William P. Jacobs

Auxin Transport, Gibberellin, and Apical Dominance

Substitution of indoleacetic acid plus gibberellic acid for Pisum shoot apices restored apical dominance more effectively than indoleacetic acid alone. Studies of translocation, in which carbon-14-labeled indoleacetic acid was used (as determined by paper chromatography and scintillation counting) revealed that gibberellic acid caused more indoleacetic acid to be present and effective far from the site of application.

What substance normally controls a given biological process?: I. Formulation of some rules

Abstract Six rules (the “PESIGS” rules) are stated for determining what naturally occurring substance normally controls a given biological process. The application of these rules, in quantitative form, is urged on developmental physiologists.

A role of auxin in phloem regeneration in Coleus internodes

Abstract The physiology of regenerative phloem formation in the number 5 internode of Coleus blumei Benth. was studied by estimating quantitatively its extent in plants or plant parts wounded so as to sever one or more of the existing phloem bundles and then allowed to regenerate for a short period (usually 5 days). Removal of all leaves and buds from plants resulted in a marked reduction (to approximately one-quarter of the value of the intact controls) in phloem regeneration. Primary leaf removal also resulted in some, though a smaller, reduction, as did the one other treatment—removal of all distal organs—which involved removal of young primary leaves. The other patterns of shoot organ removal tried—all buds removed and all proximal leaves and buds removed—had no effect on phloem regeneration. Indoleacetic acid (IAA) applied in aqueous solution (2 mg/l) to plants from which all leaves and buds had been removed completely substituted for them in phloem regeneration. Applied in lanolin paste (0.1% and 1%), it evoked more phloem regeneration than attached organs themselves. Sucrose (2%) applied alone or together with IAA in aqueous solution had no effect. IAA (0.1%) applied in lanolin to excised internodes increased phloem regeneration in these, but the anomalous appearance of some of the wound sieve tubes indicated that sugar or other material may have become limiting for full differentiation. A relationship between the number of phloem bundles cut in wounding and the extent of phloem regeneration was shown. This and other observations were interpreted as indicating the involvement in phloem regeneration of a substance or substances leaking from the cut phloem. Existing evidence is interpreted to mean that auxin is limiting for the cell divisions which immediately precede phloem differentiation and that some other factor, presently unknown, may be limiting for differentiation when auxin is in sufficient supply for cell division. However, present evidence neither implies nor excludes the possible participation of auxin in differentiation itself.

Regeneration and Differentiation of Sieve Tube Elements

Publisher Summary This chapter discusses regeneration and differentiation of sieve tube elements. The pattern of first differentiation of sieve tubes in the young leaf is continuous and acropetal. The first sieve tube element in the leaf forms at a separate locus on the outer side of the procambial strand near the base of the leaf. The regeneration of cells in the internode is controlled by the amount of basipetally moving endogenous auxin, indole-3-acetic acid (IAA), and coming from the leaves. As a leaf grows more quickly, it produces more IAA, which in turn allows more vascular tissue to differentiate in the traces supplying the leaf with the wherewithal to grow. A declining growth rate is associated with declining IAA production and, therefore, with declining differentiation of vascular tissue in the leaf traces.

A quantitative study of mitotic figures in relation to development in the apical meristem of vegetative shoots of Coleus

Abstract A quantitative, round-the-clock study was made of vegetative shoot apexes of a clonal stock of Coleus blumei Benth. A special aim of the study was to find meaningful, exact relation that would be reproducible year after year. Such relations we have found for the following: 1. 1. There is a diurnal rhythm in the initiation of leaf primordia, with a plastochron more likely to start near the middle of the dark period than at other times sampled (Fig. 4). 2. 2. The number of mitotic figures in the apical meristem (here defined as the portion of the shoot distal to the youngest leaf primordium) is closely and significantly correlated with the height of the apical meristem, but the distal 10–20 μ of the meristem consistently has fewer mitotic figures than more proximal regions (Fig. 2). 3. 3. The height of the apical meristem is closely and significantly correlated with the length of the youngest leaf primordium (Fig. 1). 4. 4. By combining the above finding with the previously reported (Jacobs and Morrow, 1957) correlation between lengths of unfolded leaves and of younger primordia, one can now collect apical meristems of reasonably specific heights by measuring the older leaves with a millimeter ruler. 5. 5. Contrary to expectation from the older literature, no evidence was found for a marked diurnal rhythm in the percentage of cells showing mitotic figures. The large increase at 11 p.m. in the absolute number of mitotic figures was a reflection, rather, of the much higher apical meristem at that time (Fig. 4). A critical examination reveals that some of the apparently disparate results in the literature could have resulted from an unrecognized, Coleus -like, diurnal rhythm in the growth of the apical meristem.

The Movement of Plant Hormones: Auxins, Gibberellins, and Cytokinins

Auxins are well known to move with polarity in sections cut from shoots. At first, physiologists thought that non-endogenous auxins like 2,4-D did not move with IAA-like polarity; however, by testing 2,4-D and IAA under identical conditions, McCready and I found polar transport of both (1963). When I proposed, at the Izmir conference on hormone transport in 1967, that polar movement was too valuable to be restricted to the auxins only and that therefore it was likely that other hormones would also move with polarity through plants, there was great resistance to the idea — so much resistance, in fact, that I decided to collect more evidence before urging the hypothesis on the transport-physiologists.

