Protein bodies from the cotyledons of Cucurbita maxima

Abstract

The structure of protein bodies from cotyledon tissue of dormant squash seeds was studied using several different preparative techniques. Four major components, the proteinaceous matrix, the protein crystalloid, the soft globoid, and the globoid crystal were found and verified to be present in protein bodies from the large mesophyll cells. Some of these four components were found to be absent in protein bodies of epidermal cells or in protein bodies near the nucleus of mesophyll cells. The globoid crystal, which was hard and electron dense after osmium fixation, is probably the site of phytin storage. The globoid crystal was usually surrounded by the soft globoid, a material of undetermined composition which tended to smear during freeze-etching. Some protein bodies contained several of these globoid regions although most protein bodies seemed to have only one globoid region. One or several protein crystalloids were usually found in protein bodies. Both the protein crystalloids and the globoid regions were surrounded by a proteinaceous matrix substance.Glutaraldehyde–permanganate fixation of tissue clearly showed the presence of the proteinaceous ground substance and the protein crystalloid but the globoid structures were absent. In tissue chemically fixed with glutaraldehyde–osmium all four components were sometimes found but interpretation, on the basis of this fixation alone, was difficult because of the presence of fixation artifacts. Isolation of protein bodies from cotyledon tissue made faster and better quality osmium or glutaraldehyde–osmium fixations possible. In such protein body pellets all four components were seen. Freeze-etching of tissue verified the presence of the globoid crystal and the soft globoid in most protein bodies from cotyledon mesophyll cells. In dormant seed tissue the two proteinaceous components usually were not distinguishable by the freeze-etching technique however. Thus only through the use of several different preparative techniques was it possible to locate and verify the presence of the various structural components in protein bodies from squash cotyledons.