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Genetic improvement of rice for water-limited environmentsGENETIC IMPROVEMENT OF RICE FOR WATER-LIMITED ENVIRONMENTS Proceedings of a workshop, 1-3 Dec. 1998, Los Baños, Laguna, Philippines Edited by J. O'toole, O. Ito, and B. Hardy Critical characteristics of rainfed rice environments and implications for adaptive strategies, phenotyping, and rice improvement L.J. Wade This chapter begins by outlining the area and productivity of rainfed rice in upland, rainfed lowland, and flood-prone ecosystems of South and Southeast Asia. Critical characteristics of these rainfed systems are then discussed, particularly in relation to soil conditions and the implications for root growth, water extraction, and other traits likely to contribute to drought tolerance. Implications for effective phenotyping of breeding populations and for selection of lines with improved adaptation to different types of water stress are discussed. The chapter concludes that further field data are needed for identifying quantitative trait loci associated with drought tolerance, and that the benefits from incorporating particular traits must be demonstrated. M. Cooper, D.W. Podlich, and S. Fukai The principles used to design multi-environment trials (METs) are discussed. Results from METs provide much of the information used to make selection decisions in plant breeding programs. The resources available for conducting them must be used to give the most accurate and precise information possible. This requires attention to (1) the adequacy of the sampling of the target population of environments, (2) experimental design, analysis, and interpretation of results, (3) data and information management, (4) understanding the causes of genotype-by-environment (G ´ E) interactions, and (5) the merits of alternative selection strategies compared with those currently used. Most breeding programs do not optimize the use of METs to achieve breeding objectives. Much can be done to improve on common practice. Our work on rainfed lowland rice in northeast Thailand and Lao PDR has revealed that large G ´ E interactions are common for grain yield. A major component of these interactions results from genetic variation for flowering time and environmental variation in the timing of water deficits. Genetic variation for yield, after adjusting for the effects of flowering time, is related to the capacity of lines to maintain a favorable leaf and panicle water status. The proposed breeding strategy involves a greater use of METs in combination with targeted screening of traits contributing to drought resistance. The value of putative drought-resistance traits must be established prior to their use as indirect selection criteria. The efficiency of indirect selection for broad and specific adaptations by trait-based or marker-assisted selection (MAS) needs to be evaluated in comparison to the potential for direct selection for yield from optimized METs. Quantitative genetics theory in combination with computer simulation provides a powerful framework for evaluating and optimizing direct and indirect selection strategies. The relative merit of direct selection and MAS strategies is influenced by the completeness of the molecular marker description of the traits and the adequacy of the MET in providing accurate and precise information on the adaptation of breeding lines for the target population of environments. Rice improvement for drought-prone upland environments of Asia| B. Courtois and R. Lafitte Drought is the main cause of yield instability in upland rice. But this general statement has to be put into perspective. Four different production systems of upland rice (slash-and-burn, integrated rice-based, perennial-based, and cash-based) are encountered in Asia. They correspond to three different agroecological zones with different risks of drought and sets of additional specific constraints. We define different breeding priorities for the different production systems and present the available germplasm in the light of improvement of resistance to water deficit. We discuss some important points for upland rice improvement, notably the strong G ´ E interactions and their implications for decentralization of breeding work, and the tradeoff between productivity and stability. More specifically, we present the current breeding strategy for drought resistance, highlighting the difficulties involved in establishing relevant drought conditions and choosing relevant selection criteria. We emphasize the use of molecular markers to improve selection efficiency. We chose to focus on improving individual traits rather than on performance under stress. The IRRI breeding program focuses on developing near-isogenic lines by introgressing QTLs for a structural root system and osmotic adjustment into elite backgrounds. This approach