Regeneration tracheary cells

These first quantitative studies of cell differentiation under the influence of hormones are among the few cases in the literature where exact substitution of synthetic hormone for the normal hormone source has been demonstrated to give full and exact replacement of the hormone source in terms of a physiological effect. The prevention of abscission by lAA is another such case (Chap. 8 in Jacobs 1979). In both cases, the full effect of the leaves was replaceable by lAA alone. The results confirmed the hypothesis that, of all the chemicals undoubtedly required to differentiate a strand of regenerated tracheary cells, lAA from the leaves was the one that acted as a limiting factor in the process. Furthermore, the results with the polarity of the stem (Fig. 4.2) supported the view that the ability of the stem to transport lAA was a further check on how much lAA reached the regenerating area (Jacobs 1954). 

Fig. 4.9. The dose-response curve to lAA (added in lanolin to the apical end of excised older internodes of Coleus) of regenerating tracheary cells solid circles) and sieve tube strands open circles). Asterisks mark the interpolated values from intact controls (as in Fig. 4.4) (Thompson and Jacobs 1966). Note that each sieve tube strand contains an average of ten cells (LaMotte and Jacobs 1962) hence, the two curves directly reflect the number of cells that regenerated...

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, 4.1. A cleared and stained whole mount, showing strands of tracheary cells (arrow) that have regenerated around a wound (left) in a Coleus stem. (Jacobs 1952)... 

Fig. 4.4. The number of tracheary cells regenerating around a slit wound in excised young internodes of Coleus in relation to increasing doses of IAA in lanolin added to the apical cut surface. The asterisk marks the interpolated value for the number of tracheary cells regenerated around such a wound in otherwise intact control plants. (After Jacobs 1970, corrected data from Thompson and Jacobs 1966)...

In other experiments, internodes were cut free of the entire rest of the plant and regeneration tested in these isolated stem pieces. lAA added to the apical end of such an excised older internode increased sieve-tube regeneration over the controls (LaMotte and Jacobs 1963, Thompson and Jacobs 1966). The dose-response curve of apically applied lAA, as it affected both tracheary and sieve cells, is shown in Fig. 4.9 with no externally added lAA, a small number of sieve-tube cells but no tracheary cells regenerated as the lAA level was increased, more vascular cells of both types regenerated (Thompson and Jacobs... 

Plants intact except for the transverse wound regenerated numbers of vascular cells equivalent to those formed by about 0.05% lAA added to the isolated internode (asterisks in Fig. 4.9). Hence, 0.05% lAA in lanolin exactly replaced the root system and the rest of the shoot system in their effects on the regeneration of both sieve tubes and tracheary cells. At any one lAA level, more sieve cells than tracheary cells regenerated. 

Like the effect of lAA on tracheary cells in the young, elongating internode, the effect of lAA on regeneration in this older internode was strictly polar (Thompson and Jacobs 1966), as was lAA movement through excised tissues of such internodes (Jacobs and McCready 1967). 

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. 

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