hence, root system architecture (RSA) is important for enhancing high-level and stable rice (Oryza sativa L.) production. However, the genetic improvement of RSA has received less attention than yield and yield components. Here, we aimed to identify and characterize quantitative trait loci (QTLs) for RSA by determining the maximum root length (MRL) of seedlings grown hydroponically under various concentrations of NH₄ ⁺. We used a total of 280 introgression lines (ILs) with an Indica-type variety IR64 genetic background, consisting of ten sibling ILs groups, to detect the QTLs. Greater variation of MRL was found in three sibling ILs groups. In total, five QTLs were detected by single marker analyses: two each on chromosomes 5 and 6 and one on chromosome 7. Among them, the most effective QTL was detected on a segment derived from IR69093-41-2-3-2 (YP5), which was localized to the long-arm of chromosome 6. The QTL, designated as qRL6.4-YP5, concerned in root elongation. MRL and total root length of a near-isogenic line (NIL) for qRL6.4-YP5 were significantly (15.2–24.6 %) higher than those of IR64 over a wide range of NH₄ ⁺ concentrations. Root number and weight of the NIL were the same as those of IR64. These results indicated that qRL6.4-YP5 was a constitutive QTL for root length in response to change in nitrogen concentrations. To enhance yield potential by improving RSA, qRL6.4-YP5 might help to improve root development in rice molecular breeding programs with marker-assisted selection.     

Accumulation of additive effects generates a strong photoperiod sensitivity in the extremely late-heading rice cultivar ‘Nona Bokra’

Many rice cultivars that originated from lower-latitude regions exhibit a strong photoperiod sensitivity (PS) and show extremely late heading under long-day conditions. Under natural day-length conditions during the cropping season in Japan, the indica rice cultivar ‘Nona Bokra’ from India showed extremely late heading (202 days to heading) compared to the japonica cultivar ‘Koshihikari’ (105 days), from Japan. To elucidate the genetic factors associated with such extremely late heading, we performed quantitative trait locus (QTL) analyses of heading date using an F₂ population and seven advanced backcross progeny (one BC₁F₂ and six BC₂F₂) derived from a cross between ‘Nona Bokra’ and ‘Koshihikari’. The analyses revealed 12 QTLs on seven chromosomes. The ‘Nona Bokra’ alleles of all QTLs contributed to an increase in heading date. Digenic interactions were rarely observed between QTLs. Based on the genetic parameters of the QTLs, such as additive effects and percentage of phenotypic variance explained, these 12 QTLs are likely generate a large proportion of the phenotypic variation observed in the heading dates between ‘Nona Bokra’ and ‘Koshihikari’. Comparison of chromosomal locations between heading date QTLs detected in this study and QTLs previously identified in ‘Nipponbare’ × ‘Kasalath’ populations revealed that eight of the heading date QTLs were recognized nearby the Hd1, Hd2, Hd3a, Hd4, Hd5, Hd6, Hd9, and Hd13. These results suggest that the strong PS in ‘Nona Bokra’ was generated mainly by the accumulation of additive effects of particular alleles at previously identified QTLs.     

Improving rice yield and quality by QTL pyramiding

To facilitate marker-assisted transfer of desirable genes for improvement of yield traits, we used a set of backcross recombinant inbred lines (BRIL) derived from two elite parental lines, ‘Zhenshan97’ and ‘93-11’, to resolve a quantitative trait loci (QTL) cluster for heading date and yield-related traits in rice. Four main-effect QTL (qHD6.1, qHD6.2, qHD7, and qHD8) and four epistatic QTL affecting heading date in the BRIL were detected in two experimental trials. The major QTL (qHD8) was confirmed in three heterogeneous inbred families (HIF) that segregated for this target region, and narrowed down to a 20-kb segment in a large HIF-derived population. qHD8 was found to interact with qHD7 and had a pleiotropic effect responsible for heading date and yield components. To test usability of the identified QTL in rice improvement, we further developed near-isogenic lines (NIL) containing one or more target genes by marker-assisted transfer of ‘93-11’ alleles at qHD8, qHD7, and qHD6.1, and the GS3 gene for grain size into ‘Zhenshan97’. The pyramid line NIL(qHD8 + GS3) had higher yield potential, longer grains, and a more suitable heading date than ‘Zhenshan97’. Comparison of the NIL showed existence of epistasis between alleles at different loci and background effect on qHD8, which are very important for pyramiding of desirable alleles at the target QTL. These results will be particularly useful not only to understand the genetic basis of yield-related traits but also to improve the efficiency of marker-assisted selection for favorable loci in rice breeding programs.     