allows us to reconcile the need for rapid impact and the testing of physiological hypotheses on the value of the various traits. Germplasm improvement for drought-prone rice environments S. Sarkarung and G. Pantuwan More than 50% of the 40 million ha of rainfed lowland rice area in South and Southeast Asia is affected by drought annually, which has contributed to significant yield losses. A breeding program to improve rice cultivars for drought-prone environments began by establishing shuttle breeding programs in different target environments. Evaluation and selection of early generation material are carried out at the key site in Ubon, northeast Thailand, and at satellite stations across the region. Breeding objectives for drought-prone rice environments were prioritized and ideotypes described. Breeding strategies to tackle drought have been formulated as follows: (1) identify component traits that are related to drought resistance, (2) generate diversified sets of germplasm adapted to drought-prone conditions, (3) develop field techniques for mass screening of rice cultivars for drought resistance, (4) develop mapping populations for the identification of molecular markers that are closely related to genes that control drought resistance, and (5) begin a marker-aided selection program. Field techniques for mass screening of rice cultivars for drought resistance were established and routinely used to evaluate breeding lines for drought resistance. Doubled-haploid and recombinant inbred populations were developed for the mapping of genes that control root-pulling resistance, root penetration ability, and osmotic adjustment. Phenotypic evaluation of these DH lines for root-pulling resistance, drought score, and important agronomic traits was accomplished. A database of varietal responses to drought has been established and is available for researchers. Many advanced breeding lines were identified to possess a high level of drought resistance and recovery ability at the vegetative stage when subjected to different moisture stresses in the dry season. Experiments to investigate the mechanisms responsible for drought resistance in those breeding lines are under way. Water uptake by plant roots: modes of regulation E. Steudle, H.M. Zimmermann, and T. Henzler A composite transport model explains variability in the ability of roots to take up water. The model is based on measurements of root hydraulics (hydraulic conductivity) at the level of both excised roots and root cells (membrane level) using pressure probes and other techniques. The composite transport model integrates apoplastic and cellular components of radial water flow across the root cylinder. It explains why the hydraulic conductivity of roots changes in response to the nature (osmotic vs. hydraulic) and intensity of water flow. The model provides for an adaptation of plants to conditions of drought and other stresses by allowing for a coarse regulation of water uptake according to soil water conditions and demands for water from the shoot. The coarse regulation is physical in nature but depends on root anatomy, e.g., on the state of suberization and the existence of apoplastic barriers in the exo- and endodermis. A fine regulation results from the activity of water channels (aquaporins) in root cell membranes, which is assumed to be under metabolic control. Physiological characterization of the rice plant for tolerance of water deficit T. Hirasawa Rice plants are grown in upland fields and in irrigated and nonirrigated fields. Soil moisture depletion suppresses the midday photosynthetic rate as well as leaf expansion and causes severe drought injury to panicles in the critical stages. The photosynthetic rate also decreases at midday and severe panicle dehydration sometimes occurs when the foehn blows at heading even under submerged soil conditions. A depletion in soil moisture also diminishes the photosynthetic rate because of senescence. Compared with other crop plants, (1) rice plants suffer from water stress markedly when soil moisture decreases because of their small root system, (2) the stomata of rice plants close noticeably in response to a reduction in leaf water potential, causing a marked reduction in photosynthetic rate, and (3) leaf senescence of rice plants is promoted with a small reduction in soil moisture. Significant differences in root system development and drought resistance have been observed among rice cultivars. Water uptake capacity might depend on root system development and on root hydraulic conductivity. Many factors, such as the rate, duration, and direction of root elongation and root branching, might contribute to root system development. Root hydraulic conductivity changes with growth conditions and age. Some root functions might affect leaf senescence under drought. We need a better understanding of root system