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.

Call number: LD2668 .T4 1986 K84 / Master of Science / Grain Science and Industry

Call number: LD2668 .T4 1957 O94 / Master of Science

Call number: LD2668 .T4 1958 C45 / Master of Science

M.Sc. / Plants have evolved a complex array of biochemical pathways that enable them to recognise and respond to signals from the environment. At present, little is known about the signal transduction pathways that are activated during a plant's response to attack by a pathogen, although this knowledge is central to our understanding of disease susceptibily and resistance. A common form of plant resistance is the restriction of pathogen proliferation to a small zone surrounding the site of infection. In many cases, this restriction is accompanied by localized death of host tissues, known as the hypersensitive response. In addition to local defense responses, many plants respond to infection by activating defenses in uninfected parts of the plant. As a result, the entire plant is more resistant to a secondary infection. This systemic acquired resistance can persist for several weeks or more and often confers crossresistance to unrelated pathogens. Fungal polygalacturonases (PGs) catalyze the fragmentation and the solubilisation of the homogalacturonan in the plant cell wall. These enzymes might have important functions during plant colonization by a fungus. PGs have also been shown to activate plant defense responses, likely because they generate oligogalacturonides with elicitor activity from the plant cell wall. A polygalacturonase inhibiting protein (PGIP), found in the plant cell wall of many plants, forms a specific complex with fungal PGs and favours the accumulation of elicitor-active oligogalacturonides in vitro. An agarose diffusion assay was used to screen the extracts from Verticillium dahliae for PG activity and ensuing inhibition by purified cotton PGIP. Quantitative determination of differences in polygalacturonase activity in the extracts were performed using a reducing sugar assay. There may be more than one isoform of PG present since the polygalacturonases produced by fungi are likely to be to a mixture of exo- and endo-PGs. Polygalacturonase was therefore isolated from 18-day-old culture filtrates of V. dahliae. The enzyme was partially purified by means of ammonium sulphate precipitation and gel chromatography. The band responsible for PG activity was identified and characterized, having a molecular weight of approximately 28-31 kDa, and a pl of 5.1 - 5.9. Kinetic studies indicate a Km of 0.33% and V,„,,of 0.85 pmoles reducing units / min. A commercial preparation of endo-PG from Aspergillus niger was used as a control. This endo-PG had a molecular weight of 68 kDa and a pl point of 3.6 and 5.1, suggesting there were at least two isoforms of endo-PG present. Kinetic studies indicate a K m of 0.33% and V,,„ of 1.07 gmoles reducing units / min.

Solanum lycopersicon (the cultivated tomato) is a commodity of great economic importance in South Africa (SA) as well as worldwide. A destructive viral disease known as Tomato curly stunt virus, ToCSV-[ZA:Ond:98], belonging to the genus Begomovirus has negatively impacted on tomato production in SA. This has brought about the need to develop resistant cultivars to ToCSV. Since all cultivated tomato cultivars are susceptible to ToCSV, resistance genes against the virus found in wild tomato plant species have been introgressed into the cultivated tomato by plant breeding techniques. Wild relatives of tomato were adapted to many pathogens (including viruses) as well as stresses from the surrounding environment. During breeding for improved fruit quality and increased yield, the gene networks giving rise to many biotic and abiotic stress resistances have been lost leaving the domesticated tomato extremely susceptible. Plant breeders have reconstituted some of the gene networks into the cultivated tomato that provide tolerance to stresses including viruses. They have achieved this by the help of marker-assisted selection (MAS), where the associated marker is used as an indirect selection criterion. This is an important process in commercial breeding programs as it allows for a speedy selection of selected traits in the development of tomato hybrids. The defence response to abiotic stresses in plants includes the expression of heat shock proteins (HSPs) that function as stress response proteins, molecular chaperones and proteases which repair or degrade damaged proteins. The objective of this study was to elucidate the type of resistance mechanism of a tomato inbred line (TAM), to ToCSV. Since TYLCV-IL shows 77% nucleotide identity with ToCSV, molecular markers already established for the detection of resistance genes for TYLCV-IL were used to screen TAM. The inbred line, TAM, was screened for the absence of any of the known resistant genes to TYLCV-IL using molecular markers already established for the screening of TYCLV-IL resistance genes. TAM was crossed with susceptible cultivar, Rooikhaki, to produce F1 hybrids. These F1 hybrids were selfed to produce an F2 population. Infection trials using ToCSV were conducted using TAM inbred line, F1 hybrids and the F2 population. Since TAM did not have any of the known resistance genes to TYLCV-IL, a possible novel resistance source to ToCSV was speculated. A clue to the resistant mechanism against ToCSV resistance in TAM was indicated by the segregation patterns of the F2 population after inoculation with ToCSV. The results suggest that the resistance is under the control of partially dominant resistant genes. The level of resistance of commercial South African tomato cultivars (Tyler and Tovi-star) against TYLCV-IL was investigated. The heat shock protein (HSP) profiles of these two SA lines including susceptible cultivar, Rooikhaki, were treated with abiotic stresses (salt and heat) and results were compared with a similar study conducted with TYCLV-IL resistant and susceptible tomato cultivars. Heat shock protein 70 accumulation patterns were similar in that HSP70 was more stable in the resistant cultivars throughout the application when abiotic stresses were applied to the SA resistant and susceptible tomato cultivars as compared to Israel resistant and susceptible breeding lines. A relation between infection severity and the pattern of HSP expression was found. A higher level of HSP 70 in resistant tomato plants could contribute to a lower symptom severity phenotype.

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