Hormones Other than Auxins

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. 

In contrast to the convincing demonstration of a specific basipetal auxin transport system, mostly conflicting results have so far been obtained concerning the translocation of other hormones. 

3 Transport and Other Modes of Movement of Hormones (Mainly Auxins) 

1973) and Zea mesocotyls and coleoptiles (El-Saidi 1972), and for segments from different floral organs of Cleome (Koevenig 1973). Moreover, Fox and Weis (1965) were not able to confirm the results of Osborne and coworkers who, in all likelihood, had used unphysiologically high hormone concentrations. Thus, a clear picture of the mechanism and polarity of cytokinin translocation is lacking. 

Abscisins and Thiamine. The few studies on ABA movement in shoot sections has likewise led to contradictory results. Dorffling and Bottger (1968) and Milborrow (1968) found fast (20-30 mm h ) ABA movement through petiole 

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In seedlings the cotyledons are remarkable for the extent and speed of the growth of roots and leaves in the dark. Therefore Went assumed that the hormone-like substances (rhizocaline, phyllocaline) other than auxin... 

For each of these approaches it is essential to measure the actual concentrations of the putative hormone. This is no trivial task. Great care must be exercised in obtaining quantitative values. There must be a correction for losses in the hormone during preparation and analysis of the sample [33]. Another problem is that sizable amounts of the hormone may be sequestered in compartments other than the one in which the hormone is physiologically active, for example, ABA is concentrated in chloroplasts, while its site of action appears to be the plasma membrane [34], Or the hormone may be in a different part of the tissue from the one where it acts. For example, the auxin levels in the stele and cortex of roots are vastly different [35] analysis of the total auxin levels in roots may give the wrong impression of the amount of auxin available for some auxin-dependent process in the cortex. 

O. have been shown to inhibit flowering, promote vegetative growth and control organ development in plant tissue culture. Effective concentrations of O. are 100-1000-fold lower than those of other plant hormones, such as auxins, cytokinins and gibberellins. 

Well before the discovery of auxins and gibberellins during the 1920 s hormone movement had been suggested as occurring in plants. As early as in 1879 Julius Sachs, considering observations on correlations between main axes and side shoots, proposed substances other than nutrients as involved in these phenom-... 

Some biological roles of auxins. The involvement of other hormones is indicated as (+) if the hormone has the same effect as auxin and (-) if it inhibits the auxin effect. Speed of response rapid (R), occurs in less than 1 hr intermediate (I), 1-24 hours slow (S), > 1 day. 

In addition to mitosis, cytokinins interact with auxins in a number of other important processes. The initiation of shoot and root primordia from calluses depends on the ratio of cytokinins to auxins rather than on the absolute amount of either hormone [60]. When the CK/auxin ratio is high, formation of shoot primordia is favored, while a low ratio promotes the formation of root primordia. The formation of lateral roots also appears to be regulated, in part, by the CK/auxin ratio [39]. The primordia develop at a location back from the root tip specified by auxin from the shoot and cytokinin from the root tip. 

Auxin mutants and cross resistance. Many hormone mutants, both in Arabidopsis and other species, display altered responses to more than one plant hormone [47]. In... 

The characteristic feature of auxins is their ability to promote cell elongation in coleoptile and stem tissue. For a detailed discussion of the auxins the reader is referred to Audus (1972). The emphasis here will be on the interaction of auxin with other plant hormones in the regulation of stem and coleoptile cell elongation. Coleoptiles are included here rather than in the section on leaf cells, since the hormonal sensitivity of coleoptiles is more stem-like than leaf-like. 

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