development, the path of water transport in roots, and root-shoot relationships and their genetics. Genotypic differences in physiological responses to water deficit in rice G. Cabuslay, O. Ito, and A. Alejar Several factors have limited progress in developing rice cultivars that can survive and yield favorably under rainfed conditions. These include the difficulty in identifying traits that would confer tolerance for water deficit and the fact that indices used by plant breeders are usually related to complex physiological mechanisms. In addition, high genotype by environment interaction makes it difficult to identify consistently superior genotypes. This study was carried out (1) to develop a system in which water-deficit conditions can be reproduced, (2) to characterize rice cultivars popularly used for drought studies in rainfed ecosystems, and (3) to identify physiological traits that confer tolerance for water deficit. In this study, a simple and repeatable method of simulating drought conditions using polyethylene glycol was effective in inducing water stress in plants. Variability in response to water deficit at the early seedling stage (2-3 wk after germination) was observed among 27 rice cultivars with diverse cultural adaptations. Visual scoring to assess damage provided a reliable measure of tolerance for water deficit and was highly correlated with transpiration. Visual scoring reflects dehydration of the plant tissue as shown by its relative water content. Stomatal closure is the immediate response of plants to avoid tissue dehydration during water stress. The stomata should remain partially open, however, to allow entry of CO2 and maintain the supply of energy needed to sustain plant growth during water stress. Results showed that cultivars with high tolerance based on visual scoring had high transpiration rates under stress conditions, implying high stomatal conductance, and also maintained leaf area. The highly significant negative correlation between initial leaf area and relative transpiration suggests that high initial leaf area promotes excessive water loss at the onset of drought. Measurement of carbon isotope discrimination is easier than tedious daily weight change measurements involved in whole-plant transpiration studies. Carbon isotope discrimination was positively correlated with relative transpiration but negatively correlated with specific leaf weight after water stress treatment. It seems that thinner leaves promote transpiration and carbon isotope discrimination in water-stressed plants. Levels of sugar and starch stored in leaf blades and leaf sheaths before the onset of drought did not correlate well with tolerance based on visual score. Osmotic adjustment did not play a significant role in tolerance for water deficit at the level and duration of stress used in this study. Characteristics of the root system and water uptake in upland rice M. Kondo, M.V.R. Murty, D.V. Aragones, K. Okada, T. Winn, and K.S. Kwak The importance of root system morphology, especially deep rooting zone and root density at depth, in ensuring water capture under drought is established in upland rice. To improve the efficiency of the root system, environmental factors and genotype × environment interactions that affect plasticity of the root system must be emphasized, as well as genetic factors. Because the development of nodal roots basically determines the rooting zone, it is important to understand the dynamics of the nodal root system in relation to tillering habit. Stimulating the growth of nodal roots from early internodes in early tillers is an important mechanism for forming a deep rooting zone. We need to further understand genetic and environmental effects on morphological characteristics of lateral roots in rice in relation to their function in water uptake. Results from field experiments indicated that root length density at depth is important in determining the water extraction rate across layers in the soil profile. The limited water-capturing capacity of rice under severe stress can be attributed to (1) the low morphological response to the stress in increasing root density, especially in deep layers, and (2) limited water absorption rate per length in the deep soil layer. A.R. Reddy, V. Ramanathan, N. Seetharama, S. Bajaj, and R. Wu Genetic enhancement of rice for improved performance under water-limited environments and high salt is of paramount importance. Producing improved rice lines for stress environments requires the rigorous application of molecular breeding and biotechnology. In this paper, we discuss current efforts in this direction and present information on the status of research on DNA marker technologies in the improvement of crops for drought and salt tolerance. To improve rice further using marker-assisted selection and map-based cloning, we require critical