Alternate routes for starch synthesis in developing grains of wheat and sorghum: Indirect evidence through its regulation by inorganic phosphates and organic acids

Abstract

The path of carbon from sucrose to starch in developing grains appears to be different in wheat (Triticum aestivum) and sorghum (Sorghum bicolor). In this paper we present indirect evidence in support of this hypothesis. Detached ears, carrying actively metabolising grains, were cultured in complete liquid media manipulated with respect to inorganic phosphates and organic acids. 14C‐incorporation studies indicated a higher level of metabolism of sucrose before starch formation in sorghum grain as compared to that in wheat grain. Starch and protein contents drastically decreased in sorghum grain in response to exogenously‐supplied inorganic phosphate and inorganic pyrophosphate. In contrast, these inorganic phosphates had no such effect on starch and protein accumulation in wheat grain, a‐ketoglutarate and citrate in sucrose‐free culture medium increased the starch content of wheat grain. Conversely, these organic acids markedly decreased the starch content of sorghum grain. This difference in the response of wheat and sorghum grains to inorganic phosphates and organic acids leads us to suggest that whereas triose phosphates act as precursors of starch in sorghum grain, hexose phosphates and/or adenosine 5'‐diphospho‐glucose may act as direct precursors of starch in wheat grain. This hypothesis is supported by the observation of higher specific activities of some selected glycolytic enzymes, such as phosphoglucoi‐somerase, ATP‐dependent phosphofructokinase, fructose 1,6‐bisphosphate aldolase, and triose phosphate isomerase in sorghum grain as compared to those in wheat grain.