No description available.

No description available.

No description available.

Tests were conducted to ascertain whether B chromosomes influence the susceptibility of maize (Zea mays L.) plants to gamma-radiation-induced DNA damage. Isogenic stocks of Black Mexican sweet corn with and without B chromosomes were premeiotically irradiated and DNA damage was assayed by measuring pollen viability. Higher pollen viabilities relative to non-irradiated control plants were consistently obtained in irradiated plants with B chromosomes when compared to irradiated plants without B's. Furthermore, among plants irradiated with 1250R those with one B chromosome produced the greatest proportion of viable pollen and plants with increasing numbers of B's produced progressively less viable pollen. An exophenotypic trait elicited by B chromosomes is also reported. Plants with 5 or more B chromosomes often display an aberrant phenotype characterized by longitudinal white leaf stripes and/or narrow leaves. This phenotype intensifies with increasing numbers of B chromosomes and is the first case of a qualitative exophenotypic trait attributable to B chromosomes reported in maize.

Thesis (PhDAgric)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: Maize is undoubtedly South Africa's most important field crop. The identification of markers and genes for traits of interest is important to sustain the improvement of maize cultivation. Northern corn leaf blight (NClB) is a disease that occurs worldwide and can dramatically reduce yield. A number of single dominant resistance genes have been identified for NClB and some have been mapped. Currently there are no simple PCR markers for any of these resistance genes, making markerassisted selection (MAS) difficult. The aim of this study was to develop PCR markers for the NClB resistance genes Ht1, Ht2, Ht3 and Htn1 in maize. To accomplish this, the AFlP (amplified fragment length polymorphism) technique was first optimised. The results indicated that the Mlul/Msel restriction enzyme combination produces a higher percentage of polymorph isms when compared to the PstllMsel enzyme combination. It was also shown that the enzyme combination plays an important role in the percentage of polymorphic fragments observed, whereas the number of restriction enzymes used in AFlP analysis only significantly affects the total number of fragments scored. Populations segregating for the different resistance genes were not available for this study. Nearly-isogenic lines (Nils) were used in combination with AFlP technology to identify markers that map close to the genes. AFlP markers common in at least two resistant or susceptible lines were cloned and converted to PCR markers. Two commercially available recombinant inbred line (Ril) populations were then used to map the identified markers. For Htn1 fifteen polymorphic fragments were present in both resistant lines. They were selected for sequence specific marker conversion. Seven of the fifteen sequence characterized amplified region (SCAR) markers were polymorphic on the Nil pairs and five mapped to one region of maize chromosome 8.05/06. Twenty-one AFlP markers were identified for Ht1 and four SCAR markers were polymorphic In the Ht1 Nils. Three of these were mapped to chromosome 2.07. Three AFlP markers were identified for Ht2 of which two were converted to SCAR markers. Both SCAR markers were polymorphic on the Ht2 Nils and mapped to chromosome 8.05/06. On the Ht3 NILs, four AFLP markers were identified and two converted SCAR markers and one microsatellite marker (bnlg1666) were polymorphic. One of the SCAR markers and the microsatellite marker were mapped to chromosome 7.04 using a RIL population. This reports the first tentative mapping position for the Ht3 locus. The next step was to determine if a set of marker alleles could be used in a number of Htn 1 resistance lines to identify a common donor region selected by the breeders. Nine markers consisting of five SCAR markers, three converted RFLP markers and one microsatellite marker were used on 16 Htn1 resistant lines. The marker allele of us3 was in 12 of the 16 lines in coupling with Htn1 resistance. Second was the marker us5 in 11 of the 16 lines. Using this data 14 of the 16 lines shared a common