Polarity of Indoleacetic Acid in young Coleus Stems

Abstract

Young internodes of Coklus blumci Benth. have long been known for their sizable amount of acropetal indoleacetic acid movement. However, plants of the same done, under improved growing conditions, now show almost absolute basipetal polarity of 14C-indoleacetic acid, as measured by liquid scintillation counting of 14C in the receiver cylinders of agar.The ratio of basipetal to acropetal movement is now as much as 85:1, instead of the 3:1 ratio found years ago under conditions providing dower growth.Although auxins such as IAA were first known for their unvarying and strongly polar basipetal movement through the organs of etiolated seedlings such as Avena coleoptiles, later work showed that this movement was subject to regulation (e.g. 3, 4, 8). Furthermore, in green shoots of plants past the seedling stage, even cases of acropetal movement were discovered. The most thoroughly investigated of these are the 3:1 ratio of basipetal to acropetal IAA movement through sections cut from young internodes of Coleus (first found with bioassay measurements by Jacobs [5,6], then confirmed with radioisotope counting of 14C_ IAA by Naqvi and Gordon [23] and Leopold and de la Fuente [19]) and the increasing acropetal movement of IAA that occurs through sections of bean petioles of increasing age (20,21). The auxin-type herbicide, 2,4-D, gave similar results in bean petioles (9,14,21).Is this acropetal movement through excised sections merely an artifact of excision? For bean petioles the only evidence on this point comes from elongation measurements after such sections were provided with IAA from the base only or from the apex only: basally applied IAA produced relatively more growth as the petioles from which the sections were cut were progressively older (10, 21). For young Coleus internodes the evidence that the 3:1 ratio of IAA movement is meaningful has been related somewhat more closely to the intact plant: if vascular strands in matching internodes were cut, the regeneration of tracheary cells was mostly basipetal, but with some strands regenerating acropetally from the cut strand below the wound (5)-thus qualitatively paralleling the 3:1 ratio of IAA movement. Quantitative data in support of the physiological significance of acropetal IAA movement came from experiments in which all leaves below the regenerating internode were excised in one set of plants and that set compared with a set which had all leaves excised above the regenerating internode. Leaves had already been shown to be the main source of endogenous auxin in the Coleus shoot (5), hence those wounds presumably had auxin available to them only from above or only from below, respectively. (5,7,15, 27), the conclusion was drawn that the acropetal movement of IAA, observed in transport sections cut from internode #2, also occurred in the more nearly intact plant, where it caused the regeneration of the 5.6 strands of tracheary cells found 1 week after the leaves above the wound were excised (6).To understand more about the polarity of hormone movement...

Graft Formation in Cultured, Explanted Internodes

“…It is now a well-established fact that the extent of differentiation of xylem elements is strongly limited by auxin level (Jacobs, 1954(Jacobs, , 1956 and therefore the connection between auxin level and the attainment of vascular continuity between stock and scion is clear. Further, although auxin can be transported acropetally this is variable in extent and, according to Jacobs (1977), forms only one-third to one-eighty-fifth of auxin transported basipetally. Clearly, in the grafted internode it is the lack of auxin because of low acropetal auxin movement which contributes to the small number of trans-graft vascular strands in individuals in which auxin is not supplied apically.…”