Marker-assisted selection to introgress rice QTLs controlling root traits into an Indian upland rice variety

A marker-assisted back-crossing (MABC) breeding programme was conducted to improve the root morphological traits, and thereby drought tolerance, of the Indian upland rice variety, Kalinga III. This variety, the recurrent parent in the MABC, had not previously been used for quantitative trait locus (QTL) mapping. The donor parent was Azucena, an upland japonica variety from Philippines. Five segments on different chromosomes were targeted for introgression; four segments carried QTLs for improved root morphological traits (root length and thickness) and the fifth carried a recessive QTL for aroma. Some selection was made at non-target regions for recurrent parent alleles. We describe the selection made in three backcross (BC) generations and two further crosses between BC₃ lines to pyramid (stack) all five target segments. Pyramids with four root QTLs were obtained in eight generations, completed in 6 years using 3,000 marker assays in a total of 323 lines. Twenty-two near-isogenic lines (NILs) were evaluated for root traits in five field experiments in Bangalore, India. The target segment on chromosome 9 (RM242-RM201) significantly increased root length under both irrigated and drought stress treatments, confirming that this root length QTL from Azucena functions in a novel genetic background. No significant effects on root length were found at the other four targets. Azucena alleles at the locus RM248 (below the target root QTL on chromosome 7) delayed flowering. Selection for the recurrent parent allele at this locus produced early-flowering NILs that were suited for upland environments in eastern India.     

QTLs associated with root traits increase yield in upland rice when transferred through marker-assisted selection

Altering root morphology of rice (Oryza sativa L.) cultivars could improve yields in drought-prone upland ecosystems. Marker-assisted backcross breeding was used to introgress four QTLs for root traits into an upland rice cultivar. The QTLs had previously been identified under experimental conditions in a different genetic background. The introgressed lines and the recurrent parent were grown for 6 years by resource-poor farmers in upland sites in Eastern India and yields recorded. In combination the QTLs significantly increased yield by 1 t ha^?1 under relatively favourable field conditions. In less favourable trials, the QTL effects were not detected due to greater heterogeneity in soil–water availability in very low yielding environments and consequent yield variability. Root studies under controlled conditions showed that lines with the introgressions had longer roots throughout tillering than the recurrent parent (14 cm longer 2 weeks after sowing). Therefore, both improved roots and increased yield can be attributed to the introgression of QTLs. This is the first demonstration that marker-assisted backcross breeding (MABC) to introgress multiple root QTLs identified under controlled conditions is an effective strategy to improve farmers’ yields of upland rice. The strategy was used to breed a novel upland rice cultivar that has been released in India as Birsa Vikas Dhan 111.     

Whole plant responses, key processes, and adaptation to drought stress: the case of rice

Most high-yielding rice cultivars developed for irrigated conditions, including the widely grown lowland variety IR64, are highly susceptible to drought stress. This limits their adoption in rainfed rice environments where there is a risk of water shortage during the growing season. Mapping studies using lowland-by-upland rice populations have provided limited information about the genetic basis of variation in yield under drought. One approach to simultaneously improve and understand rice drought tolerance is to generate backcross populations, select superior lines in managed stress environments, and then evaluate which features of the selected lines differ from the recurrent parent. This approach was been taken with IR64, using a range of tolerant and susceptible cultivars as donor parents. Yields of the selected lines measured across 13 widely contracting water environments were generally greater than IR64, but genotype-by-environment effects were large. Traits expected to vary between IR64 and selected lines are plant height, because many donors were not semi-dwarf types, and maturity, because selection in a terminal stress environment is expected to favour earliness. In these experiments it was found that some lines that performed better under upland drought were indeed taller than IR64, but that shorter lines with good yield under drought could also be identified. In trials where drought stress developed in previously flooded (lowland) fields, height was not associated with performance. There was little change in maturity with selection. Other notable differences between IR64 and the selected backcross lines were in their responses to applied ABA and ethylene in greenhouse experiments at the vegetative stage and in leaf rolling observed under chronic upland stress in the field. These observations are consistent with the hypothesis that adaptive responses to drought can effectively allow for improved performance across a broad range of water environments. The results indicate that the yield of IR64 under drought can be significantly improved by backcrossing with selection under stress. In target environments where drought is infrequent but significant in certain years, improved IR64 with greater drought tolerance would be a valuable option for farmers.     