introgressed region between the markers us3 and us5. A further common introgressed region between 11 of the inbred lines was found between the markers us14 and asg17. The last aim of this study was to propose a new marker technique that might be more successful than the AFLP technique in the identification of markers closely linked to genes. A new marker approach was identified where a MITE (Hbr) primer was used as an anchor primer in combination with resistance gene analog primers. This was found to be a highly polymorphic marker technique that could be used to identify markers and possibly candidate genes. It is a robust technique, which is affordable since amplifications occur from undigested genomic DNA and the primers mainly amplify fragments from genic regions. / AFRIKAANSE OPSOMMING: Mielies (Zea mays) is ongetwyfeld Suid Afrika se belangrikste lanbou gewas. Vir volgehoue opbrengs verbetering is die identifisering van merkers en gene vir belangrike eienskappe noodsaaklik. Noordelike blaarskroei (NBS) kan opbrengs wesenlik kan beïnvloed. Tans is daar reeds "n aantal enkel weerstandsgene geïdentifiseer, maar geen PKR-merkers is beskikbaar vir merker gebaseerde seleksie nie. Die doelwit van hierdie studie was om PKR-merkers te ontwikkel vir vier enkel weerstands gene (Ht1, Ht2, Ht3 en Htn1) teen NBS in mielies. Om die doelstelling te bereik is die AFLP-tegniek eers geoptimiseer. Op grond van waargenome aantal polimorfismes, was Mlul/Mse/"n beter restriksie ensiem kombinasie as Pstl/Msel. In die studie is ook bewys dat die aantal (meer as twee) restriksie ensieme wat gebruik word slegs die aantal fragmente, en nie die persentasie polimorfismes, wesenlik beïnvloed nie. Geen segregerende populasie was vir die verskillende gene beskikbaar nie. Naby isogeniese lyne (NILe) is daarom in kombinasie met die AFLP-tegniek gebruik om merkers te identifiseer wat naby die gene karteer. Alleenlik polimorfiese merkers wat in ten minste twee weerstand biedende of vatbare lyne voorgekom het, is gekloneer en omgeskakel na PKR-merkers. Daarna is twee kommersiële rekombinante ingeteelde lyn populasies gebruik om die gene te karteer. Vyftien fragmente is gevind wat gekoppel was met die Htn1 weerstand. Sewe van hierdie merkers is omgeskakel in polimorfiese SCAR-merkers waarvan vyf gekarteer is in een gebied op chromosoom 8.05/06. Een-en-twintig AFLP-merkers is geïndentifiseer vir Ht1 en vier is omgeskakel na polimorfiese SCAR-merkers. Drie hiervan is gekarteer op chromosoom 2.07. Drie AFLP-merkers is geïndetifiseer vir Ht2 waarvan 2 omgeskakel is na polimorfiese SCAR-merkers. Altwee hierdie merkers is gekarteer op chromosoom 8.05/06. Op die Ht3 lyne is vier AFLP-merkers geïdentifiseer waarvan twee omgeskakel is na polimorfiese SCAR-merkers. Een mikrosatelliet merker (bnlg1666) is ook gevind wat die selfde polimorfiese patroon wys op die Ht3 lyne. Die mikrosateliet en een van die SCAR-merkers het gekarteer op chromosomale posisie 7.04. Hierdie is die eerste tentatiewe posisie vir die Ht3 lokus. Die volgende stap was om te bepaal of "n stel polimorfiese merker-allele gebruik kan word om die donor DNA-segment te identifiseer wat die plantteiers geselekteer het. Nege PKR-merkers wat bestaan het uit vyf SCAR-merkers, 3 omgeskakelde RFLP merkers en een mikrosateliet is gebruik op 16 Hnt1 weerstandslyne. Us3 was die merker alleel wat in die meeste gevalle gekoppel was met die Htn1 weerstandslyne (12/16). Tweede was die merker us5 (in 11 van die 16 lyne). Uit die data blyk dit dat 14 van die 16 lyne "n donor segment het wat beide merkers us3 en us5 bevat. Merkers us14 en asg17 het in 11 van die 16 bestande lyne saam voorgekom. Die laaste doelstelling van hierdie studie was om "n nuwe tegniek te ontwikkel wat dalk meer suksesvol as AFLPs kan wees om merkers te identifiseer nabyaan gene. "n Nuwe tegniek word voorgestel waar "n MITE (Hbr) inleier gebruik kan word in kombinasie met weerstandgeen-analoog inleiers. Dit is gevind dat hierdie kombinasie van inleiers "n hoogs polimorfiese band patroon gee en dat die merkers ook dalk kandidaat-gene kan wees. Die tegniek is maklik uitvoerbaar, relatief goedkoop en maak gebruik van onverteerde genomiese DNA. Die fragmente wat geamplifiseer word is hoofsaaklik vanaf geenryke areas.

No description available.