information on component traits, accurate phenotyping, the identification of candidate genes and quantitative trait loci (QTLs), the relationship between QTLs and genes, the contribution of individual QTLs to the phenotype, and their variability across different locations and different crop seasons. Another important route to producing stress-tolerant rice varieties is transformation, which offers a powerful means of incorporating exotic or even synthetic genes with a profound ability to up- or down-regulate specific metabolic steps. Rapid progress has been made in developing transformation technologies for rice and gene transfer will be done routinely soon. Developments in new strategies of isolation and characterization of novel genes and promoters and their successful transfer into rice provide new avenues for metabolic engineering for stress tolerance. We examine the usefulness of the transgenic approach for improving stress tolerance in rice. We describe the potential genes and promoters that are associated with various stress-response pathways in plants and other organisms. Such novel genes and promoters offer unique opportunities in genetic engineering of rice for stress environments. Marker-aided selection and transgenic approaches are two powerful tools to accelerate plant breeding to produce rice varieties with improved drought and salt tolerance. DNA markers and QTL mapping in rice Zhikang Li The advent of DNA marker technology in 1980 has energized the most exciting era in biological sciences, genomics. Wide applications of DNA markers and resulting molecular linkage maps allow the molecular dissection of genetic variation of complex phenotypes through the design, execution, and analysis of quantitative trait loci (QTL) mapping experiments. To date, QTLs identified in rice appear to include two types. The first type represents major genes that affect quantitative traits, which are detected with large LOD scores (>10). The second type includes most of the QTLs identified in rice, which have relatively small effects and tend to be involved in epistasis and/or genotype ´ environment interactions. We have addressed several important issues in QTL mapping, such as background genetic variation control, epistasis, and QTL ´ environment interaction, by model comparison and computer simulation. Our results indicated that control of background genetic variation with a significant maia-effect and epistatic QTLs was required to obtain accurate and precise estimates of QTL parameters, particularly in the presence of linkage. Based on a mixed linear model, software (QTLMAPPER v. 1.0) was developed for the reliable detection and quantification of main-effect and epistatic QTLs and QTL ´ environment interactions in doubled-haploid, recombinant inbred, and backcross populations. Furthermore, the use of experimental designs and advanced backcross QTL identification are briefly discussed. Finally, we anticipate that accurate genetic and physical mapping of QTLs coupled with new developments in rice structural genomics and genome research technology will lead to a golden era of functional genomics. In this era, the comprehensive integration of genomic technologies, bioinformatics, and conventional breeding techniques will occur. This will allow the functional assignment of large numbers of genes/QTLs and quick identification of allelic diversity of important genes/QTLs and multilocus genotypes associated with desirable phenotypes. K. Yamaguchi-Shinozaki and K. Shinozaki Many genes have been shown to be induced by environmental stresses such as drought, salt loading, and freezing and function in the stress tolerance of plants. We precisely analyzed the promoter regions of a drought-inducible Arabidopsis gene, rd29A, in transgenic plants, and identified a novel cis-acting element containing 9 bp, TACCGACAT (DRE, dehydration responsive element). DRE plays an important role in dehydration and high-salt- and low-temperature-induced gene expression in Arabidopsis. Two cDNA clones that encoded DRE-binding proteins, DREB1A and DREB2A, were isolated by yeast one-hybrid screening. Both the DREB proteins specifically bind to the DRE sequence and activate the transcription of genes driven by the DRE sequence in Arabidopsis. Expression of the DREB1A gene and its two homologs was induced by low-temperature stress, whereas expression of the DREB2A gene and its single homolog was induced by dehydration. These results indicate that two independent families of DREB proteins, DREB1 and DREB2, function as trans-acting factors in two separate signal transduction pathways under low temperature and dehydration conditions, respectively. Overexpression of the DREB1A cDNA, driven by the constitutive 35S CaMV promoter in transgenic plants, activated strong expression of the target stress-inducible genes under unstressed conditions, which, in