Quantitative trait loci for yield and yield components in an Oryza sativa×Oryza rufipogon BC2F2 population evaluated in an upland environment

An advanced backcross breeding strategy was used to identify quantitative trait loci (QTLs) associated with eight agronomic traits in a BC₂F₂ population derived from an interspecific cross between Caiapo, an upland Oryza sativa subsp. japonica rice variety from Brazil, and an accession of Oryza rufipogon from Malaysia. Caiapo is one of the most-widely grown dryland cultivars in Latin America and may be planted as a monoculture or in a multicropping system with pastures. The objectives of this study were: (1) to determine whether trait-enhancing QTLs from O. rufipogon would be detected in 274 BC₂F₂ families grown under the drought-prone, acid soil conditions to which Caiapo was adapted, (2) to compare the performance with and without pasture competition, and (3) to compare putative QTL-containing regions identified in this study with those previously reported for populations adapted to irrigated, low-land conditions. Based on analyses of 125 SSLP and RFLP markers distributed throughout the genome and using single-point, interval, and composite interval mapping, two putative O. rufipogon derived QTLs were detected for yield, 13 for yield components, four for maturity and six for plant height.We conclude that advanced backcross QTL analysis offers a useful germplasm enhancement strategy for the genetic improvement of cultivars adapted to stress-prone environments. Although the phenotypic performance of the wild germplasm would not suggest its value as a breeding parent, it is noteworthy that 56% of the trait-enhancing QTLs identified in this study were derived from O. rufipogon. This figure is similar to the 51% of favorable QTLs derived from the same parent in crosses with a high-yielding hybrid rice cultivar evaluated under irrigated conditions in a previous study. In conclusion, parallel studies in rice using AB-QTL analysis provide increasing evidence that certain regions of the rice genome are likely to harbor genes of interest for plant improvement in multiple environments. Received: 3 September 1999 / Accepted: 16 May 2000     

Mapping QTLs for root traits in a recombinant inbred population from two indica ecotypes in rice

Evaluation of root traits in rainfed lowland rice is very difficult. Molecular genetic markers could be used as an alternative strategy to phenotypic selection for the improvement of rice root traits. This research was undertaken to map QTLs associated with five root traits using RFLP and AFLP markers. Recombinant inbred lines (RILs) were developed from two indica parents, IR58821–23-B-1–2-1 and IR52561-UBN-1–1-2, that were adapted to rainfed lowland production systems. Using wax-petrolatum layers to simulate a hardpan in the soil, 166 RILs were evaluated for total root number (TRN), penetrated root number (PRN), root penetration index (RPI, the ratio of PRN to TRN), penetrated root thickness (PRT) and penetrated root length (PRL) under greenhouse conditions during the summer and the fall of 1997. A genetic linkage map of 2022 cM length was constructed comprising 303 AFLP and 96 RFLP markers with an average marker space of 5.0 cM. QTL analysis via interval mapping detected 28 QTLs for these five root traits, which were located on chromosomes 1, 2, 3, 4, 6, 7, 10 and 11. Individual QTLs accounted for between 6 and 27% of the phenotypic variation. Most of the favorable alleles were derived from the parent IR58821–23-B-1–2-1, which was phenotypically superior in root traits related to drought resistance. Three out of six QTLs for RPI were detected in both summer and fall experiments and they also were associated with PRN in both experiments. Out of eight QTLs for RPT, five were common in both seasons. Two genomic regions on chromosome 2 were associated with three root traits (PRN, PRT and RPI), whereas three genomic regions on chromosomes 2 and 3 were associated with two root traits (PRT and RPI). Two QTLs affecting RPI and two QTLs affecting PRT were also found in similar genomic regions in other rice populations. The consistent QTLs across genetic backgrounds and the common QTLs detected in both experiments should be good candidates for marker-assisted selection toward the incorporation of root traits in a drought resistance breeding program, especially for rainfed lowland rice. Received: 17 November 1999 / Accepted: 19 March 2000     

