Synthesis and Function of Glyoxylate Cycle Enzymes

The glyoxylate cycle in plants has been the subject of much research for several decades, and many hundreds of primary research papers have been published. There have been numerous reviews covering different aspects of the synthesis and function of glyoxylate cycle enzymes (Beevers, 1979; Trelease, 1984; Escher and Widmer 1997; Eastmond and Graham, 2001). Here we choose firstly to summarize the main features of the glyoxylate cycle as understood from the many studies of its role in lipid metabolism in seedlings of oilseed species. Then we focus on aspects that have been discovered since the previous treatise on plant peroxisomes (Huang et al, 1983) and highlight those aspects which are still to be understood. Among these topics are the discovery that aconitase is a cytosolic enzyme while other key enzymes of the cycle are peroxisomal, and that the metabolic pathway of the cycle is still unknown, principally because we do not know how reducing equivalents are exported from the peroxisome. We review the role of the glyoxylate cycle in seed and pollen development, senescence and starvation, highlighting different potential functions. The importance of genetic aproaches for future research is illustrated by studies of knock-out mutants of both isocitrate lyase and malate synthase in Arabidopsis thaliana.