Thesis (MSc (Genetics. Institute for Plant Biotechnology)--Stellenbosch University, 2008. / Strawberry (Fragaria x ananassa) fruit quality is largely determined by the relative amounts of
sugars and organic acids present, as well as soluble solid content. This study had three components:
1) Characterisation of cytosolic carbohydrate metabolism and carbon partitioning to sugars and
organic acids in two commercial varieties, 2) analysis of transgenic strawberry fruit with increased
pyrophosphate: D-fructose-6-phosphate 1-phosphotransferase (PFP) activity and 3) analysis of
transgenic strawberry fruit with increased ß-fructosidase (invertase) activity in either cytosol or
apoplast. Analyses of transgenic strawberry may inform similar attempts in grape berries.
Festival and Ventana, two popular commercial strawberry cultivars in South Africa, were fairly
similar with respect to sugar and organic acid content. Twelve cytosolic enzymes were
investigated. Temporal differences in maximum catalytic activity were observed for invertase, PFP,
pyruvate kinase and ADP-glucose pyrophosphorylase (AGPase). Invertase, PFP and AGPase
activity also differed between the cultivars. One enzyme, SuSy, could not be analysed effectively,
due to the purification method employed. These analyses established methodology for the analysis
of transgenic berries.
Constructs were designed to constituitively express Giardia lamblia PFP (GL-PFP), or to
express Saccharomyces cerevisiae invertase (SCI) in a fruit-specific manner. A second invertase
construct was designed to target SCI to the apoplast. Strawberry (cv. Selekta) was transformed and
the presence of each transgene confirmed by PCR. Untransformed Selekta was used as control in
both transgenic studies.
Transgenic lines were selected based on GL-PFP activity in leaves and total PFP activity in ripe
fruit. Sugar and organic acid content of ripe berries with high PFP activity was determined.
Although berries displayed marked changes in sugar composition, the total sugar content was
similar to controls, in all except one line. Organic acid content was decreased, leading to a clear
reduction in organic acid-to-sugar ratio. This points to a gluconeogenic role for PFP in strawberry
fruit.
Transgenic berries were screened for SCI activity. Berries containing untargeted SCI exhibited
total invertase activity similar to controls and were not analysed further. Berries with apoplasttargeted
SCI displayed three-fold increases in invertase activity compared to controls. Total sugar
content was reduced and exhibited reduced sucrose content relative to hexoses. Despite the effect
of increased invertase activity on metabolites, maximum catalytic activity of enzymes involved in
cytosolic sucrose, hexose and organic acid metabolism were unchanged. Transgenic plants selected
in these studies were subsequently vegetatively replicated and future work will include immature
fruit.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2295 |
| Date | 12 1900 |
| Creators | Basson, Carin Elizabeth |
| Contributors | Groenewald, J.-H., Bauer, R., Stellenbosch University. Faculty of AgriSciences. Dept. of Genetics. Institute for Plant Biotechnology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | Stellenbosch University |
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