陆地棉主要产量相关性状的SSR标记关联分析

高产优质育种是我国棉花育种的主要目标。寻找与目标性状关联的分子标记,可克服常规育种的盲目性,提高分子标记辅助选择育种的准确性。本研究对118份陆地棉种质资源的衣分、单铃重、单株铃数及子指等4个产量相关性状进行2年2点的表型鉴定,并利用覆盖全基因组的、有多态性的214对SSR标记进行标记与性状的关联分析。结果表明:118份材料的4个产量相关性状表型变异丰富,平均变异系数的变幅在6.1%~19.1%之间,且在各环境中表现较为稳定;基因型分析表明,214对标记共检测到460个等位变异,基因多样性指数平均为0.5151,PIC值平均为0.4587,表明该批标记具有较多的等位变异数和较高的基因多样性;群体结构分析表明该批材料可分为4个亚群,且各类群中材料与地理来源无对应关系;关联分析结果显示,在显著条件下(-log10P1.3,P0.05),共有39个标记位点能够在2个及2个以上的环境中同时检测到,其中有4个标记位点同时与2个以上性状相关联,进一步比较发现,有7个位点与前人研究结果一致,其余32个位点为新发现的位点。研究结果可为陆地棉产量性状遗传改良的分子标记辅助选择提供理论依据。     

水、旱条件下水稻茎秆主要抗倒伏性状的QTL分析

以粳型旱稻IRAT109和粳型水稻越富杂交的116个DH株系的群体为材料,利用已构建的分子标记连锁图(包括94个RFLP标记和71个SSR标记),定位了水稻茎秆主要抗倒伏性状的QTL。在水田、旱田栽培条件下,考查了乳熟期DH系及其亲本的茎基粗、茎秆长及茎秆强度等性状。相关分析表明,茎基粗与茎秆长、茎基粗与茎秆强度及茎秆长与茎秆强度间均呈极显著正相关。利用QTLMAPPER进行水、旱田单环境定位分析及水、旱田联合定位分析定位了控制这些性状的QTL。水、旱田单环境定位分析结果表明：3个性状共检测到9个加性QTL和5对上位性QTL;联合定位分析表明：茎基粗、茎秆长共检测到6个加性QTL和6对上位性QTL,其中6个加性QTL和1对上位性QTL在两种方法下都检测到。旱田条件下检测到2个加性及2对上位性QTL(bctla、ct9、c16a-c16c和cs5-cs12)对表型变异的贡献率(简称贡献率)大于30％。这些高贡献率QTL可能对旱田条件下旱稻抗倒伏分子育种有重要意义。     

QTL mapping of root traits in a doubled haploid population from a cross between upland and lowland japonica rice in three environments

