Pollen tetrades of wild type plants (left) and heterozygous knockouts (right) in the gene for UDP-sugar pyrophoshorylase. Note that during meiosis, two out of the four pollen die because they contain the defect copy of the gene

Recycling pathways for nucleotide sugars
Most of the nucleotide sugars for cell wall polymers are synthesized de novo from photosynthesis products. However the genome sequencing of many plants shows a large number of genes which are required for recycling of sugars. This process usually involves two steps:
1) A sugar-1-kinase sugar + ATP ———-> sugar-1-phosphate + ADP
2) A UDP-sugar pyrophosphorylase sugar-1-P + UTP ———-> UDP-sugar + PPi
The latter enzyme is encoded by a single copy gene in Arabidopsis. A knockout of this gene is lethal during pollen development (see picture) and possibly also during other steps of plant development. This indicates a far more important role of recycling pathways than previously believed. We analyze a homozygous knockout mutant in this gene (only possible by applying a genetic trick) to pinpoint the defects.
We have recently cloned a novel sofar unknown sugar-1-kinase form Lilium, catalyzing the activation of glucuronic to glucuronic-1-phosphate. This enzyme is part of the MIOX pathway for nucleotide sugars. (Pieslinger et al. (2010) J Biol Chem, 285: 2902-2910)