turn, increased tolerance of freezing and drought. But the overexpression of stress-inducible genes controlled by the DREB1A protein caused severe growth retardation under normal growth conditions. The stress-inducible rd29A promoter minimized negative effects on plant growth. P.K. Subudhi, G.B. Magpantay, D.T. Rosenow, and H.T. Nguyen Drought is a major constraint to crop productivity. In sorghum, drought stress during and after the flowering stage causes premature leaf senescence that, in turn, leads to stalk lodging, charcoal rot, and significant yield loss. More than 80% of commercial sorghum hybrids in the United States are grown under nonirrigated conditions and most of them have preflowering drought resistance but do not have any significant postflowering drought resistance, typically called the stay-green trait. Stay-green is a drought-resistance mechanism that gives sorghum plants resistance to premature senescence under severe soil moisture stress during the postflowering stage. We have identified four quantitative trait loci (QTLs) for this trait using a recombinant inbred line population developed from B35 × Tx7000. Three major stay-green QTLs were consistently identified in all field trials, accounting for 46% of the phenotypic variance. Physical mapping of these QTL regions and, ultimately, the isolation of the stay-green genes will lead to a better understanding of drought resistance in sorghum. Research is under way to introgress major stay-green QTLs into elite breeding lines through marker-assisted selection. Marker-assisted selection for improving drought tolerance in tropical maize J.-M. Ribaut, G.O. Edmeades, F.J. Betrán, C. Jiang, and M. Bänziger Drought is thought to cause maize grain losses of >20 million tons annually in the tropics. Recurrent selection for improved drought tolerance has resulted in yield gains under midseason drought of 5% (about 75- 100 kg ha-1) yr-1. Grain yield under drought is negatively correlated (r = - 0.3 to - 0.6) with the anthesis-silking interval (ASI), a secondary trait observed at flowering. DNA molecular markers (MMs) were used in a segregating population from the cross of lines P1 (short ASI) ´ P2 (long ASI), evaluated under several levels of drought, to identify five QTLs that were stable over stressed environments. A backcross marker-assisted selection (MAS) scheme to improve the drought tolerance of an elite but drought-susceptible inbred line, CML 247, has been successfully completed, using PCR-based markers as a preselection tool. A second MAS experiment used MMs to detect changes in the frequency of alleles at loci having known association with drought tolerance as a result of recurrent selection in an open-pollinated population. Plant selection based on the presence/absence of these alleles whose frequency changed could give increased drought tolerance in less time than recurrent selection, and preliminary results are encouraging. New breeding schemes involving optimal combinations of MAS and conventional selection to improve drought tolerance in maize hold considerable promise for the future. R.S. Yadav, C.T. Hash, F.R. Bidinger, and C.J. Howarth Pearl millet (Pennisetum glaucum (L.) R. Br.) is an important cereal grain and fodder crop in the driest areas of the semiarid tropics. Drought at the crop's reproductive stage is one of the most important environmental factors limiting pearl millet productivity; improved adaptation to such drought stress is an important breeding objective. We used molecular marker tools to better understand the inheritance and expression of grain yield and grain yield-component traits in three terminal drought-stress environments differing in the intensity and duration of late reproductive stage stress. Test-crosses of mapping population progenies derived from a cross between two inbred lines differing in their response to drought were evaluated under paired stress and nonstress environments to understand the biological basis of drought tolerance, and to identify quantitative trait loci (QTLs) associated with drought tolerance. Detailed analysis led to the identification of flanking markers for QTLs associated with increased harvest index, improved grain filling, and maintenance of grain yield in terminal drought-stress environments. We discuss the significance of the putative QTLs identified, the steps to be followed in their marker-assisted backcross transfer to the elite parent of this mapping population, and the potential for their further use in the improvement of pearl millet productivity in water-limited environments. A. Kamoshita, L.J. Wade, J. Siopongco, Jingxian Zhang, M.L. Ali, M.S. Pathan, S. Sarkarung, and H.T. Nguyen Variation and quantitative trait loci (QTL) for a deep and thick root system were compared in anaerobic lowland soil 45 days after sowing. Two rice populations, 220 doubled-haploid lines