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To explore the potential for breeding biofortified maize (Zea mays L.), Long et al. (2019–2026) crossed 14 maize inbred lines in a diallel. F1 hybrids were evaluated at five high-productivity sites and one low-N site in Zimbabwe. Grain Fe and Zn concentrations were analyzed. General combining ability effects for flour Fe and Zn concentration were significantly more important than specific combining ability effects in high-yielding environments, indicating that per se line evaluation could identify promising lines. This differed under low N conditions where test-cross performance and per se line evaluation should be used to identify Fe- and Zn-dense materials.
Influence of Plant Height and Heading Date for Septoria tritici Blotch
Simón et al. (2078–2085) investigated the effects of plant height and heading date on the expression of the resistance to Septoria tritici blotch in near isogenic lines of wheat differing in dwarfing genes (Rht) or in genes for insensitivity to photoperiod (Ppd). Reduced plant height was usually associated with increased necrosis because the shortest lines had the highest necrosis percentage. Late heading date was not associated or positively associated with increased necrosis due to weather conditions favoring disease development in late heading cultivars. Corrections of disease severity values for heading date and plant height should be done in breeding programs when selecting for resistance.
Public Breeding Programs and Wheat Cultivar Development
Public breeding programs play a significant role in wheat cultivar development and it is important to maintain their competitiveness by implementing new technologies. DNA tags can now be used (Dubcovsky, 1895–1898) to accelerate the incorporation of valuable traits. A consortium of 12 U.S. wheat researchers and breeders is using available markers and developing new ones to transfer resistance to fungi, viruses, and insects as well as gene variants related to bread, pasta, and noodle qualities into U.S. wheat cultivars. Hundreds of improved lines from all wheat market classes are being developed by this project (http://maswheat.ucdavis.eud/).
Value of Genomics in Crop Science
Recent advances in sequencing technology have put the sequence of large crop genomes such as maize and even wheat within the reach of molecular genetics, but how will this affect crop science? Martienssen (1898–1899) used a comparison with the impact of human genome sequencing on biomedical research to demonstrate the value of genomics in crop science.
Genetic Engineering Will Not Greatly Change Plant Breeding
Plant breeding is unlikely to be radically altered by genetic engineering. It takes as long to breed a molecularly engineered, or molecularly marked, gene into a cultivar as it takes for a natural gene. The simplest advances take, on average, 10 yr from inception of breeding to placement on the farm. Worldwide, the greatest problem that needs to be solved for most crops is postharvest protection against insects and vermin; that would solve far more problems than adding carotene to rice or lysine to maize (see Goodman, 1913–1914).
Molecular Breeding Methods and Research in Progress
Refinements and changes to plant breeding methodology have and will continue to be an activity of the applied breeder (Cooper et al., 1907–1913). Currently, attention is focused on the development and use of genomic and molecular genetic methodologies in breeding. When progress is being made in elite breeding populations, the challenge is to demonstrate that the alternative methods can generate knowledge and cost-effective tools that enable germplasm enhancement and product development opportunities that are either complementary or superior to those currently in use. Molecular breeding methods are discussed from the perspective of research in progress to examine their potential to enhance the outcomes of a large commercial maize breeding program.
Genetic Diversity of Pedigree Founder Maize
Smith et al. (1935–1946) conducted a survey of genetic diversity utilizing pedigree founder information for Pioneer maize (Zea mays L.) hybrids that were widely used in the central U.S. Corn Belt during the decades 1930 to 1999. Pedigrees traced to at least 61 founder sources including at least 22 landraces. Contrasting contributions from both private and public sector breeding programs were evident in the pedigrees. Diversity in time was revealed with most new founders introduced in the 1940s, 1960s, and 1980s. Breeding networks allowed germplasm that was once exotic to the central Corn Belt to contribute to improved productivity in that region.
Breeding for Striga Resistance
Crop losses from parasitic witchweeds (Striga spp.) contribute to the growing deficit of food production in the semiarid tropics. Rich et al. (2221–2229) evaluated a collection of 55 wild and 20 cultivated sorghums for potential pre-attachment resistance mechanisms to infestation by seeds of Striga asiatica in a laboratory procedure. Unique reactions at the level of weed seed germination and haustorial induction were observed in a few wild sorghum accessions. This suggests that these new sources of host plant resistance will further enhance the efforts of breeding for striga resistance in grain sorghum.
