Epsilon Open Archive

Abstract

Carbohydrates are important nutrients and structural components in all living organisms. In plants they affect the developmental and metabolic processes throughout the plant life cycle. However, the mechanisms by which plants recognise and respond to carbohydrates are mainly unknown. Hexokinase, an enzyme that mediates the first catalytic step in hexose metabolism, has recently been suggested to be involved in sugar sensing and signalling in plants. The moss Physcomitrella patens has recently emerged as a powerful model system in plant functional genomics following the discovery that gene targeting works in it with frequencies comparable to those in yeast. The aim of this thesis was to learn more about the function of plant hexokinases, both as key metabolic enzymes and in their putative role as sensors in sugar signalling, using Physcomitrella patens as a model system. Five hexokinases from Physcomitrella patens were cloned and studied with respect to their subcellular localizations. PpHxk1 and PpHxk5 are located in the stroma of chloroplasts and are dependent on N-terminal transit peptides for correct localization. PpHxk2 and PpHxk3 both contain hydrophobic membrane anchors that localize the proteins to the outer envelope of chloroplasts. PpHxk4 contain neither a transit peptide nor an anchor, and is found in the cytosol. A targeted knockout revealed that PpHxk1 is the major hexokinase in Physcomitrella, accounting for 80% of the glucose phosphorylating activity. Consistent with this, the knockout mutant exhibits an artificial starvation phenotype with altered sensitivities to plant hormones and a disturbed development.