from the cross of upland japonica and lowland indica (CT9993/IR62266), and 184 recombinant inbred lines from the cross of lowland indica (IR58821/IR52561) were compared. Thirteen traits, categorized into three groups (shoots, deep roots, and thick roots), were analyzed in two seasons of contrasting temperature and radiation. Large transgressive variations for deep root traits in CT9993/IR62266 and for thick root traits in IR58821/IR52561 were identified. Genotype by season interaction was always smaller than genotypic variation, but these were comparable for deep and thick root traits in IR58821/IR52561, with only three QTLs for deep and thick root traits identified in common across the two plantings. In CT9993/IR62266, seven common QTLs for deep or thick root traits were found, but four of them were the same for shoot dry weight or plant height. The results suggested potential genetic improvement of constitutive root traits of rainfed lowland rice by introgressing upland japonica and by crossing within lowland indica. Careful selection of the screening environment for phenotyping is required, however, and effects of shoot growth under saturated conditions need to be taken into account in the populations from upland and lowland crosses. H.E. Shashidhar, G.S. Hemamalini, and S. Hittalmani Root morphology under well-watered conditions sampled on two occasions and under low-moisture stress was studied in a randomly chosen subset of 56 doubled-haploid lines derived from a cross between IR64 and Azucena at two growth stages during the vegetative stage. A molecular map of the same population served as the basis for locating quantitative trait loci (QTLs) controlling root morphology and associated traits. Regions flanking the restriction fragment length polymorphism markers RZ730-RZ801 on chromosome 1 were associated with plant height in all three sampling environments. This position corresponds to sd-1, a semidwarfing gene. Fifteen QTLs were detected at the two developmental stages, of which only four were common. Twenty-one QTLs for different traits were detected under low-moisture stress. Although two QTLs for plant height on chromosomes 1 and 3 were common, none of the loci for root morphological traits was common between the two different moisture regimes. The absence of common QTLs for root traits between two developmental stages and two moisture regimes suggests the existence of parallel genetic pathways operating at different growth stages and under different moisture regimes. Root volume and total root number per plant decreased significantly under stress, whereas maximum root length and plant height exhibited nonsignificant increases under stress. Mapping root and shoot traits in rice: experience in UK, IRRI, and WARDA A. Price, K. Steele, J. Townend, J. Gorham, A. Audebert, M. Jones, and B. Courtois A mapping population of 205 recombinant inbred lines derived from a cross between two drought-resistant upland rice varieties, Azucena and Bala, has been used to identify genomic regions contributing to drought resistance. We describe analyses of screens conducted in the dry season at the International Rice Research Institute (IRRI) over two seasons and at the West Africa Rice Development Association (WARDA) over one season. Performance under drought was assessed visually as leaf rolling and leaf drying and by measuring relative water content. Using a combined RFLP and AFLP map with 134 markers, quantitative trait loci (QTLs) with LOD >2.0 detected using composite interval mapping were found on all chromosomes except 4, 6, 8, and 11. The same population has been used to locate QTLs associated with root morphology assessed in two different greenhouse experiments in the UK. Maximum root length and root thickness traits were mapped and showed considerable plasticity. We discuss the results in the context of previous work on drought screening and root-morphology mapping with other populations and root penetration QTLs identified in this population. We also discuss progress toward identifying valuable regions for marker-assisted breeding. L. Shen, B. Courtois, K. McNally, S. McCouch, and Z. Li A well-developed root system is an important trait contributing to drought resistance in rice. Preliminary work allowed us to map QTLs involved in the control of several root traits in a doubled-haploid population of rice derived from the cross IR64 ´ Azucena. We are now developing near-isogenic lines of IR64 introgressed with the major QTLs. Four segments on chromosomes 1, 2, 7, and 9 were initially targeted for single QTL selection. Four doubled-haploid lines with the proper allelic combination at the targeted segments and less than 50% Azucena alleles in the rest of the genome were chosen as donors. These lines were backcrossed on IR64 up to the BC3F1 generation and then selfed. Marker-aided selection for the target