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The use of oat (Avena sativa L.) dwarfing genes in breeding programs to improve lodging resistance has been limited, mainly because of decreases in yield and grain quality in many environments. The objectives of this study were to evaluate the effects of the dominant Dw6, Dw7, and Dw8 dwarfing genes on plant height components and the gibberellic acid (GA) response of the dwarf lines. Plant height components including internode length, panicle length, and panicle exertion were measured in plants of three nondwarf and nine dwarf lines grown in the field at St. Paul in 1992 and 1993. Five experiments were performed in growth chambers to assay the response of nondwarf and dwarf oat genotypes to exogenous GA applied at the seedling stage. The Dw6 gene in line OT207 caused a 34 to 37% reduction in plant height due to the reduction in the length of the three uppermost internodes but not internode number. The 46% reduction in height caused by the Dw7 gene in line NC2469-3 resulted from decreases in both internode number and elongation. The Dw8 gene present in derivatives of seven Japanese lines shortened all internodes but did not affect internode number, reducing plant height by about 50%. The dwarf lines that carry these dwarfing loci are responsive to exogenously added G[A.sub.3], G[A.sub.1] and G[A.sub.20], and thus the mutations appear not to involve disruptions of the conversion of G[A.sub.20] to G[A.sub.1]. The results indicate that different strategies may be needed to adjust for the different plant height component effects of each of the three dwarfing genes for their use in oat cultivar development., THE DEVELOPMENT of shorter cultivars with increased resistance to lodging is of interest in oat breeding. Major dwarfing genes have been used extensively in developing semidwarf wheat (Triticum aestivum L.) [...]

A set of oat-maize chromosome addition lines with individual maize (Zea mays L.) chromosomes present in plants with a complete oat (Avena sativa L.) chromosome complement provides a unique opportunity to analyze the organization of centromeric regions of each maize chromosome. A DNA sequence, MCS1a, described previously as a maize centromere-associated sequence, was used as a probe to isolate cosmid clones from a genomic library made of DNA purified from a maize chromosome 9 addition line. Analysis of six cosmid clones containing centromeric DNA segments revealed a complex organization. The MCS1a sequence was found to comprise a portion of the long terminal repeats of a retrotransposon-like repeated element, termed CentA. Two of the six cosmid clones contained regions composed of a newly identified family of tandem repeats, termed CentC. Copies of CentA and tandem arrays of CentC are interspersed with other repetitive elements, including the previously identified maize retroelements Huck and Prem2. Fluorescence in situ hybridization revealed that CentC and CentA elements are limited to the centromeric region of each maize chromosome. The retroelements Huck and Prem2 are dispersed along all maize chromosomes, although Huck elements are present in an increased concentration around centromeric regions. Significant variation in the size of the blocks of CentC and in the copy number of CentA elements, as well as restriction fragment length variations were detected within the centromeric region of each maize chromosome studied. The different proportions and arrangements of these elements and likely others provide each centromeric region with a unique overall structure.

A class of tandemly repeated DNA sequences (TR-1) of 350-bp unit length was isolated from the knob DNA of chromosome 9 of Zea mays L. Comparative fluorescence in situ hybridization revealed that TR-1 elements are also present in cytologically detectable knobs on other maize chromosomes in different proportions relative to the previously described 180-bp repeats. At least one knob on chromosome 4 is composed predominantly of the TR-1 repeat. In addition, several small clusters of the TR-1 and 180-bp repeats have been found in different chromosomes, some not located in obvious knob heterochromatin. Variation in restriction fragment fingerprints and copy number of the TR-1 elements was found among maize lines and among maize chromosomes. TR-1 tandem arrays up to 70 kilobases in length can be interspersed with stretches of 180-bp tandem repeat arrays. DNA sequence analysis and restriction mapping of one particular stretch of tandemly arranged TR-1 units indicate that these elements may be organized in the form of fold-back DNA segments. The TR-1 repeat shares two short segments of homology with the 180-bp repeat. The longest of these segments (31 bp; 64% identity) corresponds to the conserved region among 180-bp repeats. The polymorphism and complex structure of knob DNA suggest that, similar to the fold-back DNA-containing giant transposons in Drosophila, maize knob DNA may have some properties of transposable elements.

The recovery of maize (Zea mays L.) chromosome addition lines of oat (Avena sativa L.) from oat x maize crosses enables us to analyze the structure and composition of specific regions, such as knobs, of individual maize chromosomes. A DNA hybridization blot panel of eight individual maize chromosome addition lines revealed that 180-bp repeats found in knobs are present in each of these maize chromosomes, but the copy number varies from [approximately]100 to 25,000. Cosmid clones with knob DNA segments were isolated from a genomic library of an oat-maize chromosome 9 addition line with the help of the 180-bp knob-associated repeated DNA sequence used as a probe. Cloned knob DNA segments revealed a complex organization in which blocks of tandemly arranged 180-bp repeating units are interrupted by insertions of other repeated DNA sequences, mostly represented by individual full size copies of retrotransposable elements. There is an obvious preference for the integration of retrotransposable elements into certain sites (hot spots) of the 180-bp repeat. Sequence microheterogeneity including point mutations and duplications was found in copies of 180-bp repeats. The 180-bp repeats within an array all had the same polarity. Restriction maps constructed for 23 cloned knob DNA fragments revealed the positions of polymorphic sites and sites of integration of insertion elements. Discovery of the interspersion of retrotransposable elements among blocks of tandem repeats in maize and some other organisms suggests that this pattern may be basic to heterochromatin organization for eukaryotes.

