The effect of water stress on the activities of key regulatory enzymes of the sucrose to starch pathway in wheat

Developmental changes in the starch and sucrose content of grains and the activities of enzymes of starch synthesis in wheat were studied under water stress conditions. Water stress caused a marked reduction in the sucrose and starch content of the grains. Sucrose synthase (SS) and UDP-glucose pyrophosphorylase (UDP-Gppase), showed higher catalytic activity and more resistance to water stress compared with amyloplastic enzymes. ADP-glucose pyrophosphorylase (ADP-Gppase) activity was reduced to a low level under both in situ and osmotic stress conditions in which grains failed to accumulate dry matter in vivo. Granule-bound starch synthase (GBSS) also responded rapidly to in situ water stress treatments as did ADP-Gppase. Reduction in GBSS activity at the time of growth cessation in situ was less than that of ADP-Gppase and the enzyme did not respond to severe osmotic stress. Soluble starch synthase (SSS) was the enzyme most sensitive to water stress in that it responded earlier, and to a greater extent, than the other enzymes. However, under severe dehydration conditions, leading to cessation of growth, the decline in SSS activity was less than that for ADP-Gppase. SSS showed the lowest in vitro activity followed by GBSS. These results suggest that SSS is the site of response to water stress by which the rate of grain growth can be affected, whereas growth cessation is due mainly to the inactivation of ADP-Zn Gppase.