Ultrastructure of Developing Sieve Elements in Thlaspi arvense L. II. Maturation

Abstract

Developing sieve elements of pennycress (Thlaspi arvense L.) were studied with the electron microscope. The maturation of sieve elements involved loss of ribosomes from cytoplasm; degeneration of nulcei; modification of endoplasmic reticulum (ER); loss of tonoplast; and disappearance of dictyosomes and dictyosomes vesicles, coated vesicles, microtubules, and microbodies. Such changes produce a mature, presumably conducting cell that contains no nucleus or central vacuole but which retains a thin layer of peripheral cytoplasm with plastids, mitochondria, and smooth ER. Some similar changes have been described in a variety of developing sieve elements of angiosperms, but coated vesicles and microbodies previously have not been followed through sieve-element maturation. Likewise, few developmental studies have been made of plant sieve elements that exhibit two types of P-protein, the tubular type and the granular P-protein body.