Sieve-tube differentiation

A notable feature of the development and differentiation of the cell walls in phloem tissues is the formation of pores connecting the adjacent sieve-tubes to each other and to the companion cells. At an early stage of development, walls of the sieve tubes are marked by parts of endoplasmic reticulum on both sides where the pore is to be formed. As the sieve plate develops and the wall thickens, normal materials are deposited on the cell wall, except in the areas below the endoplasmic reticulum these areas grow, instead, by the... 

A major advance made since the 1965 edition of the volumes lA-16 on developmental physiology in the first series of this Encyclopedia was prepared has been the quantitative elucidation of hormonal control of sieve-tube differentiation during regeneration in stems. In addition, the earlier quantitative studies of tracheary differentiation have been extended and amplified. An unexpectedly close connection between sieve tubes and tracheary cells in their regeneration and normal differentiation has been revealed and a unifying picture of vascular differentiation in the shoot has resulted. A start has recently been made in unraveling the controlling factors for fiber differentiation. These advances will be described and discussed below. 

Fig. 4.5. Diagram of the technique used by LaMotte and Jacobs (1962) for removing a phloem strip from one side of a Coleus stem (Jacobs 1970). Note that the smallest strands of sieve tubes do not yet have tracheary counterparts differentiated on the other side of the cambium...

In his elegant study of hormonal control of the normal differentiation of phloem fibers (described below), Aloni (1976) pointed out that the fibers did not differentiate from the parenchymatous cells of the wound callus, even though sieve tubes and tracheary cells did. The differentiation of phloem fibers was always limited to the longitudinal vascular strands, never being found in the intervening parenchyma. In that sense, one could say that they do not regenerate however, their regeneration has been seldom studied. 

Hormonal effects on the normal differentiation of primary sieve tubes in shoots are essentially uninvestigated. 

The note by Wareing (1958), reporting no differentiated tracheary cells from added GA3, some from lAA, but more when GA3 was added with lAA, to excised sections of tree stems, was extended in later papers. For five genera, Wareing et al. (1964) found the maximal width of new secondary xylem and phloem tissue when lAA and GA were added together to excised pieces of stem (Fig. 4.12). The cells considered to be on the phloem side of the cambium did not differentiate into sieve tubes or companion cells, however, whether they resulted from GA alone or GA plus lAA. lAA added alone resulted in some tracheary cells but no phloem cells. Kinetin, whether added alone or with IA A, was said to be without effect or perhaps slightly inhibitory. The... 

Jacobs WP (1970) Regeneration and differentiation of sieve-tube elements. Int Rev Cytol 28 239-273... 

Jacobs WP, McCready CC (1967) Polar transport of growth regulators in pith and vascular tissues of Coleus stems. Am J Bot 54 1035-1040 Jacobs WP, Morrow IB (1957) A quantitative study of xylem development in the vegetative shoot apex of Coleus. Am J Bot 44 823-842 Jacobs WP, Morrow IB (1967) A quantitative study of sieve tube differentiation in vegetative shoot apices of Coleus. Am J Bot 54 425 31 Jacobs WP, Danielson J, Hurst V, Adams P (1959) What substance normally controls a given biological process II. The relation of auxin to apical dominance. Dev Biol 1 534-554... 

Thompson NP, Jacobs WP (1966) Polarity of lAA effect on sieve-tube and xylem regeneration in Coleus and tomato stems. Plant Physiol 41 673-682 Torrey JG (1953) The effect of certain metabolic inhibitors on vascular tissue differentiation in isolated pea roots. Am J Bot 40 525-533 Torrey JG, Loomis RS (1967) Auxin-cytokinin control of secondary vascular tissue formation in isolated roots of Raphanus. Am J Bot 54 1098-1106 Torrey JG, Fosket DE, Hepler PK (1971) Xylem formation A paradigm of cyto-differen-tiation in higher plants. Am Sci 59 338-352... 

Examples of unequal cell division can easily be multiplied. It must suffice simply to mention the formation of sieve tubes and accompanying cells in angiosperms, of elaters and spore mother cells in liverworts, and of hyalin and chlorophyll cells in peat mosses. After this demonstration of conspicuous cases of unequal cell division, let us return to the problem we raised at the beginning of the chapter, to cell divisions which are unequal in the sense that one of the two daughter cells can continue to divide whereas the other begins to differentiate. The first manifestation of differentiation is usually elongation of the cell in question. 

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