segments was performed from BC1F1 up to BC3F2. All the selected BC3F2 progenies carried the donor alleles for at least one target region. Plants carrying two target segments were also identified. Phenotypic variation for several nontarget traits was observed in the BC3F2 plants, suggesting the presence of genetic drag during introgression. A background survey of the rest of the genome indicated that the BC3F2 plants had a very small portion of Azucena alleles in the nontarget regions, close to the expected ~3%. Crosses were made between BC3F2 plants carrying different target donor segments to remove possible genetic drag and pyramid different target QTLs. Screening for osmotic adjustment in rice R. Chandra Babu, A. Blum, Jingxian Zhang, S. Sarkarung, and H.T. Nguyen Osmotic adjustment (OA) is a major component of drought resistance. Incorporation of OA in crop improvement programs is limited by its slow and complex measurement procedures. Broadly, three different methods for determining OA in plants are in use. In a recent study, the comparative performance of these methods was tested using diverse rice lines. Significant variation in OA among cultivars was observed in all three methods. More than a fourfold difference in OA among cultivars (0.35 to 1.51 MPa) was observed using the standard regression method. The mean OA over the cultivars was 0.89, 0.51, and 0.72 MPa in the "regression," "full turgor adjusting," and "rehydration" methods, respectively. The rehydration method had a significantly higher correlation with the standard regression method and had an advantage over the full turgor adjusting method in terms of quick measurements of osmotic potential. The rehydration method served as a rapid procedure to screen large amounts of germplasm with some improvements suggested here. OA was not fully expressed under conditions of high temperature and evaporative demand because of the fast development of stress. Because of the sensitivity of the measurement systems and experimental methods employed in this study, rice required about 3 wk of stress duration for solute(s) to accumulate and for cultivar differences to develop in OA. The complexity of measuring OA in selection work tempts us to seek alternate indices for OA that are simple and rapid for measurement. Constitutive capacity for OA is one such example. The initial turgid osmotic potential in rice plants not previously exposed to drought, however, is not associated with OA after the stress treatment. On the other hand, the osmotic potential of plants rehydrated after stress showed a significant positive relation with OA. The need for a proper stress protocol, measurement procedures for plant water relations, and the different methods of OA determination are discussed, with emphasis on rice. Progress on the molecular mapping of osmotic adjustment and root traits in rice Jingxian Zhang, H.G. Zheng, M.L. Ali, J.N. Tripathy, A. Aarti, M.S. Pathan, A.K. Sarial, S. Robin, Thuy Thanh Nguyen, R.C. Babu, Bay Duy Nguyen, S. Sarkarung, A. Blum, and Henry T. Nguyen The advent of molecular mapping technology makes it possible to dissect complex traits such as drought resistance and yield. Because drought is a major limiting factor for grain production in rainfed rice ecosystems, this paper reviews the literature on molecular mapping of drought resistance in rice. Research progress has been made at Texas Tech University on tagging quantitative trait loci (QTLs) for two important drought-resistance components in rice, osmotic adjustment and root traits. When QTLs for drought-resistance traits were compared across populations, we found seven genomic regions where QTLs for various traits were clustered and shared by two or more populations. The practical applications of identified QTLs and the candidate gene approach are discussed. Drought resistance of a doubled-haploid line population of rice in the field A. Blum, J. Mayer, G. Golan, and B. Sinmena A doubled-haploid (DH) F2 line population of rice (cross CT9993-5-10-1-M/IR62266-42-6-2, designated IR68586) was developed for the molecular mapping of drought-resistance traits in rice. This study was performed to evaluate a subset of 100 lines of this population for drought resistance in terms of plant production in the field. Lines were planted at Bet Dagan, Israel, in 1997 under fully irrigated and drought-stress conditions. Drought-stress conditions were imposed by subjecting plots to two consecutive drying cycles between tillering and heading growth stages, followed by full irrigation until harvest. Data were collected on plant production, phenology, and several indicators of plant water stress. Lines differed significantly for all measured plant phenology and production traits, under both control and stress conditions. Mean heading date was delayed by 16.7 d under stress compared with the controls. The integrated