Phloem differentiation in tobacco pith culture

Abstract

Phloem cells differentiating in sterile pith cultures of tobacco have been fixed with formaldehyde-glutaraldehyde for electron microscopy. The phloem consists of sieve elements, companion cells and other parenchyma cells. The differentiation processes of these cells are similar to those in intact plants. The P-protein component is first observed in sieve elements as tubules arranged in groups which subsequently enlarge to form large P-protein bodies. In some P-protein bodies, the tubules are packed in a hexagonal arrangement, often around lightly staining centers. Fine fibrils may connect the P-protein tubules. The P-protein bodies disperse in the cells at about the same stage of differentiation as the degeneration of the nucleus and vacuole, the loss of dictyosomes and ribosomes and the reorganization of the membrane systems into the sieve element reticulum. The P-protein tubules in many cells are transformed into narrower fibrils. Occasionally the fibrils have a beaded appearance. The cultured cells differ in this regard from intact plant sieve elements where the bulk of the fibrillar P-protein has a beaded appearance. The difference may be correlated with the absence of long distance transport in the cultured material. The transformation of tubular to fibrillar P-protein is discussed in relation to the two helices model for the structure of P-protein tubules. The sieve plate pores were plugged with P-protein in some cases although many unplugged pores were encountered. Dense slime plugs were uncommon; usually the P-protein was distributed throughout the lumen of mature sieve elements.