Robert Behmüller

Abstract

This work’s focus is on a not yet characterized sugar-1-kinase from plants, in particular the two isoforms of arabinokinase and its mutant form ara1-1. The mutant ara1-1 has a characteristic lethal phenotype when fed with arabinose. The mutation is close to the substrate binding site and changes the Km-value for arabinose from 80 µM in the wild type to 17 000 µM in ARA1-1.
The previous arabinose toxicity explanation is challenged by knockout plants in arabinokinase, which accumulate higher levels of arabinose but do not show signs of arabinose toxicity. Analysis of marker genes from sugar signaling pathways (SnRK1 and Tor) suggest that ara1-1 misinterprets its carbon energy status during L-arabinose feeding. Though glucose is present in ara1-1 similar to wild type levels, it constitutively changes gene expression as typically found in wild type plants only under starvation conditions. Furthermore, ara1-1 shows increased expression of marker genes for programmed cell death as found in other lesion mimic mutants. We have also tested the effects of overexpression of arabinokinase in combination with arabinose feeding, leading to the conclusion that a functional arabinokinase domain is required to induce toxicity. Up to now the function of the glycosyltransferase remains unknown, however, possible targets of this transferase are being investigated. One of which is a new polysaccharide located in the cytoplasm, called soluble heteroglycans (SHG).