Análise de associação quanto à produtividade e seus caracteres componentes em linhagens e cultivares de arroz de terras altas

O objetivo deste trabalho foi identificar, por meio da análise de mapeamento associativo, os marcadores moleculares relacionados à produtividade do arroz de terras altas e aos seus caracteres componentes. Foram usadas 113 linhagens e cultivares de arroz de terras altas, da Coleção Nuclear de Arroz da Embrapa, com reduzido vínculo genético entre si. Os seguintes caracteres componentes da produtividade foram avaliados: número de panículas por metro, número de grãos por panícula e peso de 100 grãos. Dos 115 marcadores utilizados, 25 (21,7%) associaram-se significativamente a um ou mais caracteres. Entre os 29 SSR ("simple sequence repeats") colocalizados em QTL ("quantitative trait loci") de produtividade de arroz, 12 foram associados aos caracteres avaliados e considerados como candidatos para uso na seleção assistida por marcadores. Os marcadores NP914540, Q6ZGD1 e Q69JE3, associados ao número de grãos por panícula, ainda não foram anotados no arroz e podem constituir o ponto de partida para estudos de genômica funcional. Entre os marcadores derivados de sequências transcritas, NP914526 e NP914533 destacam-se por pertencer a rotas metabólicas relacionadas ao aumento do potencial produtivo de arroz

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Not AvailabletPoor seedling emergence and establishment are a major restraint for adopting direct seeding in rice.This is especially critical in rainfed lowlands where floods can occur immediately after sowing, creat-ing anaerobic conditions during germination and early seedling growth. Developing rice cultivars thatcan germinate in flooded soils will ensure good crop establishment. Several major quantitative trait loci(QTLs) for tolerance of anaerobic conditions during germination (anaerobic germination; AG) were iden-tified from different landraces; and near isogenic lines (NILs) containing one of these QTLs (qAG-9-2, AG1)in the background of IR64 were developed. This study is an attempt to determine the effectiveness ofAG1 QTL in improving germination of two IR64-AG NILs; IR 93312-30-101-20-3-66-6 (IR64–AG131) andIR93312-30-101-20-13-64-21 (IR64-AG132) sown under flooded conditions. The study also evaluatedselected agronomic practices anticipated to further enhance crop establishment in flooded soils. IR64-AG NILs showed 81% and 217% higher plant populations over IR64-Sub1 and IR64, respectively, and thiswas also reflected as significantly higher grain yields. IR64-AG NILs mobilized relatively more starch intosoluble sugars while submerged and maintained higher soluble sugar supply to the developing embryoresulting in faster and higher germination rates than the non-AG genotypes. Agronomic manipulationslike higher seed rate (60 kg ha−1), better seeding methods and nutrient application improved seedlinggrowth after germination and emergence, increasing plant height, leaf area, number of tillers and biomassaccumulation. These improvements were reflected as higher grain yield even in the intolerant cultivarIR64. Application of 20% more phosphorus with the recommended N-P-K resulted in 21%, 16% and 19%higher grain yield in IR64-AG, IR64-Sub1 and IR64, respectively, over the control. The results establishedthat combining proper crop establishment practices with tolerant genotype could enhance crop estab-lishment in flooded soils using direct seeding methods, with consequent reduction in production costsand water use and subsequent increase in farmers’ income.EuropeanCommission–International Fund for Agricultural Development(EC-IFAD), Bill and Melida Gates Foundation (through STRASAproject) and the Indian Council of Agricultural Research (ICAR) forfunding this research at the ICAR-NRRI, Cuttack, Odisha, throughgrants provided to the International Rice Research Institute (IRRI

A marker-assisted backcross approach for developing submergence-tolerant rice cultivars

Submergence stress regularly affects 15 million hectares or more of rainfed lowland rice areas in South and Southeast Asia. A major QTL on chromosome 9, Sub1, has provided the opportunity to apply marker assisted backcrossing (MAB) to develop submergence tolerant versions of rice cultivars that are widely grown in the region. In the present study, molecular markers that were tightly linked with Sub1, flanking Sub1, and unlinked to Sub1 were used to apply foreground, recombinant, and background selection, respectively, in backcrosses between a submergence-tolerant donor and the widely grown recurrent parent Swarna. By the BC2F2 generation a submergence tolerant plant was identified that possessed Swarna type simple sequence repeat (SSR) alleles on all fragments analyzed except the tip segment of rice chromosome 9 that possessed the Sub1 locus. A BC3F2 double recombinant plant was identified that was homozygous for all Swarna type alleles except for an approximately 2.3-3.4 Mb region surrounding the Sub1 locus. The results showed that the mega variety Swarna could be efficiently converted to a submergence tolerant variety in three backcross generations, involving a time of two to three years. Polymorphic markers for foreground and recombinant selection were identified for four other mega varieties to develop a wider range of submergence tolerant varieties to meet the needs of farmers in the flood-prone regions. This approach demonstrates the effective use of marker assisted selection for a major QTL in a molecular breeding program

Genetic advances in adapting rice to a rapidly changing climate

Rice, with its wide geographic distribution extending from 50°N to 35°S, is expected to be the most vulnerable cultivated crop to future changing climates. Among the different abiotic stresses, extreme temperatures coinciding with critical developmental stages, increasingly frequent floods and drought spells, and worsening sea water inundation are some of the major threats to sustainable rice productivity. Following the successful implementation of molecular marker-assisted backcrossing to introgress large-effect QTL for submergence tolerance in rice mega varieties, rice breeding for drought, salinity and, recently, heat tolerance is employing the same approach. Although tolerance for combined submergence and salinity has been achieved, developing rice varieties with multiple tolerance for other abiotic and biotic stresses and finding the appropriate agronomic package to exploit their performance remain a challenge. The major bottleneck is the lack of unidentified large-effect QTL for other abiotic stresses that are strongly influenced by genotype × environment (G × E) interaction. Rapid advances in the use of molecular tools, including a plethora of SNP markers, are expected to facilitate the development of major abiotic stress-tolerant rice. In response to the actual farmer field situation, progress achieved in understanding and developing independent abiotic stress tolerance is being exploited to combine tolerances (for example, heat and drought; salinity and submergence) to address emerging environmental problems across a wide range of rice ecosystems

Through the genetic bottleneck : O. rufipogon as a source of trait-enhancing alleles for O. sativa

This paper summarizes results from a decade of collaborative research using advanced backcross (AB) populations to a) identify quantitative trait loci (QTL) associated with improved performance in rice and to b) clone genes underlying key QTLs of interest. We demonstrate that AB-QTL analysis is capable of (1) successfully uncovering positive alleles in wild germplasm that were not obvious based on the phenotype of the parent (2) offering an estimation of the breeding value of exotic germplasm, (3) generating near isogenic lines that can be used as the basis for gene isolation and also as parents for further crossing in a variety development program and (4) providing gene-based markers for targeted introgression of alleles using marker-assisted-selection (MAS). Knowledge gained from studies examining the population structure and evolutionary history of rice is helping to illuminate a long-term strategy for exploiting and simultaneously preserving the well-partitioned gene pools in rice