Browsing by Author "Venter, Alida"
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- ItemThe functional analysis of Vitaceae polygalacturonase-inhibiting protein (PGIP) encoding genes overexpressed in tobacco(Stellenbosch : University of Stellenbosch, 2010-03) Venter, Alida; Vivier, Melane A.; Joubert, D. A.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.ENGLISH ABSTRACT: Agriculture worldwide is under great pressure to produce enough food in order to sustain the ever-growing world population. Among the many challenges faced by food producers, crop losses and damage caused by fungal plant pathogens is a major problem. The study of fungal pathogens and the interaction between plants and fungi is therefore essential, and has been carried out for many years. Much has been learned in this time, but the full mechanisms of the various modes of fungal attack and plant defence have still not been elucidated. Many fungi rely on the action of cell-wall degrading enzymes (CWDEs) to breach the plant cell wall and facilitate access to the nutrients within. CWDEs are among the very first enzymes to be secreted at the start of fungal attack, and many of them are considered to be essential pathogenesis factors. Endopolygalacturonases (ePGs) are CWDEs that cleave the homogalacturonan stretches of the plant cell wall and are vital virulence factors for a number of fungi, including Botrytis cinerea. An important defence mechanism of plants involves the inhibition of CWDEs in order to halt or slow down the fungal attack. Plant polygalacturonaseinhibiting proteins (PGIPs) are cell wall associated CWDE-inhibiting proteins that specifically act on fungal ePGs. Many different PGIPs from a number of diverse plant species have been described to date. They are known to have differential inhibition capabilities that often result from only a few key amino acid changes within the leucine-rich repeat (LRR) active domains. Previously, the first grapevine PGIP was isolated and characterised from Vitis vinifera cultivar Pinotage (Vvpgip1). This Vvpgip1 gene was overexpressed in the tobacco species Nicotiana tabacum, and was shown to be very effective in reducing the susceptibility of tobacco towards B. cinerea. The combined results confirmed transgene overexpression, increased PGIP activity and a strong resistance response against Botrytis, leading to the characterisation of these lines as having PGIP-specific resistance phenotypes. In a subsequent transcriptomic analysis of these lines it was found that they display differential expression of cell wall metabolism genes and biochemical characteristics that might indicate possible cell wall strengthening compared to wild-type tobacco under uninfecting conditions. The V. vinifera cultivars are all very susceptible to fungal attack, whereas other grapevine species, specifically the North American Vitis species, are known for their strong resistance and even immunity against many fungal pathogens. Thirty seven PGIPs have previously been isolated from these more resistant species. The amino acid sequences of the active domains of these PGIPs were previously aligned with that of VvPGIP1, and the proteins were found to be highly homologous with each other and with VvPGIP1. The different nonvinifera PGIPs separated into 14 subgroups based on their active domain sequences. For this study, one PGIP from each group was selected for functional analysis in tobacco. The selected PGIP-encoding genes were transformed into tobacco by means of Agrobacterium tumefaciens. Analyses of the putatively transformed plantlets were performed to test for transgene presence, transgene expression, and PGIP activity: final transgenic tobacco populations consisting of three to twelve individually transformed lines of nine different nonvinifera PGIPs were obtained. A subset of the resultant transgenic lines was infected with B. cinerea in two independent whole plant infections over 11-14 days in order to investigate the disease resistance afforded by the various PGIPs towards this fungus. A line from the previously characterised VvPGIP1 population was included as reference; all the infections were contrasted to the WT tobacco. All the infected lines overexpressing the non-vinifera PGIPs displayed very strong disease reduction in comparison to the WT control: after initial primary lesion formation, the spread of fungal infection was contained and halted in these lines, while wild-type tobacco plants were severely affected. Although the VvPGIP1 line displayed the characteristic PGIP-defense response, the non-vinifera PGIP plants displayed smaller lesions, indicating very strong resistance phenotypes. The characterised non-vinifera PGIP overexpressing lines, together with the VvPGIP1 line and the WT control were also used to further evaluate the previous observation that overexpression might lead to changes in expression of cell wall genes. Analysis of the expression of a xyloglucan endotransglycosylase (xth) gene in the transgenic population showed that this gene was down-regulated in healthy uninfected tissue from all the transgenic lines tested. This confirmed previous results and have confirmed in all grapevine PGIP overexpressing lines tested so far that this gene is downregulated. XTH is typically involved in cell wall metabolism and specifically in controlling the strength and elasticity of the plant cell wall. From previous work it is known that downregulation of this gene leads to strengthening of the wall. The results obtained in this study showed that the PGIP-specific resistance phenotype seen for VvPGIP1-overexpressing tobacco could be confirmed in transgenic tobacco overexpressing non-vinifera PGIPs from more resistant grapevine species as well. The fact that these PGIPs lines all performed even better than the VvPGIP1 lines in conferring resistance towards B. cinerea provides an interesting angle for further investigation into the structural differences between the non-vinifera PGIPs and VvPGIP1. The transgenic lines are also excellent material to study the in vivo functions of PGIPs further in the context of plant-pathogen interactions.