Mapping quantitative trait loci associated with regeneration ability of seed callus in rice, Oryza sativa L.

 Quantitative trait loci (QTL) controlling the regeneration ability of rice seed callus were detected using 245 RFLP markers and 98 BC₁F₅ lines derived from two varieties, ‘Nipponbare’ and ‘Kasalath’. Regeneration ability was evaluated by two indices: average number of regenerated shoots per callus (NRS) and regeneration rate (RR). The BC₁F₅ lines showed continuous segregation for both indices. Five putative QTL for NRS (tentatively named qRg1, qRg2, qRg4a, qRg4b and qRg4c) located on chromosomes 1, 2 and 4 were detected. Digenic interaction among these detected QTL was not significant (P<0.01). Among the five QTL detected, four ‘Kasalath’ alleles and one ‘Nipponbare’ allele increased NRS. According to an estimate based on the nearest marker loci, the five QTL accounted for 38.5% of the total phenotypic variation of the BC₁F₅ lines. For RR, four putative QTL were detected on chromosomes 2 and 4, and all of these were in the same chromosomal regions as the NRS QTL. The four RR QTL accounted for 32.6% of the total phenotypic variation. Received: 7 November 1996 / Accepted: 25 April 1997     

Genetic analysis of root elongation induced by phosphorus deficiency in rice (Oryza sativa L.): fine QTL mapping and multivariate analysis of related traits

Root elongation induced by phosphorus deficiency has been reported as one of the adaptive mechanisms in plants. Genetic differences were found in rice for the root elongation under phosphorus deficiency (REP), for which a distinct quantitative trait locus (QTL) was detected on the long arm of chromosome 6. Subsequently, the effect and position of the QTL, designated as qREP-6, were confirmed using chromosome segment substitution lines (CSSLs), in which the background of a japonica cultivar, ‘Nipponbare’ with non-REP, was partially substituted by chromosomal segments from an indica cultivar, ‘Kasalath’ with remarkable REP. Out of 54 CSSLs, two lines (CSSL28 and CSSL29) that retain a common ‘Kasalath’-derived segment on the long arm of chromosome 6 showed a significantly high REP. The high REP lines also showed high adaptabilities such as enhanced tillering ability and shoot phosphorus content. Accordingly, conditional dependencies between the related traits were assessed using a graphical Gaussian model (GGM). Direct interactions between REP and root length, and between root length and tiller number were detected under P deficiency in CSSLs. Furthermore, qREP-6 for REP and qTNP-6 for tiller number under P deficiency were fine-mapped with an F₂ population of a cross between Nipponbare and CSSL29. A region containing qREP-6 accounted for more than half of the phenotypic variance, the most plausible interval of which contained 37 candidate genes. The result provides a foundation for cloning of the qREP-6 gene which will be applicable to study P deficiency-dependent response and to improve rice’s adaptability to P deficiency stress. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Fine mapping of <Emphasis Type="Italic">Sta1</Emphasis>, a quantitative trait locus determining stele transversal area,on rice chromosome 9

The stele (root vascular cylinder) in plants plays an important role in the transport of water and nutrients from the root to the shoot. A quantitative trait locus (QTL) on rice chromosome 9 that controls stele transversal area (STA) was previously detected in an F₃ mapping population derived from a cross between the lowland cultivar ‘IR64’, with a small STA, and the upland cultivar ‘Kinandang Patong’, with a large STA. To identify the gene(s) underlying this QTL, we undertook fine mapping of the locus. We screened eight plants from BC₂F₃ lines in which recombination occurred near the QTL. Progeny testing of BC₂F₄ plants was used to determine the genotype classes for the QTL in each BC₂F₃ line. Accordingly, the STA QTL Sta1 (Stele Transversal Area 1) was mapped between the InDel markers ID07_12 and ID07_14. A candidate genomic region for Sta1 was defined more precisely between markers RM566 and RM24334, which delimit a 359-kb interval in the reference cultivar ‘Nipponbare’. A line homozygous for the ‘Kinandang Patong’ allele of Sta1 had an STA approximately 28.4% larger than that of ‘IR64’. However, Sta1 did not influence maximum or total root length, suggesting that this QTL specifically controls STA.     