One important goal in oat (A vena sativa L.) breeding is the development of shorter cultivars; with increased resistance to lodging. Although seven dwarfing genes have been identified in oat, only two genes, Dw6 and Dw7, are still readily available in breeding programs. New sources of dwarfing genes were recently identified in Japan from accessions of A. fatua L. and introgressed into the cultivar `Kanota'. The objectives of this study were to examine seven of these new sources with apparent dominant genes for dwarfness in oat and to determine their usefulness in oat breeding. Allelism tests using analysis of [F.sub.2] populations demonstrated that a new dominant dwarfing gene is present in the Japanese lines and that it is independent of the Dw6 and Dw7 loci. The symbol Dw8 has been assigned to the dwarfing gene found in the Japanese lines. [F.sub.2] populations derived from 17 crosses made among the seven Japanese lines did not segregate for height, indicating that the same dwarfing gene or closely linked genes were present in these lines. Based on the numbers of progenies analyzed from these crosses, we concluded that if there are distinct linked dominant dwarfing genes among the Japanese lines, there would be no more than 20% recombination among them. [F.sub.1] progenies from four Dw8 lines crossed with tall elite lines all segregated in a 3 dwarf: 1 tall ratio. The Dw8 gene, combined with other height modifier genes, may provide new opportunities for breeding for reduced plant height and improved lodging resistance in oat., Oat is an important crop grown in temperate regions of the world and is valuable in human and animal nutrition and for soil conservation (Forsberg and Shands, 1989). One of [...]

Mature embryos of oat (A vena sativa L.) have been used to establish regenerable tissue cultures with potential use for transformation. The objective of this study was to investigate tissue cultures established from mature embryos of oat as an alternative source of totipotent target cells for microprojectile bombardment-mediated transformation. Mature embryos of a specific genotype, GAF/Park-1, were incubated on a tissue culture induction medium for 1, 4, 8, or 9 wk before either being directly bombarded after 1 and 4 wk, or bombarded as tissue cultures initiated after 8 and 9 wk incubation. The 8- and 9-wk-old tissue cultures yielded the greatest numbers of transgenic tissue cultures (3.2 transgenic tissue cultures per microprojectile bombardment treatment). Three additional transformation experiments were conducted with mature embryo-derived 8- to 9-wk-old tissue cultures to determine the regeneration capacity and production of fertile transgenic plants. Overall, fertile plants were regenerated from 35 of 85 independently derived transgenic tissue cultures. Identification of mature embryo-derived tissue cultures as a source of transformable totipotent cells should reduce the expense and labor involved in oat transformation. Moreover, the uniformity and convenience of this explant likely will stimulate further investigations in oat transformation efficiency., Oat transformation has utilized friable, embryogenic callus initiated from immature embryos as a source of totipotent target cells for microprojectile bombardment (Somers et al., 1992, 1996; Torbert et al., 1995). [...]

Milling yield, or the grain weight from which 100 kg of rolled groats is obtained upon milling, is an important quality characteristic of cultivated oat (Avena sativa L.). Kernel morphology and the groat (caryopsis) percentage of the whole kernel including hull are factors that influence milling yield. We mapped QTLs for kernel area, kernel length, kernel width, and groat percentage in two populations of 137 recombinant inbred lines by RFLP and AFLP analysis to evaluate the prospects of marker-assisted selection (MAS). Phenotypic correlations between kernel morphology traits and groat percentage were not significant. For kernel morphology traits and groat percentage, one to five QTLs were detected, explaining 7.0–60.7% of the total phenotypic variance depending on the trait. One QTL for kernel length in each population and one QTL for kernel width in one population were found at the same location as a QTL for groat percentage, indicating that a change in kernel size or shape could have an influence on groat percentage. The positions and effects of QTLs for kernel morphology and groat percentage were compared to QTLs detected previously for chemical grain composition (oil andβ-glucanconcentration) and agronomic traits to evaluate the selection response on these traits through MAS. Several regions of the oat genome were identified that contained clusters of QTLs influencing two or more traits. While the allele from one parent at a QTL could simultaneously improve two or more traits in one population, it could have opposite effects on the same traits at another QTL or in the other population. Associations among traits were complex and will require careful consideration when employing QTL-marker associations in MAS to avoid negative selection response. Future research to discover candidate genes for those QTL clusters could provide information about trait associations and help in designing selection programs. [ABSTRACT FROM AUTHOR]

The availability of molecular genetic maps in oat (Avena spp.) and improved identification of chromosomes by C-banding are two recent developments that have made locating linkage groups to chromosomes possible in cultivated hexaploid oat, 2n=6x=42. Monosomic series derived from Avena byzantina C. Koch cv Kanota and from Avena sativa L. cv Sun II were used as maternal plants in crosses with the parents, Kanota-1 and Ogle-C, of the oat RFLP mapping population. Monosomic F1 plants were identified by root-tip cell chromosome counts. For marker analysis, DNAs of eight F2 plants from a monosomic F1 were combined to provide a larger source of DNA that mimicked that of the monosomic F1 plant. Absence of maternal alleles in monosomic F1s served to associate linkage groups with individual chromosomes. Twenty two linkage groups were associated with 16 chromosomes. In seven instances, linkage groups that were independent of each other in recombination analyses were associated with the same chromosome. Five linkage groups were shown to be associated with translocation differences among oat lines. Additionally, the results better-characterized the oat monosomic series through the detection of duplicates and translocation differences among the various monosomic lines. The F1 monosomic series represents a powerful cytogenetic tool with the potential to greatly improve understanding of the oat genome. [ABSTRACT FROM AUTHOR]

Examines genetically engineering elite oat cultivars. Tissue culture response of elite genotypes; Genetically engineering an elite cultivar; survey of initiation frequencies of embryogenic callus from mature embryos of different oat genotypes.