Effects of Auxin

Auxins modify mitotic activity in cell types ranging from yeast (Sharma and Baruah 1975) to bovine muscle cells (Basrur etal. 1976). The high levels of auxin in human cancer tissue have even led to the suggestion that lAA plays some role in the acceleration of division in animal cancer cells (Yamaki et al. 1974). 

Auxin is essential for cell division in higher plants, but the evidence for direct control of the cell cycle is weaker for auxin than for cytokinin. In tobacco cell suspensions, for example, withdrawal of either cytokinin or auxin leads to cessation of division but, unlike cytokinin, replenishment of auxin has no synchronizing effect on the resumed cell division (Jouanneau 1971). However, auxin does play a role in the timing of cytokinin-induced synchronous division cultures pretreated with auxin before cytokinin restoration begin synchronous cell division earlier than those in which auxin and cytokinin are replenished simultaneously (Peaud-Lenoel and Jouanneau 1971). This indicates that auxin may be necessary, perhaps indirectly, for one or more phases of the cell cycle, possibly the RNA and DNA synthesis associated with the S phase (Nitsch and Lance-Nougarede 1967). 

In cultured carrot cells, temporary auxin deprivation induces at least partial synchronization of cell division (Nishi et al. 1977), a response not shown by cultured tobacco cells. Cell arrest during auxin deprivation appears to be in Gj, suggesting that auxin is needed to initiate the S phase of the cycle. 

In some cases, changes in auxin levels show a positive correlation with changes in mitotic activity. Nishinari and Yamaki (1976) noted that, in synchronized tobacco cell cultures, there is an increase in the activity of lAA-synthesiz-ing enzymes preceding an increase in the level of auxin, and that these events occur just prior to the increase in the number of dividing cells. The authors concluded that synthesis of free lAA may initiate mitosis. A dependence of cell division on internal auxin concentration in Acer pseudoplatanus was also shown by Lequay and Guern (1977). 

Gamburg and Osharova (1973) reported that NAA caused a sharp increase in both mitotic activity and DNA synthesis in tobacco cells. Although DNA synthesis was induced by brief exposure to NAA, mitosis did not follow without 

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Table I. Effects of auxin treatment on pea epicotyl apices. Apical 5 mm regions of etiolated epicotyls were delineated, seedlings sprayed once at zero time with or without 4.5 mM 2,4-D and marked regions examined after 48 h. Data compiled from refs. 28 and 32...

The stimulatory effect of auxin on ethylene biosynthesis is the basis for the use of synthetic auxins (Figure 5.11) to promote the abscission of... 

Since the same effects occurred with the non-herbicidal (2,6-dichloro-phenoxy) acetic acid as with toxic compounds, glucose metabolic changes are not the important singular effects of auxin herbicides. 

It has long been known that detergents increase the effectiveness of auxin herbicides. This may now be interpreted as perhaps not due entirely to the wetting properties of the detergents and it would be of interest to compare fatty acid ester detergents with those not containing fatty acid groups in herbicide trials. 

The effects of auxin upon the growth of plants were truly remarkable. As a result of the concerted effort and interest that plant scientists devoted to this highly fashionable and exciting field of research, much of what we know of the control of plant growth and development by auxin was discovered in these highly productive years. The hormone was derived from the aromatic amino acid tryptophan and was relatively abundant in the rapidly growing meristematic parts of plants such as apical buds and root tips. Inactive bound forms existed, as in seeds, where hydroly-... 

A study has been reported on the effect of auxins on the solasodine content of S. platanifolium,48... 

How IAA can elicit profound changes in the size and form of a plant is totally unknown. At the cellular level it is known that the plant cell walls must be "softened" for growth promotion to occur (cf. 18J. Such effects, however, may be concomitants of growth and not the "primary" effects of auxin (19). 

Ethylene as a stimulator of growth and development. The most observed actions of ethylene on growing plants involves growth inhibition, or acceleration of senescence. These actions are especially evident in the antagonism or opposition of ethylene to auxins, gibberellins and cytokinins (27), as already outlined above. Actually ethylene stimulates growth in many types of cells, especially in water plants (Table II). When ethylene acts to stimulate cell elongation, as in water plants, auxins and CC>2 enhance the ethylene effect (38,39). This interaction is the reverse of that observed on land plants wherein ethylene opposes the effects of auxin, GA3 and cytokinins. 

