► Rice root growth encompasses a remarkable range of genetic diversity. ► A detailed background of rice root growth at a range of scales is reviewed. ► Key root traits for drought response in rice ...are presented. ► Physiology and breeding perspectives for drought improvement in rice are considered.
Rice root growth encompasses a remarkable genetic diversity in terms of growth patterns, architecture, and environmental adaptations. In order to harness this valuable diversity for improving rice response to drought, an understanding of key root traits and effective drought response mechanisms is necessary. A trait-based approach with precise understanding of the target environment, including temporal and spatial heterogeneity, is a possible path toward the use of roots and dehydration avoidance traits for improved drought resistance in rice. The ability to grow deep roots is currently the most accepted target trait for improving drought resistance, but genetic variation has been reported for a number of traits that may affect drought response. Here, we review variation in rice root response to drought from a physiological perspective in terms of morphology and function with respect to the different growth environments (upland and lowland) commonly used by farmers. Recent advances in linking physiology and breeding are also presented.
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.
•Genetic gain for rice grain yield for International Rice Research Institute drought breeding program was estimated.•Positive trend of 0.68 %, 0.87 %, 1.9 % under irrigated control, moderate and ...severe drought achieved.•Superiority of new rice varieties over currently grown demonstrated on farmers’ fields.•International Rice Research Institute developed rice varieties can protect farmers from crop losses under drought conditions.
The complexity of genotype × environment interactions under drought reduces heritability, which determines the effectiveness of selection for drought tolerance and development of drought tolerant varieties. Genetic progress measured through changes in yield performance over time is important in determining the efficiency of breeding programmes in which test cultivars are replaced each year on the assumption that the new cultivars will surpass the older cultivars. The goal of our study was to determine the annual rate of genetic gain for rice grain yield in a drought-prone rainfed system in a series of multi-environment trials conducted from 2005 to 2014 under the Drought Breeding Network of Indian sites in collaboration with the International Rice Research Institute (IRRI). Our results show a positive trend in grain yield with an annual genetic yield increase of about 0.68 % under irrigated control, 0.87 % under moderate reproductive stage drought stress and 1.9 % under severe reproductive stage drought stress due to breeding efforts. The study also demonstrates the effectiveness of direct selection for grain yield under both irrigated control as well as managed drought stress screening to improve yield in typical rainfed systems. IRRI's drought breeding programme has exhibited a significant positive trend in genetic gain for grain yield over the years under both drought stress as well as favorable irrigated control conditions. Several drought tolerant varieties released from the programme have outperformed the currently grown varieties under varied conditions in the rainfed environments on farmers’ fields.
Rice double-haploid (DH) lines of an indica and japonica cross were grown at nine different locations across four countries in Asia. Genotype-by-environment (G x E) interaction analysis for 11 ...growth- and grain yield-related traits in nine locations was estimated by AMMI analysis. Maximum G x E interaction was exhibited for fertility percentage number of spikelets and grain yield. Plant height was least affected by environment, and the AMMI model explained a total of 76.2% of the interaction effect. Mean environment was computed by averaging the nine environments and subsequently analyzed with other environments to map quantitative trait loci (QTL). QTL controlling the 11 traits were detected by interval analysis using mapmaker/qtl. A threshold LOD of >/=3.20 was used to identify significant QTL. A total of 126 QTL were identified for the 11 traits across nine locations. Thirty-four QTL common in more than one environment were identified on ten chromosomes. A maximum of 44 QTL were detected for panicle length, and the maximum number of common QTL were detected for days to heading detected. A single locus for plant height (RZ730-RG810) had QTL common in all ten environments, confirming AMMI results that QTL for plant height were affected the least by environment, indicating the stability of the trait. Two QTL were detected for grain yield and 19 for thousand-grain weight in all DH lines. The number of QTL per trait per location ranged from zero to four. Clustering of the QTL for different traits at the same marker intervals was observed for plant height, panicle number, panicle length and spikelet number suggesting that pleiotropism and or tight linkage of different traits could be the possible reason for the congruence of several QTL. The many QTL detected by the same marker interval across environments indicate that QTL for most traits are stable and not essentially affected by environmental factors.