To genetically dissect drought resistance associated with japonica upland rice, we evaluated a doubled haploid (DH) population from a cross between two japonica cultivars for seven root traits under three different growing conditions (upland, lowland and upland in PVC pipe). The traits included basal root thickness (BRT), total root number (RN), maximum root length (MRL), root fresh weight (RFW), root dry weight (RDW), ratio of root fresh weight to shoot fresh weight (RFW/SFW) and ratio of root dry weight to shoot dry weight (RDW/SDW). The BRT was significantly correlated with the index of drought resistance, which was defined as the ratio of yield under the stress of the upland condition to that under the normal lowland condition. A complete genetic linkage map with 165 molecular markers covering 1,535 cM was constructed. Seven additive quantitative trait loci (QTLs) and 15 pairs of epistatic loci for BRT and RN were identified under upland and lowland conditions, and 12 additive QTLs and 17 pairs of epistatic QTLs for BRT, RN, MRL, RFW, RFW/SFW and RDW/SDW were identified under the PVC pipe condition. Four additive QTLs and one pair of epistatic QTLs controlling IDR were also found. These QTLs individually explained up to 25.6% of the phenotypic variance. QTL × environment (Q × E) interactions were detected for all root traits, and the contributions of these interactions ranged from 1.1% to 19.9%. Five co-localized QTLs controlling RFW and RDW, RFW/SFW, RDW/SDW and IDR, BRT and RN, RN, MRL and IDR were found. Four types of QTLs governing BRT and RN were classified by their detection in the upland and lowland conditions. Some common QTLs for root traits across different backgrounds were also revealed. These co-localized QTLs and common QTLs will facilitate marker-assisted selection for root traits in rice breeding programs.     

Locating QTL for osmotic adjustment and dehydration tolerance in rice

Research was conducted to identify and map quantitative traitloci (QTL) associated with dehydration tolerance and osmoticadjustment of rice. Osmotic adjustment capacity and lethal osmoticpotential were determined for 52 recombinant inbred lines grownin a controlled environment under conditions of a slowly developedstress. The lines were from a cross between an Indica cultivar,Co39, of lowland adaptation and a Japonica cultivar, Moroberekan,a traditional upland cultivar. The QTL analysis was conductedusing single marker analysis (ANOVA) and interval analysis (Mapmaker/QTL).The measurements obtained and the QTL identified were comparedto root traits and leaf rolling scores measured on the samelines. One major locus was associated with osmotic adjustment. Theputative locus for osmotic adjustment may be homoeologous witha single recessive gene previously identified for the same traitin wheat. The putative osmotic adjustment locus and two of thefive QTL associated with dehydration tolerance were close tochromosomal regions associated with root morphology. In thispopulation, osmotic adjustment and dehydration tolerance werenegatively associated with root morphological characters associatedwith drought avoidance. High osmotic adjustment and dehydrationtolerance were associated with Co39 alleles and extensive rootsystems were associated with Moroberekan alleles. To combinehigh osmotic adjustment with extensive root systems, the linkagebetween these traits will need to be broken. Alternatively,if the target environment is a lowland environment with onlybrief water deficit periods, selection for drought tolerancecharacteristics without consideration of the root system maybe most appropriate. Key words: Drought, rice, osmotic adjustment, dehydration tolerance, molecular markers, QTL, breeding     

Transmission of important chromosomal regions under selection revealed in rice pedigree breeding programs

Genetic analysis across a whole plant genome based on pedigree information offers considerable potential for enhancing genetic gain from plant breeding programs through quantitative trait loci (QTL) mapping and marker-assisted selection. Here, we report its application for graphically genotyping varieties used in Chinese japonica rice (Oryza sativa L.) pedigree breeding programs. We identified 34 important chromosomal regions from the founder parent that are under selection in the breeding programs, and by comparing donor genomic regions that are under selection with QTL locations of agronomic traits, we found that QTL clustered in important genomic regions, in accordance with association analyses of natural populations and other previous studies. The convergence of genomic regions under selection with QTL locations suggests that donor genomic regions harboring key genes/QTL for important agronomic traits have been selected by plant breeders since the 1950s from the founder rice plants. The results provide better understanding of the effects of selection in breeding programs on the traits of rice cultivars. They also provide potentially valuable information for enhancing rice breeding programs through screening candidate parents for targeted molecular markers, improving crop yield potential and identifying suitable genetic material for use in future breeding programs.     