COMPARISON OF THE MOVEMENT AND VASCULAR DIFFERENTIATION EFFECTS OF THE ENDOGENOUS AUXIN AND OF PHENOXYACETIC WEEDKILLERS IN STEMS AND PETIOLES OF COLEUS AND PHASEOLUS

Ever since Went first reported the polar movement of auxin ( 1928), physiologists have been interested in studying the polarity of auxin movement as a way of helping them understand the general phenomenon of polarity. Until a few years ago, detailed quantitative study of auxin movement was hampered by the limitations of the available techniques. The Avena curvature bioassay set drastic limits on the size of the experiments that could be Tun and, in addition, it was too imprecise for fine discriminations. The use of radioisotopes has greatly improved transport studies. After Gordon and Eib (1956) had demonstrated the feasibility of studying the movement of indoleacetic acid (IAA) labeled with 14C by counting with Geiger tubes, Goldsmith developed the technique further (1959; Goldsmith & Thimann, 1962). During the last four to five years, several groups have been using these thin-window Geiger counters for transport studies. Techniques have been improved still further by some of my students and research associates. This paper reports some of the advances they have made, both in technique and in understanding the phenomena of polarity and hormone movement. McCready and I (1963a & b) used Geiger counters to compare the movement of 1AAJ4C with that of the weedkiller 2,4-dichlorophenoxyacetic acid (2,4-D-14C) in bean leaves. When added in hormonal concentrations to young bean petioles, the weedkiller showed very similar transport properties to those of IAA: its movement was strongly basipetally polar in young petioles and, once corrections had been made for the slower net loss of 2,4-D from its donor blocks, the acropetal movement of 14C from the two substances was essentially the same (FIGURE 1). The velocities of basipetal movement, as determined by a quantified form of the van der Weij intercept technique (1932), were different: IAA moved at 6 mm/hr, 2,4-D at 0.6-1.0 mm/hr (see also McCready, 1963). Note, also, that, after 12 hr there is a secondary decrease in counts in the blocks receiving IAA-14C that is moving basipetally. If one looks at details of the earlier portion of the curve showing basipetally moving IAAJ4C, the increase in the receiver blocks is clearly linear for one through five hr (FIGURE 2) . Lower concentrations in the donor blocks give less transport, mostly (or entirely) due to less flux, as measured by slope of the fitted line. Similar studies on primary leaves of increasing age revealed interesting correlations between polarity, age, and growth (McCready & Jacobs, 1963b). Sections from progressively older leaves showed a steady decline in the polarity of movement of both IAAJ4C and 2,4-D-14C, coupled with a progressive decrease in the ability of the sections to grow in length (FIGURE 3). The decline in polarity with increasing age was due mainly to a steady increase of auxin movement in the acropetal direction. FIGURE 4 provides evidence that auxin movement, as measured by amounts collected in either apical or basal receiver blocks, is meaningfully correlated with the relative elongation of the sections through which the auxin is moving.

Auxin in Coleus Stems: Limitation of Transport at Higher Concentrations

Previous indirect evidence that the endogenous concentration of plant hormones of the auxin type is controlled by a limitation of transport over the physiological range of concentration was confirmed by measuring directly the transport of the native auxin, indoleacetic acid, through segments of Coleus stems.

DEVELOPMENT AND REGENERATION OF THE ALGAL GIANT COENOCYTE CAULERPA

The vast majority of animals and plants are multicellular, particularly if they develop any size and complexity. The cell theory reflects the ubiquity of this cellular structure. Theories of development often involve the idea that organforming substances can become localized within specific cells, and that cellular structure makes developmental polarity easier. The existence of Caulerpa, therefore, represents a gauntlet flung in the face of the cell theory. Caulerpa prolifera (FIGURE l) , the most frequently studied species, is a large and highly differentiated coenocyte. A single plant can grow for many feet in tropical lagoons, making three different organs, each with a different orientation to gravity and light. There is a horizontally growing rhizome, from the bottom side of which rhizoids develop to fasten the organism to the substratum. Leaves develop from the upper side of the rhizome and grow into flat, bladelike structures oriented toward light and away from the sand or mud. This entire, large, differentiated plant is a single cell: there are typically no cross walls in the entire organism. In addition, the multinucleate cytoplasm shows some of the most striking streaming known in plants as it flows steadily in and out of the three types of organs. Our special interest in Caulerpa, therefore, consists in trying to understand how its development differs from that of multicellular plants of similar morphological complexity, and how it manages to differentiate despite its coenocytic structure.

Accumulation of amyloplasts on the bottom of normal and inverted rhizome tips of Caulerpa prolifera (Forsskål) Lamouroux

Caulerpa prolifera (Chlorophyta) exhibits a gravimorphogenetic response to inversion by switching the site of new rhizoid initiations to correspond with the new direction of the gravity stimulus. When plants were fixed at 6 and 24 h after being held in either a normal or an inverted position the switch in the site of organ differentiation, upon inversion, was found to be preceded by the accumulation of starch-containing amyloplasts on the bottom of the rhizome. Approximately 1.5–2.0 times as many amyloplasts were found at the bottom of normal or inverted rhizomes as compared with the top in a region extending from 200 to 1,000 μm behind the rhizome tip. All new rhizoid initials were located in the region of amyloplast accumulation and each rhizoid initial contained numerous amyloplasts.