LOCALIZATION OF PHOSPHORYLASE AND OF STARCH FORMATION IN SEEDS

The discovery of the enzyme phosphorylase provides an important clue toward the solution of the problem of starch transformation in plants (5). The enzyme has been purified and its properties and reactions in vitro are fairly well understood (7) (11). Only its physiological role in the living plant remains obscure. One approach to this problem is to investigate the distribution of the enzyme in different tissues and cells to correlate it with the site of normal starch synthesis. For this purpose a histochemical method has been devised and described (13). With this method a number of observations have been made on various plant parts. The present report embodies the results of the work on germinating seeds. Briefly the method for the detection of phosphorylase (13) consists of incubating free-hand sections of plant parts in a buffered medium of glucose1-phosphate (prepared according to the method of HANES (6) and of subsequent staining with iodine in potassium iodide. The amount of starch formed is taken as a measure of the phosphorylase activity. Incubation is conducted at 25° C and lasts for a half to one hour, or longer in case the enzymic activity is particularly weak. Soybean was chosen as the main experimental material because of the absence of detectable starch, which interferes with the test, in the dormant seeds and during the first days of germination. Castor bean which is also practically starch-free was also tried. For comparison, broad bean was used, but in this case the embryo was removed from the cotyledons and starved for several days to rid it of stored starch.