Thesis (M. Sc.)--University of Stellenbosch, 2002. / Includes bibliographical references. Also available via the Internet.

Thesis (doctoral)--University of Göttingen, 2002. / Includes bibliographical references (p. [102]-118).

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Soilborne plant pathogens are responsible for many of the major crop diseases worldwide. However, plant root tips are generally resistant to pathogen infections. The goal of this dissertation research is to understand the mechanism of this natural resistance by testing the hypothesis that root caps and root border cells control the rhizosphere community through the biological products which they deliver to the soil. Specific objectives of this dissertation project are 1) identifying, isolating, and characterizing the genes important for border cell development and for root exudates delivery, and 2) analyzing the function of extracellular macromolecules in root exudates in root tip-fungal pathogen interaction. The expression of a primary cell wall synthesis gene, PsFut1, encoding Pisum sativum fucosyltransferase, was characterized during border cell production, and the impact of silencing this gene on border cell development was examined. Another gene, BRDgal1, encoding β-galactosidase, was identified and characterized in Pisum sativum during this study. It was shown that this β-galactosidase is specifically produced in and secreted from root border cells. The microarray transcriptional profiling in M. truncatula and mRNA differential display analysis in pea plants were carried out following the induction of border cell production to gain a broader understanding of the genes which potentially influence border cell development. In order to study the commonality of border cell production across different plant species, the expression of rcpme1, the marker gene for border cell production, was compared between the garden pea and a gymnosperm species, the Norway spruce (Picea abies). To accomplish the second objective, the focus of this study was shifted from border cell development to mucilaginous root exudates excreted by border cells and root cap cells. This resulted in a breakthrough in the understanding of the mechanisms of root tip resistance. The presence of extracellular DNA in the root mucilage was discovered and its requirement for root tip resistance to fungal infection was demonstrated. Extracellular proteins in the root mucilage were identified and they were shown to be also required for the root tip resistance to fungal infection. This work provided new insights into understanding plant defense mechanisms.

Įvertinus 12 juodųjų serbentų (Ribes nigrum L.) veislių ir klonų derlių nulemiančių svarbiausių komponentų įtaką: atsparumą šalčiui, grybinėms ligoms, serbentinei (pumpurinei) erkutei, lapų žiotelių parametrus ir nustačius jų tarpusavio ryšį – išskirtos veislės tinkamos mechanizuotam derliaus nuėmimui, vertingi agronominiu požiūriu donorai, naujoms veislėms kurti. Veislė Tines yra vėlyvo žydėjimo. Veislės Titania, Tiben ir klonas 98 – 279 – 25 darnaus uogų sunokimo donoras. Mechanizuotam uogų skynimui tinkamiausios veislės Tines, Ruben ir Dailiai. Daugiakiakiškumu išsiskyrė veislės Ores, Ceres, Ruben ir klonas 98 – 279 – 25. Mažiausias sėklų procentas uogoje veislių Titania ir Tisel. Veislės Tines, Bona, Ruben, Dailiai ir klonas 98 – 279 – 25 yra stambiauogiškumo, o klonas 93 – 157 – 1 yra tiek stambiauogiškumo, tiek produktyvumo donoras. Vaiskrūmiai buvo santykinai atsparūs miltligei, vidutiniškai atsparūs šviesmargei ir deguliams bei skirtingo atsparumo serbentinei (pumpurinei) erkutei. Žiotelių ilgis susijąs su krūmų pažeidimu grybinėmis ligomis. Krūmų pašalimas susijąs su pažeidimu deguliais ir serbentine (pumpurine) erkute. / The aim of the research: to investigate and derive the varieties suitable for the machine harvesting valuable as agronomic donors for the development of new varieties. For this reason 12 different blackcurrant varieties and clones were investigated to determine the most important factors influencing their yield: frost , fungal diseases, currant gall mite, leaf stomata parameters and the relationship between them. Tines is a variety of late – flowering. Varieties Titania, Tiben and a clone 98 – 279 – 25 are donors for a sustainable fruit production. The research revealed that the varieties Tines, Ruben and the Dailiai are the most suitable for the machine harvesting. Varieties of Ores, Ceres, Ruben and clone 98 – 279 – 25 proved to produce maximum number of bunches. The minimum quantity of seeds was found in berries of Titania and Tisel varieties. Varieties Tines, Bona, Ruben, Dailiai and clone 98 – 279 – 25 proved to be a large – berry producing variety, whereas a clone 93 – 157 – 1 was determined to be the most productive and large- berry producing variety. Blackcurrant berry shrubs were relatively resistant to powdery mildew, the average resistance was revealed to leaf septoria and stem canker, and the cultivars demonstrated different resistance to the gall mite. According to the research, the stomata length is associated with the damage of the cultivars by fungal diseases. Plant frost damage proved to be closely related to plant being infected by canker and gall mite... [to full text]

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No description available.