Rhizodeposition and C-partitioning of Lolium perenne in axenic culture affected by nitrogen supply and defoliation

This study investigated the effects of N-supply and partial defoliation on C-partitioning, root morphology and soluble rhizodeposition, for Lolium perenne grown in axenic sand culture systems percolated with nutrient solution. Plants were grown for 36 d in nutrient solutions with differing N concentrations (4 mM or 0.02 mM NH₄ ⁺NO₃ ^-), and effects of repeated defoliation to 4 cm were determined. The ‘low N’ supply reduced (P < 0.05) dry matter accumulation, with proportionately increased partitioning to the root systems. Root morphology was also altered at ‘low N’, with development of a finer root system, manifest as increased (P < 0.05) specific root length. Concurrent with these effects on growth of L. perenne, ‘low N’ increased (P < 0.05) exudation of C-compounds from roots on a per g root basis. Defoliation was found to increase exudation (P < 0.05) of soluble compounds for periods of 3-5 d following each cut, at both N-supply rates. The effects of N-supply and defoliation are of importance in understanding the coupling of plant productivity to nutrient cycling in soils with differing N availabilities and for grassland systems which are subject to grazing. This revised version was published online in June 2006 with corrections to the Cover Date.     

Molecular mapping of a resistance-specific PCR-based marker linked to a gall midge resistance gene (Gm4t) in rice

 A PCR-based marker (E20₅₇₀) linked to the gene Gm4t, which confers resistance to a dipteran pest gall midge (Orseolia oryzae), has been mapped using the restriction fragment length polymorphism (RFLP) technique in rice. Gm4t is a dominant resistance gene. We initially failed to detect useful polymorphism for this marker in a F₃ mapping population derived from a cross between two indica parents, ‘Abhaya’בShyamala’, with as many as 35 restriction enzymes. ‘Abhaya’ carries the resistance gene Gm4t and ‘Shyamala’ is susceptible to gall midge. Subsequently, E20₅₇₀ was mapped using another mapping population represented by a F₂ progeny from a cross between ‘Nipponbare’, a japonica variety, and ‘Kasalath’, an indica variety, in which the gene Gm4t was not known to be present. Gm4t mapped onto chromosome 8 between markers R1813 and S1633B. Our method, thus, presents an alternative way of mapping genes which otherwise would be difficult to map because of a lack of polymorphism between closely related parents differing in desired agronomic traits. Received: 1 April 1997 / Accepted: 13 May 1997     

Molecular and phenotypic characterization of near isogenic lines at QTL for quantitative resistance to Leptosphaeria maculans in oilseed rape (Brassica napus L.)

The most common and effective way to control phoma stem canker (blackleg) caused by Leptosphaeria maculans in oilseed rape (Brassica napus) is by breeding resistant cultivars. Specific resistance genes have been identified in B. napus and related species but in some B. napus cultivars resistance is polygenic [mediated by quantitative trait loci (QTL)], postulated to be race non-specific and durable. The genetic basis of quantitative resistance in the French winter oilseed rape ‘Darmor’, which was derived from ‘Jet Neuf’, was previously examined in two genetic backgrounds. Stable QTL involved in blackleg resistance across year and genetic backgrounds were identified. In this study, near isogenic lines (NILs) were produced in the susceptible background ‘Yudal’ for four of these QTL using marker-assisted selection. Various strategies were used to develop new molecular markers, which were mapped in these QTL regions. These were used to characterize the length and homozygosity of the ‘Darmor-bzh’ introgressed segment in the NILs. Individuals from each NIL were evaluated in blackleg disease field trials and assessed for their level of stem canker in comparison to the recurrent line ‘Yudal’. The effect of QTL LmA2 was clearly validated and to a lesser extent, QTL LmA9 also showed an effect on the disease level. This work provides valuable material that can be used to study the mode of action of genetic factors involved in L. maculans quantitative resistance.     

