Mechanisms of Sugar Uptake Into Endosperm and Aleurone Protoplasts Isolated From Developing Wheat Grains

Abstract

Grain-filling in cereals depends upon imported assimilates, and in wheat all assimilates entering the grain pass through the endosperm cavity which is part of the apoplast separating the maternal from the filial generation. The outermost layer of the endosperm, the aleurone layer is in contact with the contents of the endosperm cavity. Entry into the endosperm necessitates passage through the plasmalemma. Putative mechanisms involved in the transport of carbohydrate into the endosperm have been evaluated by characterising the mechanism(s) transporting sugars into protoplasts prepared from cells isolated from the aleurone layer and the starchy endosperm separately.

Protoplasts obtained from both types of cells absorbed sucrose, glucose and fructose by diffusion. Only in the case of sucrose uptake by aleurone protoplasts was there evidence for a saturable carrier mechanism. Based on calculated rates of uptake by the protoplasts it can be estimated that transport by diffusion is sufficiently fast to support observed rates of starch synthesis, and that carrier mediated transport of sucrose can account for only a small fraction of the total import.