final effect of the two consecutive drying cycles was outstanding, causing an average reduction of 44% in biomass and 71% in yield. Drought stress as imposed here was therefore severe. There was a significant (P<0.05) environment by line interaction for biomass and yield. Yield of lines under stress ranged from nil to 140% of that in the controls, whereas yield drought-resistance index ranged from 0.04 to 5.06. Lines differed significantly for all measured indicators of plant water stress-namely, midday canopy temperature, leaf rolling score, leaf death score, and leaf relative water content (RWC, measured only in a subset of 20 lines). For example, RWC at the peak of the first drying cycle ranged from 64.8% to 27.2%. Plant stress indicators were significantly correlated across lines. Significant correlations across lines were found between biomass and yield under stress and canopy temperature, leaf death score, and RWC. These associations support the conclusion that the results of this test indeed reflected real and large genetic variation in drought resistance within this population. Jingxian Zhang, R. Chandra Babu, G. Pantuwan, A. Kamoshita, A. Blum, L. Wade, S. Sarkarung, J.C. O'Toole, and H.T. Nguyen Improving drought resistance through conventional breeding approaches has been slow because of its complexity. By using current advances in molecular marker technology, we tried to dissect this complex but very important trait. For the first time, we present here a comprehensive report on associations of QTLs for drought resistance and rice performance under water stress in the field. By using a 154-doubled-haploid line population developed from a cross between CT9993-5-10-1-M and IR62266-42-6-2, we did large-scale phenotyping at Texas Tech University (USA), IRRI (Philippines), the Ubon Rice Research Center (Thailand), and the Volcani Center (Bet Dagan, Israel) under well-watered or drought conditions for traits related to drought resistance. Yield and biomass under stress were negatively associated with phenotypic expressions of plant water stress in the field. But the locations of QTLs for yield under stress and yield resistance index at Bet Dagan were different from those of QTLs for root penetration ability, osmotic adjustment, and root depth and mass as measured under greenhouse conditions and for canopy temperature and drought scores as measured in the field. These results suggest that drought resistance in terms of yield and biomass at Bet Dagan was not related to osmotic adjustment or root penetration. Most likely, yield under this field test was related to field rooting depth and the ability to extract deep soil moisture. It is interesting to point out, however, that the genomic region of RZ67-EM15_1 on chromosome 5 harbored QTLs for stress canopy temperature, drought score, and recovery score. Clearly, there is an urgent need for further quality phenotyping of the mapping population in target drought-prone environments in the field where genetic variations in OA and root traits might affect plant productivity. The acquisition of data on maximum root depth in the field and the future development of near-isogenic lines carrying QTLs for different traits might provide a fuller physiological and genetic explanation of drought resistance in this population. Genetic improvement of rice for water-limited environments: constraints and research opportunities R. Lafitte The tools available to evaluate genetic variation in tolerance for water stress have expanded tremendously over the past decade. This workshop sought to identify opportunities to apply those tools to reduce the impact of water deficit on rice yields. Many of the previous chapters report results obtained to date. This chapter summarizes the next steps that must be taken to translate these results into improved varieties for rice farmers in regions affected by water stress. |
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Rice improvement for drought-prone upland environments of Asia Germplasm improvement for drought-prone rice environments Water uptake by plant roots: modes of regulation Physiological characterization of the rice plant for tolerance of water deficit Genotypic differences in physiological responses to water deficit in rice Characteristics of the root system and water uptake in upland rice DNA markers and QTL mapping in rice Marker-assisted selection for improving drought tolerance in tropical maize Mapping root and shoot traits in rice: experience in UK, IRRI, and WARDA Screening for osmotic adjustment in rice Progress on the molecular mapping of osmotic adjustment and root traits in rice Drought resistance of a doubled-haploid line population of rice in the field Genetic improvement of rice for water-limited environments: constraints and research opportunities |
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