Localization of the rice stripe disease resistance gene, Stv-bi, by graphical genotyping and linkage analyses with molecular markers

 We used graphical genotyping and linkage analyses with molecular markers to determine the chromosomal location of the rice stripe disease resistance gene, Stv-b ⁱ . The stripe resistance gene from the indica rice (Oryza sativa) cv ‘Modan’ was introgressed into several Japanese rice varieties. We found 4 RFLP markers in ‘Modan’, five susceptible parental rice varieties (‘Norin No. 8’, ‘Sachihikari’, ‘Kanto No. 98’, ‘Hokuriku No.103’ and ‘Koganebare’) and four resistant progeny varieties (‘St. No. 1’, ‘Aichi No. 6’, ‘Aoisora’ and ‘Asanohikari’). Graphical genotyping of the resistant progeny revealed a chromosomal segment ascribable to ‘Modan’ and associated with stripe resistance. The chromosomal segment from ‘Modan’ was located at 35.85 cM on chromosome 11. Linkage analysis using 120 F₂ individuals from a cross between ‘Koshihikari’ (susceptible) and ‘Asanohikari’ (resistant) revealed another 8 RFLP markers in the same chromosome. We performed a bioassay for rice stripe resistance in F₃ lines of the F₂ individuals using infective small brown planthoppers and identified an 1.8-cM segment harboring the rice stripe disease resistance gene, Stv-b ⁱ , between XNpb220 and XNpb257/ XNpb254. Furthermore, Stv-b ⁱ was linked by 0.0 cM to a RFLP marker, ST10, which was developed on the basis of the results of RAPD analysis. These DNA markers near the Stv-b ⁱ locus may be useful in marker-assisted selection and map-based cloning of the Stv-b ⁱ gene. Received: 26 September 1997 / Accepted: 4 November 1997     

Genetic dissection of root growth in rice (Oryza sativa L.) I: a hydrophonic screen

 Root growth is an important component of the adaptation of rice to drought-prone environments. A hydroponic screen was used to study root growth of 28 rice varieties. Both maximum root length and adventitious root thickness varied widely between varieties. In general, japonica varieties had larger root systems than indica varieties. Two F₂ populations involving the thick- and long-rooted upland japonica variety ‘Azucena’ and two poor-rooting varieties, namely the upland indica‘Bala’ and the Italian japonica‘Maratelli’, were made and screened in hydroponics. Generation means analysis revealed significant additive and dominance main effects for the root length traits with a prevalence of dominance gene effects in both crosses. The dominance×dominance type of non-allelic interactions were important for maximum root length from day 7 to day 28, root volume, root thickness and root cell length in the cross ‘Bala’בAzucena’. The heritability (broad-sense) estimates varied from low to high for the traits and displayed differences between populations. This suggested that recombinant lines with improved root traits can be developed from the two crosses with selection methods that involve some form of progeny evaluation. In a companion paper, we report the mapping of quantitative trait loci (QTLs) for root growth traits in the ‘Bala’בAzucena’ population using restriction fragment length polymorphisms (RFLPs). Received: 5 May 1996 / Accepted: 14 February 1997     

Genetic basis and mapping of the resistance to rice yellow mottle virus. II. Evidence of a complementary epistasis between two QTLs

 The genetic basis of resistance to rice yellow mottle virus (RYMV) was studied in a doubled-haploid (DH) population derived from a cross between the very susceptible indica variety ‘IR64’ and the resistant upland japonica variety Azucena. As a quantitative trait locus (QTL) involved in virus content estimated with an ELISA test has been previously identified on chromosome 12, we performed a wide search for interactions between this QTL and the rest of the genome, and between this QTL and morphological traits segregating in the population. Multiple regression with all identified genetic factors was used to validate the interactions. Significant epistasis accounting for a major part of the total genetic variation was observed. A complementary epistasis between the QTL located on chromosome 12 and a QTL located on chromosome 7 could be the major genetic factor controlling the virus content. Resistance was also affected by a morphology-dependent mechanism since tillering was interfering with the resistance mechanism conditioned by the epistasis between the two QTLs. Marker-assisted backcross breeding was developed to introgress the QTLs of chromosome 7 and chromosome 12 in the susceptible ‘IR64’ genetic background. First results confirmed that if both QTLs do not segregate in a backcross-derived F₂ population, then the QTL of chromosome 12 cannot explain differences in virus content. A near-isogenic line (NIL) approach is currently being developed to confirm the proposed genetic model of resistance to RYMV. Received: 20 April 1990 / Accepted: 30 April 1998     

