Elevated temperature reduces starch deposition in wheat endosperm by reducing the activity of soluble starch synthase

Abstract

Temperatures of more than 25° C adversely affect the activity of soluble starch synthase (SSS), an amyloplastic enzyme, in endosperm of wheat (Triticum aestivum L. cv. Mardler). Enzyme rate was found to have a temperature optimum between 20 and 25°C. This effect was apparently reversible after a short period of exposure to elevated temperature. We also found that with a prolonged period of exposure to elevated temperature there was another temperature-related phenomenon which caused a loss of enzyme activity that appeared to be much slower to reverse. We have termed this effect of temperature on SSS activity “knockdown”. The knockdown in SSS activity also occurred in-vivo. However, elevated temperature did not affect the activities of several other enzymes in the pathway of starch synthesis (ADP-glucose pyrophosphorylase, UDP-glucose pyrophosphorylase, sucrose synthase, phosphoglucomutase, phosphoglucose isomerase, bound starch synthase or hexokinase). Because the knockdown effect appeared to be specific to the enzyme SSS, we quantified the effect of knockdown on flux of carbon into starch and used these data to calculate the flux-control coefficient for SSS. Using data at 10–20°C the flux-control coefficient was CStarch10–20C = 0.50, whereas at 20–30° C the flux-control coefficient was CStarch20–30C = 1.38, and between 30–40°C the flux-control coefficient was CStarch30–40C = 0.69. Using data at 10–30°C the flux-control coefficient was CStarch10–30C = 1.15, and at 10–40°C the flux-control coefficient was CStarch10–40C = 0.82. In conclusion, we suggest that SSS is a major site of regulation of starch synthesis in developing wheat grain. During periods of high temperature the control point in the pathway of starch synthesis is apparently not associated exclusively with ADP-glucose pyrophosphorylase. In field conditions, in which temperatures are fluctuating, there will likely be periods of control of starch synthesis being exerted predominantly by SSS. During periods at lower temperature, control of flux may be exerted by SSS, perhaps in combination with other flux-controlling enzymes in the pathway. Our data point-out a crucial new aspect of quantifying control strengths of enzymes in plants: the determination of enzyme control strengths should be done in carefully regulated temperature conditions. Thus, since temperature is a major determinant of real flux through a pathway and the individual enzymes can respond differently to changing temperature conditions, the control strengths of individual steps in a pathway may vary with changing environmental conditions. This is particularly pronounced in starch deposition, because of the temperature instability of SSS.