Eucalyptus, pine and wattle are the predominant exotic wood species used in the production of dissolving pulp in South Africa. On entering the mill, wood is chipped and stored in outdoor piles where it becomes vulnerable to microbial degradation and spontaneous combustion. Major losses of stored chips are due to high temperatures and combustion caused by heat energy released by microbial fermentation. Changes in the chemistry of the wood chips caused by the metabolic activity of indigenous microflora combined with the inherent chemical characteristics of each wood species could have a potential impact on final pulp quality and yield. Therefore the objective of this study was to analyse the microbial (bacteria and fungi) communities present in commercial wood chip piles and correlate this with changes in the chemistry of the wood in summer and winter. The molecular fingerprinting technique of Denaturing Gradient Gel Electrophoresis (DGGE) was optimized for the detection of microbial diversity in commercial wood chips. Wood chips were collected from an industrial wood yard and milled to different specifications. A total of four primer sets with GC-clamps were tested in nested PCR for DGGE analysis. 16S and 18S rRNA genes were amplified using 338f-GC/518r; 933F-GC/1387R (bacteria) and NS26/518R-GC; EF4F/518R-GC (fungi), respectively. Several gel gradients were examined to determine optimal separation of bacterial (40/60%, 35/50%, 30/60%) and fungal (35/50%, 20/45%, 25/50%) PCR-DGGE products. Comparison of the DGGE profiles revealed greater diversity in the milled wood chips amplified using primer sets; 338F-GC/518R (16S) and NS26/518R-GC (18S) with gradients of 30/60% (16S) and 25/50% (18S). Once optimized, this standardized protocol was tested against five samples to assess its applicability to woodyard samples. 16S and 18S DGGE profiles were generated and amplicons excised from gels, re-amplified, sequenced and the microorganism from which the DNA originated was determined. In the second phase a cross-sectional study of wood chip piles from a commercial dissolving pulp mill was conducted with sample collected in summer and winter using the optimized PCR-DGGE technique. Microbial strains were identified after sequencing of 16S and 18S rRNA amplicons separated by DGGE. Chemical characteristics of the wood chips were evaluated by conducting extractive analyses using HPLC. Due to unpredictable combinations of different wood species in commercial wood chip piles, the third phase involved the investigation of individual Eucalyptus species. The microflora indigenous to the two Eucalyptus species (E. dunnii and E. nitens) and a combination of the two were subjected to winter and summer simulations for one month during which samples were tested for wood chemistry properties, microflora and the final samples were used to generate dissolving pulp. Using the PCR-DGGE method eighteen bacterial and twelve fungal species were identified from the five samples collected from the commercial wood chip pile, compared to the ten bacterial and nine fungal isolates which were identified using the culturing technique and standard 16S and 18S rRNA gene sequence analysis. Predominant genera in the optimization phase of this study were Klebsiella spp. (×3), Bacillus spp. (×2), Pantoea spp. (×2), Pseudomonas spp. (×2) and Paecilomyces spp. (×2). Application of the optimized DGGE technique to samples collected from the commercial pulping mill in summer and winter revealed variable profiles indicating a range of bacterial and fungal strains that varied in intensity in the areas and seasons sampled. Seventy nine (45 in summer and 34 in winter) and 29 (20 in summer and 9 in winter) distinct amplicons representing bacteria and fungi, respectively, were visualized. Predominant genera in summer were Pantoea rodasii, Inquilinus limosus, Streptococcus sp., Klebsiella spp., Diversispora sp., Boletaceae sp., Scutellospora sp., and Ophiostoma bicolour. In winter the prevailing genera were Leuconostoc palmae, Streptococcus sp., Bacillus spp., Diversispora sp., Boletaceae sp., and Bullera sp. Lower cellulose levels in summer correlated significantly with high microbial loads and the predominance of Bacillus spp., suggesting that in warm humid environments storage should not exceed 1-2 weeks. No correlations were determined between the decreased hot water levels in winter and microbial activity, however they were correlated to increased exposure of those samples to environmental factors. Chemistry data on the wood chips imparts the quality of the wood which only permitted projection of final pulp quality. This inadequacy was addressed in the third phase which included identification of microbial strains, originating from the individual Eucalyptus species, after sequencing of 16S and 18S rRNA amplicons separated by DGGE. Fungal and bacterial species were also isolated, cultured, identified and screened for lignocellulolytic enzyme activity. Ninety two and 88% of the fungi isolated were capable of producing cellulase and xylanase, respectively. Significant correlations exist between the microflora, seasons (greater diversity and loading in summer) and the chemical and physical properties of wood chips (lower cellulose and viscosity in summer) as well as Eucalyptus species (significantly higher cellulose and viscosity for the combination and E. nitens). Indigenous microflora of each wood species may be one of the contributing factors to poor/good pulp quality, as significant correlations were made between enzyme production of microorganisms and wood chemistry which ultimately has an impact on the final pulp quality and yields. This investigation provides proof of concept that combining wood species with different deterioration rates results in an overall improvement in pulp quality and thus paves the way for a practical and applicable approach to managing quality of chips. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2013.

This research has focused on the determination of natural populations in the fields, the effect of different inoculum densities on lettuce growth and a study of the association of this fungus with two nematodes (Pratylenchus penetrans Cobb and Meliodogyne hapla Chitwood). Under conditions of artificial infestation of soil the results were satisfactory, but in trials with naturally infested soil the fungus could not be detected. The effect of different inoculum densities was measured at different stages of growth, and only in those plants inoculated 2 weeks after seeding were differences significant and consistent. Some evidence of the detrimental effect of wounding the root system prior to attack by the fungus led to studies of the relationship between this fungus with either P. penetrans or M. hapla. In the first case a negative interaction seemed to exist; no significant increase of the damage caused to the lettuce was observed. In contrast, when the root-knot nematodes and P. tracheiphilum were combined there was a marked reduction of lettuce growth. The interaction was found to be additive.