水、旱条件下稻米蒸煮和营养品质性状与土壤水分环境互作分析及其QTL定位

以旱稻品种IRAT109与水稻品种越富杂交构建的DH群体的116个株系及其亲本为材料,在水、旱2种栽培条件下种植,研究了稻米蒸煮和营养品质性状的变化规律,在水、旱2个土壤水分环境下对直链淀粉含量(AC)、胶稠度(GC)、碱消值(GT)和蛋白质含量(PC)4个蒸煮和营养品质性状进行QTL定位及QTLs与环境互作分析。结果表明,以上4个品质性状在水、旱2种不同栽培条件下差异较大,说明这些性状受水分条件影响较大,旱栽条件下稻米蒸煮和营养各品质性状均有不同程度的升高,其中蛋白质含量平均提高37.9%。QTL分析结果表明,4个稻米品质性状在2个环境中的表现型值都为连续分布,均存在超亲遗传类型,共检测到7个加性效应QTL与稻米蒸煮和营养品质性状4项指标有关,分别位于第1、2、3、6、8、11染色体上,单个QTLs对性状的贡献率在1.91%～19.77%之间。位于第3染色体上控制碱消值的QGt3,第6染色体上控制直链淀粉含量的QAc6,在2个不同土壤水分条件下均与环境存在显著互作,对环境互作的贡献率分别为8.99%和47.86%。控制直链淀粉含量的2对上位性QTLs与土壤水分环境显著互作,贡献率较大,分别为32.54%和11.82%。并筛选到5个主效QTL(QGt6b、QGt8、QGt11、QGc1和QPc2)在抗旱育种中可用于蒸煮和营养各品质性状MAS改良。     

Freezing tolerance and tuber production in selfed and backcross progenies derived from somatic hybrids between Solanum tuberosum L. and S. commersonii Dun.

 Selfed and backcross progenies developed from tetraploid somatic hybrids between Solanum tuberosum (tbr) and S. commersonii (cmm) were characterized for nonacclimated freezing tolerance (NA) and acclimation capacity (ACC) (two independent genetic components of freezing tolerance) under controlled environments. The segregation covered 28% and 71% of the parental range for NA and ACC, respectively, with the distribution skewed toward the tbr parent. Therefore, ACC appeared to be relatively easier to recover in the segregating generation. Some first backcross progeny had greater freezing tolerance than the cultivated parent primarily through the increase in ACC. When grown in the field, the improved freezing tolerance observed in the selfed progeny under controlled conditions was confirmed. Among NA, ACC, and freezing tolerance after acclimation (AA, which is the cumulative performance of NA and ACC), AA exhibited the highest correlation coefficient with field frost tolerance. In addition to freezing tolerance, vine maturity and tuber traits including tuber yield, tuber number per plant, mean tuber weight, and specific gravity were also segregating. No significant correlation between undesirable tuber traits and freezing tolerance was detected. Vine maturity and freezing tolerance were significantly correlated, so more careful selection for earliness was necessary in incorporating freezing tolerance. Yield comparable or superior to the backcross parent Wis AG 231 and an early Canadian cultivar, ‘Sable’, was found in many backcross progeny and some selfed progeny. The observed high yield can be attributed to the increase in mean tuber weight as well as tuber number. Moreover, a high portion of progeny had a specific gravity higher than 1.085, and some greater than 1.1. The implications derived from this study in breeding for freezing tolerance and further use of these materials are discussed. Received: 22 August 1998 / Accepted: 4 January 1999     

Mapping QTLs associated with drought avoidance in upland rice grown in the Philippines and West Africa