Fine mapping of quantitative trait loci Hd-1, Hd-2 and Hd-3, controlling heading date of rice, as single Mendelian factors

 Fine mapping was carried out on three putative QTLs (tentatively designated as Hd-1 to Hd-3) of five such QTLs controlling heading date in rice that had been earlier identified using an F₂ population derived from a cross between a japonica variety, ‘Nipponbare’, and an indica variety, ‘Kasalath’, using progeny backcrossed with ‘Nipponbare’ as the recurrent parent. One BC₃F₂ and two BC₃F₁ plants, in which the target QTL regions were heterozygous and most other chromosomal regions were homozygous for the ‘Nipponbare’ allele, were selected as the experimental material. Self-pollinated progeny (BC₃F₂ and BC₃F₃) of the BC₃F₁ or BC₃F₂ showed continuous variation in days to heading. By means of progeny testing based on BC₃F₃ or BC₃F₄ lines, we determined the genotypes of each BC₃F₂ or BC₃F₃ individual at target QTLs. Their segregation patterns fitted Mendelian inheritance ratios. When the results obtained by RFLP analysis and progeny tests were combined, Hd-1, Hd-2 and Hd-3 were mapped precisely on chromosomes 6, 7 and 6, respectively, of a rice RFLP linkage map. The results demonstrated that QTLs can be treated as Mendelian factors. Moreover, these precise locations were in good agreement with the regions estimated by QTL analysis of the initial F₂ population, demonstrating the high reliability of QTL mapping using a high-density linkage map. Received: 5 November 1997 / Accepted: 10 February 1998     

Indian rice “Kasalath” contains genes that improve traits of Japanese premium rice “Koshihikari”

Rice (Oryza sativa L.) chromosome segment substitution lines (CSSLs), in which chromosomal segments of the Indian landrace “Kasalath” replace the corresponding endogenous segments in the genome of the Japanese premium rice “Koshihikari”, are available and together cover the entire genome. Chromosome regions affecting a trait (CRATs) can be identified by comparison of phenotypes with genotypes of CSSLs. We detected 99 CRATs for 15 agronomic or morphological traits. “Kasalath” had positively acting alleles in 53 CRATs. Its CRATs increased panicle number per plant by up to 23.3%, grain number per panicle by up to 30.8%, and total grain number by up to 15.1%, relative to “Koshihikari”. CRATs were identified for grain size (grain thickness and width), with positive effects of about 5.0%. A CRAT on chromosome 8 almost doubled the weight of roots in uppermost soil layers compared to “Koshihikari”. Additionally, “Kasalath” possessed CRATs for higher lodging resistance (reduction in plant height and increase in stem diameter). In some cases, multiple CRATs were detected in the same chromosome regions. Therefore, CSSLs with these chromosome segments might be useful breeding materials for the simultaneous improvement of multiple traits. Five CRATs, one for plant height on chromosome 1, one for stem diameter on chromosome 8, and three for heading date on chromosomes 6, 7, and 8 overlapped with the corresponding QTLs that already had been mapped with back-crossed inbred lines of “Nipponbare” and “Kasalath”. In both “Koshihikari” CRATs and “Nipponbare” QTLs, “Kasalath” had similar effects. Both Y. Madoka and T. Kashiwagi have contributed equally to this article.     