Stimulatory effects of auxins on the activity of polymerases of lentil roots have been reported (45). Hardin s model (46), in which factors that "modify" RNA polymerases are released as a result of the auxin-plasma membrane interaction, has not been confirmed, but the finding that auxin action involves regulation... 

I. Effects of auxin on template capacity and chromatin-bound RNA polymerase. Mech. Ageing Dev., 1978, 8, 97-112. 

Palazon, ]., Bonfill, M., Cusido, M. and Morales, C. (1995) Effects of auxin and phe-nobarbital on morphogenesis and production of digitoxin in Digitalis callus. Plant Cell Physiol, 36, 247-52. 

Effects of auxins on growth and phenolic production in adventitious root cultures of Sanguisorba officinalis... 

Defoliation. Interest in defoliation has been low in recent years. One relatively new development is the "wiltant which is applied only shortly before harvest (51). As an outgrowth of some basic studies, several auxin transport inhibitors, TIBA, DPX-1840, Alanap (N-l-naph-thylphthalamate), and morphactins (2-chloro-9-hydroxyfluorene-9-car-boxylic acid), were shown to promote ethylene- and ethephon-mediated leaf abscission (52, 53). Subsequently, CA3 was found to be even more active in promotion of ethylene-induced abscission (54). It now appears that the CA3 counteracts the inhibitory effect of auxin on ethylene-induced leaf abscission (55) thus, CA3 might improve the performance of any defoliant that achieves part of its action by stimulating stress-induced ethylene production and lowering the natural auxin content of the dam-... 

Ethylene promotes a large array of responses in seeds, plants, and fruits (Table I), some similar to and some dissimilar to effects of auxins. Those with actual or potential commercial application include (a) growth promotion of seedlings (rice), (b) inhibition of height growth, (c) root initiation, (d) chlorophyll destruction (citrus), (e) flower initiation (pineapple, bromeliads), (f) stimulation of fruit growth (figs). 

Table I. Effects of Applied Ethylene on Plants and Plant Parts Compared with Similar and Dissimilar Effects of Auxins (5, 67)...

After 8 weeks from the inoculation, shoots appeared on all the media, demonstrating maximum 5 shoots per segment on B5 medium containing 0.01 mg/1 NAA plus 3 or 5 mg/1 BA, Fig. (3). Although the effect on addition of single cytokinin was also examined, cytokinin in combination with low concentration of auxin was even more effective, which may be attributable to the shoot growth stimulant effect of auxin in low concentration [18]. Shortened intemodal length of shoots was observed by the addition of BA, Fig. (3). 

The effect of auxins on the alkaloid production in ipecac root cultures... 

Callus and root organ cultures of Duboisia have been studied with a view to develop a new method to obtain tropane alkaloids more efficiently [64-66]. In solanaceous plants, which produce tropane alkaloids, cultured roots produce them but calli do not [65] or only in very small amounts [66]. Root organ culture might be a useful method to produce tropane alkaloids and suitable culture conditions for growth and alkaloid production must be established. In this section, the effects of auxins including lAA chloro-derivatives on the growth and alkaloid yields in root organ cultures of Duboisia myoporoides - D. leichhardtii hybrid are described [67, 68]. 

The adventitious root segments (1-2 cm) were inoculated into 50 ml MS liquid medium supplemented with 0.5 mg/1 lAA in a 100 ml Erlenmeyer flask and maintained in the dark on a rotary shaker (100 rpm) at 25 C. The roots ca. 30 mg fresh weight) were subcultured at four weekly intervals in order to obtain sufficient materials for experiments. To examine the effect of auxins in root culture, roots ca. 30 mg fresh weight) were inoculated into 50 ml MS liquid medium supplemented with various concentrations of auxins under the conditions described above. Growth and alkaloid contents were measured after certain culture periods. 

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