Effect of Temperature on Enzymes in the Pathway of Starch Biosynthesis in Developing Wheat and Maize Grain

Abstract

Soluble starch synthase (SSS) is shown to be a major site of control of flux through the pathway of starch synthesis in developing wheat and maize grain. Temperatures above 25ºC adversely affect flux, and therefore, limit yield. This process is linked to SSS which is heat sensitive. Two apparently different properties of SSS can be identified which differ in the period required before full activity is restored after heat treatment. First, enzyme rate is adversely affected by elevated temperature, an effect which is reversible on returning to a lower temperature. The effect on enzyme rate was quantified using enzyme Q₁₀ which was found to begin to be sub-optimal above 20ºC. Second, with a prolonged period of exposu;e to elevated temperature there is a loss of enzyme activity which is not freely reversible which we have termed thermal inactivation. Although this occurs at temperatures in excess of 20ºC in wheat, higher temperatures of more than 30ºC are needed in maize SSS. Elevated temperature did not affect the inherent stability or Q₁₀ characteristics of other enzymes in the pathway of starch synthesis except for branching enzyme which we believe has minimal flux-control strength. SSS thermal inactivation may not be a major problem in field conditions for developing maize grain, because temperatures rarely are high enough. However, we suggest that the effect on enzyme Q₁₀ is more physiologically relevant, since maize SSS is operating sub-optimally as temperatures exceed 20ºC. Calculations of the reductions in maize US corn-belt yield showed that significant yield improvement might be obtained by a 5ºC shift in the temperature optimum. Thus selections for a more temperature tolerant form of maize SSS were conducted using enzyme Q₁₀ as a selection tool. Of several hundred maize specimens screened, two were found to be significantly different. However, attempts to use backcross breeding to transfer this trait from the tropical donor to another line have not yet succeeded. Transgenic approaches to altering relations of starch deposition are now underway.