Quantitative trait locus responsible for resistance to Aphanomyces root rot (black root) caused by Aphanomyces cochlioides Drechs. in sugar beet

Aphanomyces root rot, caused by Aphanomyces cochlioides Drechs., is one of the most serious diseases of sugar beet (Beta vulgaris L.). Identification and characterization of resistance genes is a major task in sugar beet breeding. To ensure the effectiveness of marker-assisted screening for Aphanomyces root rot resistance, genetic analysis of mature plants’ phenotypic and molecular markers’ segregation was carried out. At a highly infested field site, some 187 F₂ and 66 F₃ individuals, derived from a cross between lines ‘NK-310mm-O’ (highly resistant) and ‘NK-184mm-O’ (susceptible), were tested, over two seasons, for their level of resistance to Aphanomyces root rot. This resistance was classified into six categories according to the extent and intensity of whole plant symptoms. Simultaneously, two selected RAPD and 159 ‘NK-310mm-O’-coupled AFLP were used in the construction of a linkage map of 695.7 cM. Each of nine resultant linkage groups was successfully anchored to one of nine sugar beet chromosomes by incorporating 16 STS markers. Combining data for phenotype and molecular marker segregation, a single QTL was identified on chromosome III. This QTL explained 20% of the variance in F₂ population (in the year 2002) and 65% in F₃ lines (2003), indicating that this QTL plays a major role in the Aphanomyces root rot resistance. This is the first report of the genetic mapping of resistance to Aphanomyces-caused diseases in sugar beet.     

Variation in Streamwater Chemistry Throughout the Hubbard Brook Valley

Streamwater chemistry was measured at 100-m intervals in all streams of the Hubbard Brook Valley, NH during ‘spring’ (May–July) and during ‘fall’ (October–December) 2001. Overall, streamwater chemistry was very similar during these two periods, but fall median concentrations were consistently higher than spring values, except for ANC, pH, NO₃⁻ and PO₄³⁻, which had lower values in fall. Median concentrations for NH₄⁺ were approximately the same in spring and fall. Stream chemistry varied throughout the Hubbard Brook Valley by elevation, channel length, drainage area and type of drainage, but most of the variability in stream chemistry was subtle and relatively small. Overall, there were relatively large (two- to 10-fold) changes in chemistry with longitudinal distance of wetted channel, elevation and/or size of drainage area in some streams and for some elements (e.g., H⁺, Alⁿ⁺, DOC), but other chemical concentrations changed relatively little (e.g., Cl⁻, dissolved Si). The main Hubbard Brook, a fifth-order stream at the mouth of the Valley, was remarkably constant in chemistry throughout its length, except where human disturbance near the mouth changed the chemistry. Differences in vegetation, geologic substrates and wetland areas were related to changes in pattern of streamwater chemistry throughout the Valley.     

A consensus linkage map identifies genomic regions controlling fruit maturity and beta-carotene-associated flesh color in melon (<Emphasis Type="Italic">Cucumis melo</Emphasis> L.)

The nutritional value and yield potential of US Western Shipping melon (USWS; Cucumis melo L.) could be improved through the introgression of genes for early fruit maturity (FM) and the enhancement of the quantity of β-carotene (QβC) in fruit mesocarp (i.e., flesh color). Therefore, a set of 116 F₃ families derived from the monoecious, early FM Chinese line ‘Q 3-2-2’ (no β-carotene, white mesocarp) and the andromonoecious, late FM USWS line ‘Top Mark’ (possessing β-carotene, orange mesocarp) were examined during 2 years in Wisconsin, USA to identify quantitative trait loci (QTL) associated with FM and QβC. A 171-point F_2–3 based map was constructed and used for QTL analysis. Three QTL associated with QβC were detected, which explained a significant portion of the observed phenotypic variation (flesh color; R ² = 4.0–50.0%). The map position of one QTL (β-carM.E.9.1) was uniformly aligned with one carotenoid-related gene (Orange gene), suggesting its likely role in QβC in this melon population and putative relationship with the melon white flesh (wf) gene. Two major (FM.6.1 and FM.11.1; R ² ≥ 20%) and one minor QTL (FM.2.1; R ² = 8%) were found to be associated with FM. This map was then merged with a previous recombinant inbred line (RIL)-based map used to identify seven QTL associated with QβC in melon fruit. This consensus map [300 molecular markers (187 co-dominant melon and 14 interspecific; 10 LG)] provides a framework for the further dissection and cloning of published QTL, which will consequently lead to more effective trait introgression in melon.     

