ALPHA-AMYLASE ISOFORM PATTERN CHANGES DURING THE WINTER SEASON IN THEWINTER-RESTING STEM INTER-NODES OF VITIS-VINIFERA

In order to better characterize the potential role played by alpha-amy lases (EC 3.2.1.1) during the winter, changes in alpha-amylase isoform pattern were first analyzed as the season progressed. Crude protein e xtracts from winter-resting: stems of the grapevine plant Vitis vinife ra were thus separated by isoelectric focusing under native conditions . Two groups of alpha-amylases isoforms could be seen. One group was c haracterized by high pi's (around 6.5) and was mainly expressed during the first phase of the winter (post dormancy period) whereas the seco nd one had lower pi's (around 4.5) and was expressed during the second half of the winter (pre-bursting period). Alpha-amylase activity incr eases corresponded to parallel losses in starch content and rises in t he concentration of some soluble sugars in the woods studied. A direct assessment of the influence of drops in temperature on alpha-amylase activity changes and isoforms expression was obtained after placing gr apevine stems at different laboratory-controlled temperatures. Lowerin g the temperature from 12 degrees C to 4 degrees C led to the maximum induction of both groups of alpha-amylase isoforms with a parallel hyd rolysis of starch and release of some soluble sugars. Interestingly, a controlled temperature drop from 12 degrees C to 4 degrees C also led to a maximum increase in sucrose content whereas a further drop to su b-freezing temperatures (-15 degrees C) was necessary to trigger raffi nose accumulation.