Genetic Variation Reduced in Modern U.S. Rice
Genetic variation at both RFLP and SSR loci was considerably reduced in modern U.S. rice varieties compared with older U.S. varieties or to a world collection of diverse rice accessions. Xu et al. (1947–1959) identified a subset or core collection of 31 rice cultivars (13% of the total 236 cultivars analyzed) that embodied 95% of RFLP and 74% of SSR alleles detected in the entire dataset. This subset offers an efficient source of genetic diversity for future crop improvement.
Diversity in Barley Yield under Water Stress
High yield potential and drought adaptation are based on different and sometimes conflicting mechanisms. It is an open question if breeding for stable production in Mediterranean areas should favor traits related to drought resistance or to high yield potential. Rizza et al. (2127–2137) conducted a field study in southern Italy for 3 yr using 89 barley genotypes. An integrated growing season Water Stress Index (WSI) was used to quantify the degree of water limitation of production. For each genotype, yield potential and stability were estimated by the yield vs. WSI regression. High yield potential played a major role in yield performance.
Pearl Millet and Grain Sorghum Yield Components under Varying Water Regimes
Yield components of pearl millet [Pennisetum glaucum (L.) R. Br.] and grain sorghum [Sorghum bicolor (L.) Moench] vary with environmental conditions. Maman et al. (2138–2145) conducted studies in a low-rainfall environment in western Nebraska and a higher rainfall area in eastern Nebraska with varying water regimes and yield components were measured. The number of panicles and kernel weight were associated with grain yield for both crops, with kernel weight being of greater importance for grain sorghum. Plant breeding and production research to increase pearl millet and grain sorghum yield should consider all yield components, but increased emphasis on kernel weight is merited for grain sorghum.
Foxtail Millet Grain Yields Are Limited by Inbreeding
Foxtail millet [Setaria italica (L.) P. Beauv.] grain yields are marginal with current varieties. Siles et al. (1960–1965) hypothesized that yields are limited by inbreeding. Estimates of heterosis were made by crossing several genotypes and comparing progeny to parents. It was estimated that, by releasing varieties that take advantage of heterosis, grain yield could be improved by as much as 68%.
Biological and Chemical Elicitors of Induced Resistance
During the past 20 yr, our improved understanding of the underlying processes leading to the preconditioning of plants against pathogens and parasites has been largely driven by the discovery of biological and chemical agents that are able to elicit the innate defenses of plants. Several biological and chemical elicitors of induced resistance are now commercially available for use in conventional agriculture. Vallad and Goodman (1920–1934) review the concept of induced plant resistance and the literature, evaluating the effectiveness of these elicitors at controlling diseases of various crops under field conditions. They then examine the benefits and drawbacks of such technology and future considerations for the improved use of chemical and biological elicitors of induced resistance.
Seeding Bermudagrass and Zoysiagrass
Bermudagrass [Cynodon dactylon var. dactylon (L.) Pers.] and zoysiagrass (Zoysia japonica Steud.) cultivars established by seed are desirable for golf and athletic turfs because of minimum establishment costs, but little is known about their management during establishment. Patton et al. (2160–2167) studied seeding dates, seeding rates, postseeding N fertility, and herbicide safety on seedling ‘Mirage’ bermudagrass and ‘Zenith’ zoysiagrass in Indiana and Kentucky. Species differed in their optimum seeding dates, rates, and their growing degree day requirements for establishment. Seeding bermudagrass or zoysiagrass instead of sprigging or sodding will allow managers to establish these turfs quickly and at minimal cost.
QuCim Simulation Tool for Genetics and Breeding
Because of the complicated inheritance of breeding traits, it is difficult to evaluate the performance of a breeding method or to compare the efficiencies of different breeding methods within an ongoing breeding program. Wang et al. (2006–2018) have developed a QU-GENE module called QuCim that can help breeders predict the outcome from different crossing and selection strategies under various genetic models. QuCim is an effective tool to compare selection strategies and to validate some theories in quantitative genetics. It can be used to optimize the selection process used in a breeding program, and to find out when and how to use new technologies such as marker-assisted selection and doubled haploid in an ongoing breeding program.