Identification and linkage mapping of complementary recessive genes causing hybrid breakdown in an intraspecific rice cross

One outcome of hybrid breakdown is poor growth, which we observed as a reduction in the number of panicles per plant and in culm length in an F₂ population derived from a cross between the genetically divergent rice (Oryza sativa L.) cultivars ‘Sasanishiki’ (japonica) and ‘Habataki’ (indica). Quantitative trait locus (QTL) analysis of the two traits and two-way ANOVA of the detected QTLs suggested that the poor growth was due mainly to an epistatic interaction between genes at QTLs located on chromosomes 2 and 11. The poor growth was likely to result when a plant was homozygous for the ‘Habataki’ allele at the QTL on chromosome 2 and homozygous for the ‘Sasanishiki’ allele at the QTL on chromosome 11. The results suggest that the poor growth found in the F₂ population was due to hybrid breakdown of a set of complementary genes. To test this hypothesis and determine the precise chromosomal location of the genes causing the hybrid breakdown, we performed genetic analyses using a chromosome segment substitution line, in which a part of chromosome 2 from ‘Habataki’ was substituted into the genetic background of ‘Sasanishiki’. The segregation patterns of poor growth in plants suggested that both of the genes underlying the hybrid breakdown were recessive. The gene on chromosome 2, designated hybrid breakdown 2 (hbd2), was mapped between simple sequence repeat markers RM3515 and RM3730. The gene on chromosome 11, hbd3, was mapped between RM5824 and RM1341. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The causes for barley root growth retardation in the presence of ammonium and glutamate

In barley (Hordeum vulgare L.) seedlings, the rate of root growth, osmotic pressure (Π), hydraulic conductance (L _p), and longitudinal (δl) and transverse (δD/D) extensibility of root cells were measured. The seedlings were grown on Knop solution with nitrate or without nitrate with addition of 5–10 mM NH₄⁺ or 0.5–1.0 mM glutamate. Root growth retardation on the 1st–4th days of exposure to NH₄⁺ was determined by a decrease in δl in the zone of elongation, whereas root thickening was evidently related to an increase in Π. Biphasic dynamics of δl in the presence of NH₄⁺ was imitated by medium acidification near the root surface to pH 3.7, which confirms a conclusion, we have done earlier, about a non-monotonous pH-dependence of longitudinal extensibility. Root growth retardation during the first day of exposure to Glu was also determined by a decrease in the δl, which was, however, accompanied by an increase in the δD/D and L _p. Fast Glu-induced changes of measured root parameters were imitated by root exposure to oryzalin, ionomycin, and inhibitors of the H⁺-pump. It was supposed that a decrease in δl in the presence of NH₄⁺ and Glu was related to cortical microtubule disorganization with the involvement of cytosolic calcium Ca_cyt²⁺. A decrease in the δD/D and L _p in the presence of NH₄⁺ was related to apoplast acidification and a high activity of the plasmalemmal H⁺-pump. An increase in the δD/D and L _p in the presence of Glu indicates the inhibition of the plasmalemmal H⁺-pump. On the 2nd–4th days of exposure to Glu, root growth ceased, as distinct from treatment with NH₄⁺. This complete root growth inhibition by Glu was possibly related to a rapid uptake of Ca²⁺ through Glu-sensitive Ca²⁺-channels, Ca²⁺-dependent inhibition of the plasmalemmal H⁺-pump, and a decrease in mitotic activity.     

Identification of a new QTL for <Emphasis Type="Italic">Fusarium</Emphasis> head blight resistance in the wheat genotype “Wang shui-bai”

Fusarium head blight (FHB) is one of the most devastating wheat diseases, causing both yield loss and quality reduction. To detect quantitative trait loci (QTL) responsible for FHB resistance, plants of the F _2:3 population derived from a ‘Wangshui-bai’ × ‘Sy95-7’ cross were artificially inoculated. Of 396 simple sequence repeats (SSRs), 125 amplified fragment length polymorphisms were used for FHB resistance QTL analysis. Five QTLs for FHB resistance were detected on chromosomes 3B, 6B, 7A, 1B and 2D. The effect of the QTL located on chromosome 3B on phenotypic variation was 31.69%, while that of the QTL found on 2D was the smallest and only accounted for 4.98% of the variation. The resistance alleles originated from ‘Wangshibai’ and association of the QTLs using these SSR markers may facilitate marker-assisted selection to improve FHB resistance in the wheat breeding programs of southwest China.