Intact plant auxin

This chapter is intended to provide an overview of the functions of the five major plant growth hormones in the intact plant auxin (lAA), gibberellins (GA), cytokinins (K), ethylene, and abscisic acid (ABA). The effects of these hormones have been described in detail on numerous occasions over the last 10 years (see general references at the end of this chapter). This review utilizes these previous reviews as well as including a survey of the literature published between January 1977 and January 1980. The most recent and surely the most detailed treatment of the subject is to be found in Phytohormones and Related Compounds A Comprehensive Treatise. D.S. Letham et al., Volumes 1 and 2, 1978. The intent of the present review is to provide a more general overview and to consider overall patterns of regulatory controls by hormones in the whole plant. 

Cloning. Asexual propagation (cloning) of plants ordinarily occurs by virture of the ability of embryonic meristematic tissue to differentiate into roots and shoots. If isolated phloem cells or other more differentiated cells are cultured, the result is often the formation of a callus, a dedifferentiated mass of cells somewhat reminiscent of embryonic cells. Under proper conditions, e.g., in a coconut milk culture and in the presence of the correct auxin-to-cytokinin ratio, some carrot root phloem cells revert to embyronic cells and develop into intact plants.99 This experiment provided proof that the differentiated carrot phloem cells... 

In the belief that the failure of the stem section to grow meant that some essential cofactor must normally be supplied to the stem section by the rest of the plant, we began investigating the effect of plant extracts. A mixture of glycerides isolated from the pea nearly doubled the growth response of the section to auxin and gibberellin, even when applied at the extremely low concentration of 10 mg. per liter (3). Even so, the growth attained by the section is still less than that of the intact plant, and a cofactor other than those discussed in this paper may be involved. 

The adventitious root cultures of Datura innoxia, Duboisia hybrid M-II-8-14 (a cross-bred between D. myoporoides and D. leichhardtii), and Scopolia tangutica were established from the axenic shoot cultures or intact plants (in the case of Duboisia) on MS solid medium containing 0.1 mg/1 NAA or 1.0 mg/1 lAA. The adventitious roots were maintained in MS liquid medium containing the same phytohormones (0.1 mg/1 NAA or 0.5 mg/1 lAA) in the dark. Addition of auxin in the culture medium has been employed for the maintenance of adventitious root cultures [14], however, the adventitious roots of H. albus and H. niger were induced and maintained in hormone-free 1/2 MS medium [15]. 

In C. roseus seedlings, TDC activity was enhanced by auxin treatment (301). Also, in a habituated C. roseus cell line and a cell line normally grown on an auxin-containing medium, it was found that low levels of auxins (0.01-0.1 ppm 2,4-D, lAA, or NAA) result in increased ajmalicine accumulation, whereas higher concentrations resulted in decreased alkaloid levels (319). This shows that extrapolation of results found in cultured cells to an intact plant must be done with great caution. 

Early Studies with Auxin in Decapitated, yet Otherwise Intact Plants... 

Our original focus was on auxins and how they elicit growth in intact plants. Accordingly, we employed a semi-intact pea epicotyl system in which the plumule and hook were cut off to remove meristematic cells and the major supply of endogenous auxin, and the cut stump was painted with lanolin containing various additives. Our earlier findings with this system showing that auxin induces cellulase synthesis in vivo and in vitro have been reported [12]. 

Surprisingly, this treatment (excision of the apex) led to polysome formation which was greater 1 h after excision than 10 h after auxin treatment of the intact plant [8]. In efforts to circumvent this wound-evoked polysome formation, we excised the tissue at a point 5 cm below the apex and were again surprised. Polysome formation in the apical 1 cm was almost as massive as in tissue wounded at the 1 cm point. In fact, polysome formation took place in the apical 1 cm within 15 min of inflicting a wound at a point 20 cm distant. This led to our realization that there was a rapidly-generated (and bidirectionally-transmitted) wound signal that could very rapidly elicit polysome formation in distant tissue [8, 18]. 

