Sucrose synthase and sucrose phosphate synthase, but not acid invertase, are regulated by cold acclimation and deacclimation in cabbage seedlings

Cabbage seedlings acquire freezing tolerance during exposure to low temperature (acclimation), however, it is lost by returning the plants to moderate temperature (deacclimation). Soluble sugars, except myo-inositol, accumulated during cold acclimation in the leaves of cabbage seedlings. During deacclimation for 5 days, sucrose, glucose and fructose contents decreased rapidly to the same levels as before cold acclimation. To identify the enzymes responsible for changes in sugar concentrations, we investigated changes in activities of sucrose synthase (E.C. 2.4.1.13), sucrose phosphate synthase (E.C. 2.4.1.14) and acid invertase (E.C. 3.2.1.26) during cold acclimation and deacclimation. Activity of sucrose synthase during cold acclimation increased up to 3 times the activity before cold acclimation, but decreased to the level of activity before cold acclimation during deacclimation. The activity of sucrose phosphate synthase also increased during cold acclimation, but decreased to the level of activity before cold acclimation during deacclimation. However, the activity of acid invertase decreased gradually during cold acclimation and did not increase to the level of activity before cold acclimation during deacclimation. These results suggest that SS and SPS, but not acid invertase, are regulated by cold acclimation and deacclimation and play important roles in sugar accumulation and acquisition of freezing tolerance in the leaves of cabbage seedlings.