A Study of Sieve Element Starch Using Sequential Enzymatic Digestion and Electron Microscopy

The fine structure of plastids and their starch deposits in differentiating sieve elements was studied in bean (Phaseolus vulgaris L.). Ultrastructural cytochemistry employing two carbohydrases specific for different linkages was then used to compare the chemical nature of "sieve tube starch" (the starch deposited in sieve elements) with that of the ordinary starch of other cell types. Hypocotyl tissue from seedlings was fixed in glutaraldehyde, postfixed in osmium tetroxide, and embedded in Epon-Araldite. Treatment of thin sections on uncoated copper grids with alpha-amylase or diastase at pH 6.8 to cleave alpha-(1 --> 4) bonds resulted in digestion of ordinary starch grains but not sieve element grains, as determined by electron microscopy. Since alpha-(1 --> 6) branch points in amylopectin-type starches make the adjacent alpha-(1 --> 4) linkages somewhat resistant to hydrolysis by alpha-amylase, other sections mounted on bare copper or gold grids were treated with pullulanase (a bacterial alpha-[1 --> 6] glucosidase) prior to digestion with diastase. Pullulanase did not digest sieve element starch, but rendered the starch digestible subsequently by alpha-amylase. Diastase followed by pullulanase did not result in digestion. The results provide evidence that sieve element starch is composed of highly branched molecules with numerous alpha-(1 --> 6) linkages.