Free Auxins

Indole-3-acetic acid is rather readily oxidized by peroxidases and is, in fact, probably not present in the plant in the free form to any appreciable extent. The nature of the complexing groups is not clear. The inherent instability of the compound in living tissue has made experimental observations difficult, and (the more stable) 1-naphthaleneacetic acid has often been used instead, although it is by no means certain that the biological activities are comparable. One view held is that auxin herbicides are effective either because they do not readily form conjugate systems, or because the conjugate retains the phytotoxic properties. 

Silatranylmethyl ester has a higher auxin activity with respect to the culture of soya tissue than free 2-methylphenoxyacetic acid. At the same time, it has a low activity with respect to tobacco and potato cultures. 1-Silatranylesterof 4-chlorophenoxyacetic... 

From a critical study of the metabolism of poly (A) in auxin-treated pea epicotyl, Verma and Maclachlan (73) showed that discreet classes of poly (A) (presumably part of mRNA s) are differentially associated with free and membrane-bound polysomes. The induction of specific mRNA s, the decline in the rate of synthesis of mRNA s, the polysome content per cell, and the formation of cellulase were all related to the membrane-bound polysomes. Although the rate of in vivo enzyme synthesis is... 

Auxins are defined as organic substances that promote cell elongation when applied in low concentrations to plant tissue segments in a bioassay. By this definition, there are several other native auxins that have been reported to occur in plants in addition to the most often studied auxin, IAA. These include the halogen-substituted 4-C1-IAA,23 as well as phenylacetic acid and indole-3-butyric acid.24 All native auxins are found in planta as both free acids and conjugated forms through ester or amide linkages. IAA, the auxin most extensively studied, will be the focus of this chapter. 

The natural auxins are represented by a single compound, indole-3-acetic acid (IAA), which occurs as the free acid and in various "bound" forms, including a thioglucoside, glycosyl esters, and IAA peptides. For a recent discussion see the book chapter by Bandurski and Nonhebel (2) in Wilkins (3). 

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]. 

Adventitious shoot formation occurred when the roots were cultured onto auxin-free medium as mentioned above. Since the decrease of auxin levels seemed to be important for the adventitious shoot formation, the influences of auxin inhibitors on shoot formation on the cultured root segments were investigated. The auxin polar transport inhibitor, 2,3,... 

The activity of the compounds substituted in the 2,4,6 position at the same time disproves the earlier theory that a precondition of auxin action was a free ortho position. 

Earlier experiments of Evans and Ray (1969) with substances inhibiting protein synthesis also seem to disprove the theory that disturbance of protein synthesis is a fundamental determinant of biological action. Rubery and Northcote (1970) proved by in vivo experiments that herbicides with auxin action stimulate cell wall synthesis. The investigations carried out on cell-free plasma membranes isolated from onion stalk treated with 2,4-D, furnished further proof of the stimulation of polysaccharide synthesis. 

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