Probability Model to Calculate the Chance of Losing Alleles
A major objective when regenerating germplasm collections is to maintain as many diverse alleles as possible. Wang et al. (2246–2253) developed a probability model to calculate the chance of losing alleles after one cycle of regeneration, and a simulation program was developed to determine the number genes lost after any number of cycles of regeneration for some commonly used pollination methods. This approach indicated that harvesting and maintaining more seeds than are required for subsequent regeneration cycles will have some negative effects on the retention of genetic diversity. On the basis of retaining diversity at the maximum level, self-pollination was recommended for species where inbreeding depression is not serious and the seeds of individual plants from an accession can be stored separately. Otherwise, paired crosses without reciprocals, by taking two to four seeds from each pollinated plant, were recommended.
QTLs Associated with Resistance to Brown Planthopper in Rice
Host plant resistance to insects is a complex trait and results from various resistance mechanisms. To understand the genetic basis of plant resistance to brown planthopper, Nilaparvata lugens (Stål) in rice, Soundararajan et al. (2214–2220) evaluated a set of doubled haploid lines derived from the cross IR64/Azucena using a series of phenotypic tests that cover seedling resistance and resistance mechanisms: antixenosis, antibiosis and tolerance. A QTL analysis detected six QTLs on chromosomes 1, 2, 6, and 7 associated with resistance to brown planthopper in this population. Their results showed that detailed phenotypic analyses of plant resistance would help in improving the efficiency of QTL detection and in understanding the quantitative resistance to insect pests in crop plants.
Maize Genetic Responses to Two Striga Species
Striga hermonthica (Del.) and Striga asiatica (L.) Kuntze are parasitic weeds that parasitize cereals causing serious losses in yield. Gethi and Smith (2068–2077) studied the genetic responses to the two striga species in Kenya in single crosses of maize (Zea mays L.) in a diallel cross involving 16 inbred lines from different genetic backgrounds. General combining ability mean squares were higher than specific combining ability mean squares for host damage rating, striga emergence counts, striga vigor rating, and yield, indicating additive genetic effects were more important.
Resistance to Fungi in Synthetic Wheat
Tan spot [caused by Pyrenophora tritici-repentis (Died.) Drechs] and Stagonospora nodorum blotch (SNB) [caused by Phaeosphaeria nodorum (E. Müller) Hedjaroude] are two important foliar diseases of bread wheat (Triticum aestivum L.) and durum wheat [T. turgidum subsp. durum (Desf.) Husn.]. These two diseases have the ability to cause serious yield losses. Because the majority of current bread and durum wheat cultivars in the USA are susceptible, there is a need to find new sources of high-level resistance to tan spot and SNB. Xu et al. (2238–2245) evaluated 120 elite lines of CIMMYT synthetic hexaploid wheat and their durum wheat parents for seedling resistance to the two diseases. The evaluation data showed that 56 (46.7%) and 36 (30.0%) synthetics were resistant to tan spot and SNB, respectively, whereas resistance was almost absent in the durum parents. The results suggest that the elite CIMMYT synthetics are an excellent source of resistance to tan spot and SNB for developing new resistant cultivars and mapping populations in bread wheat.
Productivity and Stability Relationships in Mowed Pastures
Recent ecological research suggests that increasing the diversity of plant mixtures could increase pasture productivity and stability. Yield responses and changes in species abundance were measured by Tracy and Sanderson (2180–2186) across 4 yr in mixtures of pasture plants ranging from 1 to 15 species. Adding species diversity to pasture mixtures did not significantly improve yields or stability. Increased pasture yields and stability might be best achieved by matching two or three forage species to specific environmental conditions rather then planting a complex mixture of forages species.
Physiological Basis of Heterosis for Yield in Maize
Heterosis has been an essential aspect of the North American maize industry since the early 1900s, but very little is known about the physiological basis of heterosis. Heterosis was examined by Tollenaar et al. (2086–2094) in a set of hybrids and their inbred lines in terms of grain yield and the physiological processes underlying grain yield. Results showed that heterosis in maize (Zea mays L.) is attributable to (i) increased dry matter accumulation before silking, resulting from greater light interception due to higher leaf area; (ii) increased dry matter accumulation during the grain-filling period, resulting from greater light interception due to greater leaf area and increased stay green; and (iii) increased dry matter partitioning to the grain (harvest index).