The rapid progress in rice genotyping must be matched by advances in phenotyping. A better understanding of genetic variation in rice for drought response, root traits, and practical methods for ...studying them are needed. In this study, the OryzaSNP set (20 diverse genotypes that have been genotyped for SNP markers) was phenotyped in a range of field and container studies to study the diversity of rice root growth and response to drought. Of the root traits measured across more than 20 root experiments, root dry weight showed the most stable genotypic performance across studies. The environment (E) component had the strongest effect on yield and root traits. We identified genomic regions correlated with root dry weight, percent deep roots, maximum root depth, and grain yield based on a correlation analysis with the phenotypes and aus, indica, or japonica introgression regions using the SNP data. Two genomic regions were identified as hot spots in which root traits and grain yield were co-located; on chromosome 1 (39.7-40.7 Mb) and on chromosome 8 (20.3-21.9 Mb). Across experiments, the soil type/ growth medium showed more correlations with plant growth than the container dimensions. Although the correlations among studies and genetic co-location of root traits from a range of study systems points to their potential utility to represent responses in field studies, the best correlations were observed when the two setups had some similar properties. Due to the co-location of the identified genomic regions (from introgression block analysis) with QTL for a number of previously reported root and drought traits, these regions are good candidates for detailed characterization to contribute to understanding rice improvement for response to drought. This study also highlights the utility of characterizing a small set of 20 genotypes for root growth, drought response, and related genomic regions.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
One hundred twenty six doubled-haploid (DH) rice lines were evaluated in nine diverse Asian environments to reveal the genetic basis of genotype x environment interactions (GEI) for plant height (PH) ...and heading date (HD). A subset of lines was also evaluated in four water-limited environments, where the environmental basis of G x E could be more precisely defined. Responses to the environments were resolved into individual QTL x environment interactions using replicated phenotyping and the mixed linear-model approach. A total of 37 main-effect QTLs and 29 epistatic QTLs were identified. On average, these QTLs were detectable in 56% of the environments. When detected in multiple environments, the main effects of most QTLs were consistent in direction but varied considerably in magnitude across environments. Some QTLs had opposite effects in different environments, particularly in water-limited environments, indicating that they responded to the environments differently. Inconsistent QTL detection across environments was due primarily to non- or weak-expression of the QTL, and in part to significant QTL x environment interaction effects in the opposite direction to QTL main effects, and to pronounced epistasis. QTL x environment interactions were trait- and gene-specific. The greater GEI for HD than for PH in rice were reflected by more environment-specific QTLs, greater frequency and magnitude of QTL x environment interaction effects, and more pronounced epistasis for HD than for PH. Our results demonstrated that QTL x environment interaction is an important property of many QTLs, even for highly heritable traits such as height and maturity. Information about QTL x environment interaction is essential if marker-assisted selection is to be applied to the manipulation of quantitative traits.
Complex inheritance of high iron (Fe) and zinc (Zn) traits in polished grains, coupled with large genotype (G) × environment (E) interaction components are major challenges for rice biofortification ...programmes. Understanding such G × E interactions through multi-location trials and quantifying their magnitude using appropriate statistical models is the major pre-requisite in identification of stable donors for micronutrient traits for development of nutrient rich varieties. In the present study, we evaluated a set of 28 rice genotypes for Fe, Zn and key agronomical traits during wet season 2017 in 5 diverse environments in India. Combined analysis of variance revealed significant main effects due to genotypic, environmental and G × E interaction effects. Grain Fe showed maximum contribution from the effect of the genotype while in case of Zn, it was influenced significantly by environmental effect. Mega-environments were identified for stable evaluation of genotypes for Zn and Fe content, based on GGE biplot analysis. Yield stability index identified the genotype, G14 (Karuppunel) to be superior for grain Zn content (41.1 ppm) with high mean performance and high stability across the environments followed by the genotypes G4 (Taraori Basmati), G18, G25 (Tilakasturi and IC36704) and G2 (Edavankudipokkali). The findings from the study have significant implications for the development of high grain Zn containing rice varieties, so that the hidden hunger can be addressed in the right perspective.