Localizing genes that contribute to drought avoidance in a quantitative way should enable the exploitation of these genes in breeding through marker-assisted selection, and may lead to the discovery of gene identity and function. Between 110 and 176 F₆ recombinant inbred lines from a mapping population derived from a cross of upland rice varieties Bala and Azucena have been evaluated for indicators of drought avoidance in sites in the Philippines and West Africa over two dry seasons. A molecular map with 102 RFLP, 34 AFLP and six microsatellite markers has been used to map (by composite interval mapping) quantitative trait loci (QTLs) for the visual scores of leaf rolling and leaf drying and leaf relative water content. QTLs were mapped for each site and across sites. A total of 17 regions were identified which contained QTLs with a LOD score greater than 3.2. For leaf rolling, Bala was the parent contributing the majority of positive alleles whilst for the other traits, Bala and Azucena contributed more evenly. Six of the 17 regions influenced more than one trait, explaining the phenotypic correlations between traits that were observed. Three QTLs appeared to be specific to the Philippines experiments. One QTL had opposing effects in the Philippines and West Africa. QTLs for relative water content were detected on chromosome 8, congruent with an osmotic adjustment QTL identified in another population. Only three of the QTLs identified here have not been reliably identified in the two other populations that have been screened for drought avoidance. By using several populations assessed for drought avoidance in different sites, the distribution and utility of QTLs for drought avoidance in rice is being elucidated.     

Silicon deficiency and the adaptation of tropical rice ecotypes

Although silicon (Si) is found at much higher concentrations in healthy rice crops than N, P or K, it has received far less study, particularly for upland rice. There are few reports on the existence, causes, and effects of varying Si supplies in different environments. Chemical analyses of soil, water and plant tissue samples from experiments grown on a typical weathered, acidic upland soil in Colombia found concentrations of Si which were 80-90% lower than those in a typical lowland environment. These results corroborate published findings from West Africa and Hawaii, and lend support to a conclusion that acid-soil upland rice environments in the tropics tend to be deficient in Si, increasing disease damage, among other effects. Critical values for diagnosis of Si deficiency in soils, water and rice husk tissue are suggested. These are reasonably consistent with, but extend the application of previously published values derived from lowland rice studies to upland environments, and use simpler sampling and analysis methods. A strong correlation was found (r = -0.91) between high husk Si concentration and low husk discoloration disease damage, among diverse rice genotypes grown in the uplands. These genotypic differences were mainly explained by their ecotypic affinities: those belonging to the tropical japonica ecotype exhibited 93% higher husk Si concentrations than indica ecotypes (ecotypic means of 23 vs. 12 mg kg^-1 ). This is consistent with a hypothesis that the tropical japonicas may have adapted to Si deficiency in their native upland environment by evolving mechanisms to attain relatively higher tissue Si concentrations than indicas, which are believed to have evolved in the lowlands, where the Si supply is generally ample. Increased understanding of Si-mediated disease resistance in different rice environments and ecotypes could help breeders combine the high yield potential of indica types with the more durable disease resistance of the japonicas, and could contribute to the development of integrated disease management strategies.     

Has Selection for Improved Agronomic Traits Made Reed Canarygrass Invasive?

Plant breeders have played an essential role in improving agricultural crops, and their efforts will be critical to meet the increasing demand for cellulosic bioenergy feedstocks. However, a major concern is the potential development of novel invasive species that result from breeders' efforts to improve agronomic traits in a crop. We use reed canarygrass as a case study to evaluate the potential of plant breeding to give rise to invasive species. Reed canarygrass has been improved by breeders for use as a forage crop, but it is unclear whether breeding efforts have given rise to more vigorous populations of the species. We evaluated cultivars, European wild, and North American invader populations in upland and wetland environments to identify differences in vigor between the groups of populations. While cultivars were among the most vigorous populations in an agricultural environment (upland soils with nitrogen addition), there were no differences in above- or below-ground production between any populations in wetland environments. These results suggest that breeding has only marginally increased vigor in upland environments and that these gains are not maintained in wetland environments. Breeding focuses on selection for improvements of a specific target population of environments, and stability across a wide range of environments has proved elusive for even the most intensively bred crops. We conclude that breeding efforts are not responsible for wetland invasion by reed canarygrass and offer guidelines that will help reduce the possibility of breeding programs releasing cultivars that will become invasive.     