By use of these various techniques it has been shown, especially for auxins, that hormones applied to the unfolded leaves of intact plants are generally... 

Specificity of the Auxin Transport System. Comparative studies with chemically closely related compounds reveal a positive correlation between auxin activity and degree of basipetal polarity in the transport of these substances (e.g.. Went and White 1939, Leopold and Lam 1961, Jacobs 1967, Hertel etal. 1969, Veen 1972). Further, the basipetal transport of auxins is specifically inhibited by substances such as TIBA, naphthylphthalamic acid (NPA), 3,3a-dihydro-2-(p-methoxyphenyl)-8H-pyrazolo[5,la] isoindol-8-one (DPX 1840) (see Sect. 3.3.4.3) not only in treated sections but also when they are applied to intact plants at the loci of natural auxin production (see p 100). In summary, the numerous transport studies make certain the presence of a unique transport system, which is specific for auxin molecules, and which moves the hormone basipetally from the natural auxin sources in the shoot and then on to the root to regulate growth and other developmental processes. It is clear that this system is fully functional in tissue sections which is the material which has been used most in transport studies. 

Bonnemain JL, Bourbouloux A (1973) The transport and metabolism of C-indoleacetic acid in intact plants. Proc Res Inst Pom Skierniewice Pol Ser E Conf Symp 3 207-214 Bonnett HT Jr, Torrey JG (1965 a) Auxin transport in Convolvulus roots cultured in vitro. Plant Physiol 40 813-818... 

An open question is whether auxin stimulates wall loosening by acting at the level of gene transcription. Auxin certainly stimulates the rate of RNA synthesis in plant section,454-456 isolated nuclei,457-459 and chromatin,457,460 and new protein species appear after treatment of intact tissues with the hormone.461-463 However, the findings of Haschemeyer464 and Evans and Ray465 strongly indicated that the induction of cell-wall exten-... 

Even with gibberellin and auxin the growth of the defoliated or decapitated plants is significantly less than that of the intact ones, suggesting that, as with Gerbera, still other growth factors must be accounted for. In two other cases dual roles of auxin and gibberellin in mitosis are clearly indicated. 

While the results with auxin as inhibitor are clear, the release of inhibition by reduced pressure is much less clear when the inhibition is due to the intact apex. The elongation of the upper internodes of the intact Vida plants under reduced pressure was at least as rapid as that of controls, but the liberation of basal buds was relatively weak and began only after a lapse of three days. It may be surmised that withdrawal of nutrients and water by the continued rapid development of the upper parts of the plant makes less xylen sap available to the lateral buds. Under reduced pressure the additional transpiration would further intensify this effect. To some extent this concept comes close to the early interpretation, of apical dominance by Goebel [4]. 

Evans ML (1976b) A new sensitive root auxanometer Preliminary studies of the interaction of auxin and acid pH in the regulation of intact root elongation. Plant Physiol 58 599-601... 

DEWEY 1957, 1964), with radioactive lAA and to follow its behavior in the plant, left as intact as possible. The natural auxin source was replaced by a lanolin paste containing lAA (in this case, " C-IAA). The new auxin source sustained natural elongation and growth movements of the stalk (Kaldewey 1957, 1962, 1965b). Seven hours later, the axis of the plant was cut several centimeters below the apex and supplied at the cut end of its base with agar receivers, twice each for 15-min periods. Then, a 5-mm section was excised and stood on fresh agar receivers to allow all the mobile auxin to move out, and the new basal cut surface of the remaining axis was successively supplied with another pair of receivers for 15-min periods. The radioactivity collected in the 15-min receivers and the section receivers, respectively, allowed an estimation of the transport intensity and transport density. It was possible, therefore, to calculate the transport velocity from these quantities [see Eq. (2)]. Further, auxin immobilized within the 5-mm sections could be determined by extraction (see Fig. 3.6). 

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