Gerd Albrecht

Sucrose synthase activity does not restrict glycolysis in roots of transgenic potato plants under hypoxic conditions

Abstract. The effect of hypoxia on root development and carbon metabolism was studied using potato (Solanum tuberosum L.) plants as a model system. Hypoxia led to a cessation of root elongation, and finally to the death of meristematic cells. These changes were accompanied by a 4- to 5-fold accumulation of hexoses, suggesting that insufficient carbohydrate supply was not the cause of cell death. In addition, prolonged hypoxia (96 h) resulted in a 50% increase in activity of most glycolytic enzymes studied and the accumulation of glycerate-3-phosphate and phosphoenolpyruvate. This indicates that endproduct utilisation may restrict metabolic flux through glycolysis. As expected, the activities of alcohol dehydrogenase (EC 1.1.1.1) and pyruvate decarboxylase (EC 4.1.1.17) increased during hypoxia. Apart from the enzymes of ethanolic fermentation the activity of sucrose synthase (SuSy; EC 2.4.1.13) was enhanced. To investigate the in-vivo significance of this increase, transgenic plants with reduced SuSy activity were analysed. Compared to untransformed controls, transgenic plants showed a reduced ability to resume growth after re-aeration, emphasising the crucial role of SuSy in the toleration of hypoxia. Surprisingly, analysis of glycolytic intermediates in root extracts from SuSy antisense plants revealed no change as compared to wildtype plants. Therefore, limitation of glycolysis is most likely not responsible for the observed decreased ability for recovery after prolonged oxygen starvation. We assume that the function of SuSy during hypoxia might be to channel excess carbohydrates into cell wall polymers for later consumption rather than fuelling glycolysis.

Sucrose Utilization via Invertase and Sucrose Synthase with Respect to Accumulation of Cellulose and Callose Synthesis in Wheat Roots under Oxygen Deficiency

The sucrose cleavage by sucrose synthase (SuSy) and neutral invertase was studied in wheat roots (Triticum aestivum L.) subjected to hypoxia or anoxia for 4 days. By in situ activity staining, increased SuSy activity was observed in the tip region and stele of root axes while the activity of invertase decreased. Cellulose content significantly increased in hypoxically treated roots. The cellulose deposition was correlated with regions of high SuSy activity, being mainly located in the pericycle and endodermis. Invertase activity was distributed along the root without clear difference between cortex and stele. Under root hypoxia, a significant increase in the structural carbohydrates, callose and especially cellulose, was shown. Increasing levels of soluble carbohydrates were partially used to synthesize cellulose for secondary wall thickening and callose to counteract the tissue injury following low-oxygen stress. Under strict anoxia, the roots were much more injured but sustained a high level of cellulose and callose while the soluble carbohydrates almost disappeared.

CONTENT OF WATER SOLUBLE CARBOHYDRATES UNDER OXYGEN DEPRIVATION IN PLANTS WITH DIFFERENT FLOODING TOLERANCE

We investigated the carbohydrate concentration of plant species naturally growing in habitats with a high risk of oxygen shortage (Senecio aquaticusHill,Myosotis palustris (L.) L. em.Rchb) and congeneric species from drier sites (Senecio jacobaea L.,Myosotis arvensis (L.)Hill) Plants from the four species were cultivated in either nitrogen-flushed or aerated nutrient solution.Following oxygen shortage in the root environment a two—fourfold increase of carbohydrate content was found in the roots as well as in the shoots of the species examined. Although, the tendency in the response of both genera to the hypoxic conditions in the nutrient solution was the same, there were differences in concentration and composition of the water soluble carbohydrates. The flooding tolerantSenecio aquaticus accumulated the highest amounts of carbohydrates, in particular fructans (nearly 60% of the soluble carbohydrates, compared with 30% under aerated conditions). The increasing amount of sugars found under hypoxia-inducing conditions proved that substrate availability was not the limiting factor for survival under wetland conditions.