Heterosis for Leaf CO2 Exchange Rate in Maize
Heterosis for grain yield in maize (Zea mays L.) is, in part, the result of greater seasonal dry matter accumulation of hybrids vs. inbred lines. A study by Ahmadzadeh et al. (2095–2100) was conducted to quantify the leaf CO2 exchange rate (CER) in 12 F1 maize hybrids and their seven inbred parents during the grain-filling period. Mean leaf CER of hybrids was not different from that of their parental inbred lines at silking, but significant differences became apparent 2 wk after silking, and differences became larger as plants advanced toward maturity. Maintenance of leaf CER throughout a plant's life cycle, rather than potential leaf CER, is positively associated with differences in dry matter accumulation during the grain-filling period between hybrids and their parental inbred lines.
Beyond the Oat Winter Hardiness Plateau
Progress in the development of winter hardy oat (Avena sativa L.) germplasm has been slow. Data from 65 yr of field testing of two freezing-tolerant parents suggested that transgressive segregates for freezing tolerance would likely be produced when crossed. Mass selection by Livingston et al. (1966–1969) using controlled freeze testing identified four freezing-tolerant genotypes that were significantly more freezing tolerant than both parents and were not associated with later flowering. This level of increase in freezing tolerance could result in more stable production in current growing regions as well as a significant northward movement of winter oat production.
Agrobacterium-Mediated Multiple Gene Transformation
Three foreign genes of agronomic importance, a selectable marker gene, and a reporter gene, as linked expression cassettes, were simultaneously introduced into rice via Agrobacterium by Cao et al. (2206–2213). A large number of transgenic lines were obtained through selection for hygromycin resistance. Approximately 90.2% of the transgenic lines harbored all the transgenes and the frequency of coexpression was about 85%. Statistically significant increases were observed in plant growth and productivity of transgenic lines compared with the control. The study demonstrates that this transgenic approach has great potential in developing new rice cultivars with increased productivity and enhanced tolerance to herbicide.
Polyploidy Variation among Kentucky Bluegrass Cultivars
Understanding variation in ploidy level of facultative apomictic Kentucky bluegrass (Poa pratensis L.) could facilitate breeding of the species and the study of its genetics. Ploidy level variation was assessed by Eaton et al. (2168–2174) by first using flow cytometry to quantify the DNA content in Kentucky bluegrass cultivars and then finding a correlation between DNA content and chromosome number. Twenty-two cultivars were selected that had genome sizes ranging from 5.39 pg to 17.69 pg/2C, which corresponds to the estimated chromosome number ranging from 35 (2n = 5x) to 105 (2n = 15x), respectively. Correlating DNA content with chromosome number in Kentucky bluegrass results in the detection of both euploid chromosome numbers with x = 7 and aneuploid numbers. These results could aid breeders in effectively screening and organizing germplasm in the initial collection for Kentucky bluegrass improvement.
Optimizing Pollen Confinement in Maize
Detasseling and spatial isolation reduced pollen-mediated gene flow in field experiments by Stevens et al. (2146–2153) that modeled possible production practices for certain regulated transgenic maize. In the model production system, hybrid seeds would be produced from inbred parents. Tests were conducted in Missouri at three locations for 2 yr. When no tassels were removed from female maize, the highest gene flow from yellow kernel source blocks to white kernel receptors was 0.03% at 200 m, and 0.02% at 300 m. Detasseling afforded a high level of containment. At 300 m isolation, no kernels were found from a detasseling treatment having no intended female tassels.
Genetic Diversity of Little Bluestem
A common, but rarely addressed, question in the improvement of native plant germplasm for rangeland seeding is: Should tillers or seeds be sampled from natural populations? Fu et al. (2254–2260) performed a genetic diversity study using an amplified fragment length polymorphism technique to assess the effectiveness of sampling little bluestem [Schizachyrium scoparium (Michx.) Nash] tillers and seeds from six natural stands in Manitoba and Saskatchewan in capturing genetic variation and revealed that collecting seeds may not be as effective as collecting tillers in sampling genetic diversity.
Diversity among Lowland Tropical Maize Inbreds
Tropical lowland inbred maize lines developed by CIMMYT were investigated by Xia et al. (2230–2237) using 79 simple sequence repeat (SSR) markers to observe the genetic diversity among these lines and delimit heterotic groups in this germplasm. Marker diversity was very high, but cluster analysis of the tropical yellow and white lines revealed a lack of structure within this germplasm, which can be explained by the mixed origin of the populations used to extract these lines and the specific choice of testers for reciprocal recurrent selection. The SSRs can be used to choose appropriate testers in this germplasm for more efficient hybrid production.
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