The ultrastructure and development of tubular and crystalline P-protein in the sieve elements of certain papilionaceous legumes

Summary The ultrastructure of the primary sieve elements of several papilionaceous legumes was studied using hypocotyl and young internode segments fixed in glutaraldehyde followed by osmium tetroxide. In particular, the study sought to determine whether the crystalline flagellar inclusions characteristic of these species are developmentally related to the P-protein bodies present in the phloem of these and other legumes and of angiosperms generally. The crystalline inclusions consist of a central body terminated at one or both ends by a gradually tapering tail. The central body is usually spindle shaped in longitudinal section and square in cross section. In all species examined, the inclusion is first seen as a small, thin crystal in the cytoplasm of young sieve elements. The crystal enlarges and acquires tails as the sieve element develops. In certain species, exemplified byDesmodium canadense, numerous tubules are formed in the cytoplasm near the crystal and appear to be concerned in its growth. The observations on the structure and interactions of these two components, tubules and crystalline inclusions, suggest that both represent forms of P-protein: the tubules are continuous with the crystal and are striated like the crystal near the tubule-crystal junction, suggesting that they are adding onto the crystal body; the tubules closely resemble the P-protein tubules described in the literature in that they measure 157 Å in diameter, accumulate in spindle-shaped bundles, and disperse into striated fibrils late in the ontogeny of the sieve element; and finally, the crystal also disperses into fine filaments. The crystalline inclusion therefore probably represents still another aggregation state of P-protein, one that is characteristic of papilionaceous legumes. The different stages of crystal aggregation and the diverse forms of P-protein now known are discussed briefly in relation to the control of macromolecular assembly and subunit packing.