Identification of agronomically favorable quantitative trait loci alleles from a dent corn inbred Dan232 using advanced backcross QTL analysis and comparison with the F<Subscript>2:3</Subscript> population in popcorn

Specific traits are an important consideration in plant breeding. In popcorn, inferior agronomic traits could be improved using dent or flint corn backcrossed with popcorn. In this study, we used advanced backcross quantitative trait locus (AB-QTL) analysis to identify trait-improving QTL alleles from a dent maize inbred Dan232, and compared the detection of QTL in the BC₂S₁ population with QTL results using F_2:3 families of the same population. Two hundred and twenty BC₂S₁ families developed from a cross between Dan232 and an elite popcorn inbred N04 were evaluated for nine plant traits in replicated field trials under two environments. Using composite interval mapping (CIM), a total of 28 significant QTL were detected, and of these, 23 (82.14%) had favorable alleles contributed by the dent corn parent Dan232. Nine QTL (32.14%) detected in the BC₂S₁ population were also located in or near the same chromosome intervals in the F_2:3 population. All of the favorable QTL alleles from Dan232 could be used in marker-assisted selection (MAS) to improve the respective plant traits in popcorn breeding. In addition, their near isogenic lines (QTL-NILs) could be obtained through selfing or another 1–2 backcross with N04. Also, N04 improved for the studied plant traits could be developed from the BC₂S₁ families used in this study. This study demonstrated that the AB-QTL method can be applied to identify favorable QTL from dent corn inbred in popcorn breeding and, once identified, the alleles could be used in marker-assisted selection to improve the respective plant traits.     

QTL analysis and mapping of pi21, a recessive gene for field resistance to rice blast in Japanese upland rice

Field resistance is defined as the resistance that allows effective control of a parasite under natural field conditions and is durable when exposed to new races of that parasite. To identify the genes for field resistance to rice blast, quantitative trait loci (QTLs) conferring field resistance to rice blast in Japanese upland rice were detected and mapped using RFLP and SSR markers. QTL analysis was carried out in F₄ progeny lines from the cross between Nipponbare (moderately susceptible, lowland) and Owarihatamochi (resistant, upland). Two QTLs were detected on chromosome 4 and one QTL was detected on each of chromosomes 9 and 12. The phenotypic variation explained by each QTL ranged from 7.9 to 45.7% and the four QTLs explained 66.3% of the total phenotypic variation. Backcrossed progeny lines were developed to transfer the QTL with largest effect using the susceptible cultivar Aichiasahi as a recurrent parent. Among 82 F₃ lines derived from the backcross, resistance segregated in the expected ratio of resistant 1 : heterozygous 2 : susceptible 1. The average score for blast resistance measured in the field was 4.2 ± 0.67, 7.5 ± 0.51and 8.2 ± 0.66, for resistant, heterozygous and susceptible groups, respectively. The resistance gene, designated pi21, was mapped on chromosome 4 as a single recessive gene between RFLP marker loci G271 and G317 at a distance of 5.0 cM and 8.5 cM, respectively. The relationship to previously reported major genes and QTLs conferring resistance to blasts, and the significance of marker-assisted selection to improve field resistance, are discussed. Received: 8 June 2000 / Accepted: 24 November 2000     