ROP GTPases are crucial regulators of pollen tube growth. The Rop GTPase family in maize consists of nine known rop genes, ropl-rop9. A subset of these genes (rop2, rop8, and rop9) are expressed in pollen. The rop2 and rop9 genes are a highly conserved duplicate gene pair of ancient origin. The rop2/rop9 duplicate gene pair displays differential expression in mature and germinated pollen, suggesting different roles for the genes in the process of male gametophyte development. To explore ROP2 function in maize, five Mutator transposon insertions in the rop2 gene were isolated (rop2::Mu alleles). I showed that three of the rop2::Mu alleles displayed reduced transmission through the male and were associated with reduced levels of ROP2-mRNA. Interestingly, the rop2::Mu male-specific transmission defect was apparent only when wild-type pollen was also present, an indication that the mutation reduces the competitive ability of the rop2 gametophytes. Dual pollination and pollen mixing experiments indicated that this competitive disadvantage is expressed by the majority of the mutant gametophytes, and that expression of the phenotype is associated with a delay in the ability to accomplish fertilization. Using the waxy phenotypic marker (linked to rop2 via a reciprocal translocation) to distinguish between rop2::Mu and wild-type pollen derived from heterozygous plants, I demonstrated that the delay is associated with a defect in early progamic development (i.e, germination and early pollen tube growth). The defect was detectable in vivo as early as 15 minutes after pollination. However, quantitative measurements provided no indication that the rop2 mutation affects pollen tube growth in the style. Finally, investigations focusing on the final stages of pollen function raise the possibility that a defect in the very last stages (i.e. either pollen tube guidance through the micropyle to the egg sac, or fertilization of the egg sac) may also contribute to the rop2 mutant delay. This work provides direct in vivo evidence confirming a role for Rop in male gametophyte development, and is the first study to demonstrate a role for Rop in the early stages of post-pollination gametophytic function. / Graduation date: 2005

Embryogenesis in plants, as in animals, requires the regulated expression of sets of genes involved in developmental processes. To gain insight into the processes regulating gene expression during embryogenesis differential screening was used to identify embryo-specific sequences in a cDNA library constructed from Zea mays embryo RNA. Four embryo-specific sequences and one constitutive sequence were characterized further by RNA blot hybridization and DNA sequence determination. The constitutive sequence and two of the embryo-specific sequences were found to encode parts of the previously-reported chloroplast 23S rRNA, Oleosin KD-18, and RAB-17 genes. Two sequences, named Emb5 and Emb564, were found to encode novel maize homologs of a gene expressed during late embryogenesis in a wide range of seed plants. These 5 genes exhibited differential temporal and spatial accumulation during development. Moreover, analysis of RNA from cultured embryos suggested that 4 of these genes were regulated by abscisic acid. The ABA-responsive genes could be divided into 3 classes, based on their developmental expression, tissue-specificity, and sensitivity to ABA. Antibodies raised against a $ beta$-galactosidase:EMB564 fusion protein were used to analyze the accumulation of the EMB564 and/or EMB5 proteins. These polyclonal antibodies detected one or several polypeptides with a molecular weight less than 14 kD which exhibited patterns of developmental accumulation and regulation similar to Emb5 and Emb564 transcripts.

The introduction of Quality Protein Maize (QPM), hard endosperm opaque-2 maize, into northern temperate maize growing areas is a desirable breeding objective. In topcrosses with opaque-2 testers, in diallel combination, as inbreds per se, and in inbred disease screening nurseries, some QPM lines performed better than or equal to the best local checks. In general, while agronomic potential is high for some lines and gains from selection are statistically possible, longer days to flowering intervals and higher levels of moisture at harvest than check hybrids indicated a need to improve adaptation for the locations studied. Methodology experiments indicated that detasselling of check hybrids is a suitable experimental method to facilitate the inclusion of normal endosperm local checks into QPM performance tests. The screening for Fusarium graminearum resistance in the seedling stage has not been proven to be a viable alternative to field scale ear inoculation screening procedures. (Abstract shortened by UMI.)

Thesis (Ph. D.)--University of Hawaii at Manoa, 1995. / Includes bibliographical references (leaves 228-240). / Microfiche. / xx, 240 leaves, bound ill. (some col.) 29 cm