Sugar kinases for cell wall biosynthesis

The products from photosynthesis can be used for energy metabolism, for growth and for the formation of storage compounds in plants. The partitioning of carbohydrates for the synthesis of cell wall polymers, required for growth, will be analysed in this proposal. Nucleotide sugars for the synthesis of biomass are synthesized de novo or are derived from a recycling pathway. In this proposal, enzymes required for the recycling of nucleotide sugars will be biochemically characterized and functionally tested. One of the involved sugar kinase has a novel domain of unknown function. Its role in transient starch storage will be analysed. The results are important for the breeding of energy crops, which predominantly use plant cell walls for energy conversion.

All plant cells are surrounded by a cell wall, which is essentially a network of different sugar polymers and sugar-containing glycoproteins. During the development of plants from young seedling to differentiated plants, the characteristics of the plants change. Cell walls in young seedlings are thin and flexible to enable appropriate growth. During development, the plants become larger and more stable, which can be achieved by strengthening cell walls. This requires a reorganization of cell wall components in which unused polymers are degraded. The released sugars are typically recycled and reactivated in plants so that they are available for incorporation into new polymers. This recycling route begins with sugar kinases, which attach a phosphate group to the C1 atom of sugar. These kinases can only phosphorylate one specific sugar at a time. Another enzyme that follows can then convert many different sugar phosphates into UDP sugars, the building blocks of polymer synthases. This project investigated the role of arabinose kinase. A long known point mutant (ara1-1) in the enzyme led to the assumption that arabinose is toxic to plants. We have characterized the enzyme biochemically and molecularly. It could be shown that arabinose is not toxic per se. A knockout in the arabinose kinase accumulates much higher amounts of arabinose than the point mutant without showing any signs of toxicity. This paradox could be clarified in parts. The ara1-1 mutant has minimal residual activity, which nevertheless leads to the accumulation of large amounts of UDP arabinose when feeding plants with arabinose, while the wild-type enzyme is inactivated by an unknown mechanism. As a result, the equilibrium of the various UDP sugars in a cell is disturbed. There are major changes in gene regulation, which are typically found in situations of hunger. Nevertheless, there is no objective hunger situation because the standard sugars glucose and sucrose are also available in sufficient quantities in ara1-1. A toolbox of transgenic plants has been established, which allows the testing of different explanations. Similar complex answers can be found in humans with regard to sugar toxicity by galactose (galactosemia in newborns), which is also only partially understood in mechanistic terms.