Drought tolerance in rice: morphological and molecular genetic consideration

Rice is one of the most important food crop drastically affected by drought in lowland rice ecosystem. Dissecting out the traits of importance and genomic regions influencing the response of drought tolerance and yield traits on grain yield will aid the breeders to know the genetic mechanism of drought tolerance of rice leads to the development of drought tolerant varieties. Grain yield and its components on drought situation of recombinant inbred population (IR 58821/IR 52561) were investigated under lowland managed stress situation in 2003 and 2004 by given importance to the relative water content. Water deficit resulted in significant effect on phenology and grain yield. Best lines were selected for further varietal development programme. Variability studies showed the traits viz., days to 70% relative water content, leaf rolling, leaf drying, harvest index, biomass yield and grain yield offer high scope for improvement for drought tolerance by way of simple selection technique. Correlation and path analysis indicated that, to harness high yielding combined with drought tolerance breeders should give selection pressure on relative water content, panicle length, grains per panicle, harvest index, biomass yield, root/shoot ratio and root length in positive direction, and low scores of leaf rolling, leaf drying and drought recovery rate. Analysis of quantitative trait loci for drought tolerance, yield and its components allowed the identification of 38 regions associated with both drought tolerant and yield traits. Out of these, 18 were closely linked with DNA markers could be used for marker assisted selection in breeding for drought tolerance in rice. Pleiotropism and G × E effects interaction were noticed in some of the traits. Parent IR 58821 contributed favorable alleles for the entire drought related and most of the yield component traits. Identification of traits of importance and their nature of relationship by morphological and molecular level under lowland condition will be useful to improve drought tolerance of rice.     

A global barley panel revealing genomic signatures of breeding in modern Australian cultivars

The future of plant cultivar improvement lies in the evaluation of genetic resources from currently available germplasm. Today’s gene pool of crop genetic diversity has been shaped during domestication and more recently by breeding. Recent efforts in plant breeding have been aimed at developing new and improved varieties from poorly adapted crops to suit local environments. However, the impact of these breeding efforts is poorly understood. Here, we assess the contributions of both historical and recent breeding efforts to local adaptation and crop improvement in a global barley panel by analysing the distribution of genetic variants with respect to geographic region or historical breeding category. By tracing the impact that breeding had on the genetic diversity of Hordeum vulgare (barley) released in Australia, where the history of barley production is relatively young, we identify 69 candidate regions within 922 genes that were under selection pressure. We also show that modern Australian barley varieties exhibit 12% higher genetic diversity than historical cultivars. Finally, field-trialling and phenotyping for agriculturally relevant traits across a diverse range of Australian environments suggests that genomic regions under strong breeding selection and their candidate genes are closely associated with key agronomic traits. In conclusion, our combined data set and germplasm collection provide a rich source of genetic diversity that can be applied to understanding and improving environmental adaptation and enhanced yields.     

Yield response to water deficit in an upland rice mapping population: associations among traits and genetic markers

A population of recombinant inbred rice lines from a cross between the upland japonica cultivar Azucena and the upland indica cultivar Bala was evaluated in a series of upland field experiments. Water stress was imposed during the reproductive stage by managed irrigation during the dry season, while control treatments were maintained in aerobic, well-irrigated conditions. Water deficit resulted in a yield reduction of 17 to 50%. The genetic correlation between stress and control yields was quite high when stress was mild, and the heritability of yield was similar in stress and control treatments across both years of this study. Genetic correlations between secondary traits such as leaf rolling and drying and yield under stress varied from high (leaf drying) to insignificant (leaf rolling). Lines with superior yield tended to have fewer panicles and larger grain size than the high-yielding parent, Bala, even though the panicle number was positively correlated with yield and the thousand-grain weight was not associated with yield for the population as a whole. Analysis of quantitative trait loci (QTLs) for yield and yield components allowed the identification of 31 regions associated with growth or yield components. Superior alleles came from either parent. Several of the regions identified had also been reported for root mass at depth or maximum root length in this population in other studies made under controlled environments, and for leaf drying (LD) in field studies. However, the direction of the effect of QTLs was not consistent, which indicates that there was not necessarily a causal relationship between these secondary traits and performance. We conclude that mapping populations can provide novel insights on the actual relationships between yield components and secondary traits in stress and control environments and can allow identification of significant QTLs for yield components under drought stress.Abbreviations DAS Days after sowing - GPP Grains per panicle - QTL Quantitative trait locus - RWC Relative water content - SPP Spikelets per panicle - TGW Thousand-grain weight - VPD Vapor pressure deficit