Apoplastic invertase: A dissection of its role in photosynthate translocation and partitioning

Date of Completion

January 1998

Keywords

Biology, Molecular|Biology, Cell|Biology, Plant Physiology

Degree

Ph.D.

Abstract

Based on its location, localization, kinetic properties, and relationship to membrane associated hexose symporters, the cell wall bound acid invertase from higher plants has been implicated in playing a key role in the partitioning of carbohydrates between the various sink organs. The pressure flow hypothesis states that the translocation of sucrose is driven by osmotic potentials established at the source and sink regions of the plant. This passive pressure driven long-distance transport of phloem sap is dependent on the active short-distance transport mechanisms of phloem loading and unloading. In plants, where the apoplast is part of the transport pathway, extracellular sucrose hydrolysis by the activity of cell wall bound acid invertase with the concomitant uptake of free hexose molecules would provide a strong directionality to the overall transport pathway.^ In order to test this hypothesis transgenic plants were generated via Agrobacterium mediated gene transfer utilizing sense and antisense Ti binary plasmid constructs for the controlled or localized over-expression or inhibition of apoplastic invertase. The results of these experiments are discussed.^ An acid invertase encoding genomic sequence from Arabidopsis thaliana ecotype Colombia was isolated and characterized. The theoretical isoelectric point of the encoded protein (9.74) and its high sequence similarity to other known cell wall acid invertases indicate that this gene encodes an extracellular acid invertase isoenzyme. Arabidopsis contains two cell wall invertase isoenzymes which share 54.5% homology at the level of amino acids. Their corresponding mRNAs are shown to exhibit independent tissue specific expression patterns. Isoenzyme number one is predominantly expressed in developing roots and young seedlings. The second isoenzymes is detected primarily in flower tissues. Based on a phylogenetic analysis it appears that the extracellular invertases from Arabidopsis belong to two distinct evolutionary groups. The bifurcation event leading to the two groups probably predates the monocot-dicot divergence, and most likely occurred previous to the evolution of the various taxonomic families used in the analysis. Moreover, there appears to be a loose correlation between the phylogenetic placement and tissue specific expression patterns associated with the higher plant extracellular invertases from the two lineages. ^

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