This research focused on the evaluation of twelve newly bred rice genotypes with diverse morphological traits under aerobic cultivation to develop strategies for water conservation while maintaining ...grain productivity and quality. The experiment was conducted during the summer season of 2016, employing a Randomized Complete Block Design (RCBD) with wetland cultivation as a control. For analysis, various vegetative and reproductive traits namely, Plant height, Number of Tillers, Shoot Weight, Days to 50% flowering, Days to Maturity, Number of Productive Tillers, Panicle Length, Grain Weight, Length of the seed, Breadth of the seed, Ratio of Length and Breadth of the Seed, and Test weight of 100 seeds were recorded. Data on analysis of variance revealed the significant difference for days to flowering, maturity, plant height, panicle length, test weight, and grain yield under aerobic and wetland cultivation methods during the summer season, 2016, suggesting these traits are under high genetic control. Genetic parameters such as percentage of GCV, PCV, Heritability, and GAM were estimated across 13 parameters comprising vegetative and reproductive characters. The findings suggest a considerable amount of genetic variability for most traits, providing a foundation for the potential implementation of an effective selection procedure in breeding new rice varieties within the selected genotypes.
Drought is the predominant cause of yield reduction in rainfed rice production systems, but until recently, little systematic effort has been made to breed drought-tolerant cultivars. The complex ...nature of drought tolerance, genotype
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environment interaction, and the difficulty of effective drought tolerance screening complicate the development of drought tolerant varieties. This study reports on progress made by a collaborative network of Indian rice breeding programs, in partnership with International Rice Research Institute (IRRI), to develop lines combining high yield potential with drought tolerance by employing direct selection for grain yield under drought stress, and testing their performance in rainy season managed-drought screenings (RSMDS) of different stress severity levels at several locations during the main season in rainfed rice production areas in India. Advanced-generation breeding lines of less than 100 days, 100–120 days, and greater than 120 days duration generated at eight national institutes and IRRI using diverse drought tolerant donors were evaluated in 2005–2007 under RSMDS. Stress was imposed at the reproductive stage, either by draining paddies shortly after transplanting for lowland trials, or by stopping irrigation before flowering for upland trials. Yield reduction of 34–53% under moderate stress and 65–88% under severe stress was achieved in comparison to irrigated controls. Several varieties that have been widely grown in rainfed environments in India for many years, including IR 36, IR 64, Mahamaya, Sambha Mahsuri, and Swarna, were shown to be highly susceptible. Breeding lines were identified that perform consistently better than these and other widely grown rainfed varieties under both moderate and severe drought stress without any yield penalty under fully irrigated conditions. In the combined analysis over locations and years, these promising breeding lines outyielded the widely grown controls by about 0.5
t
ha
−1 under moderate stress and 0.7–1.0
t
ha
−1 under severe stress. IRRI breeding lines contributed to the network were developed using a selection protocol involving replicated dry season managed-drought screening (DSMDS) in dry season at Los Banos, Philippines. For a subset of lines extensively tested both at IRRI and in India, correlations between yield under DSMDS in the Philippine and yield in RSMDS in India were moderately high, indicating the efficacy of dry-season MS screening at IRRI in identifying tolerant lines for India. Drought tolerant lines developed by the network members have been released in several Indian states and in the Philippines. Overall, these results indicate that managed stress screening for drought tolerance, with a focus on selection for yield under stress, is effective in generating drought-tolerant cultivars.
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