Polarity basipetal

The precise mode of PCIB action in Funaria is unknown as yet. In the rhizoids, 10 pM PCIB reduces the polar, basipetal transport to about 40% of the control [20]. In maize coleoptiles and Cucurbita hypocotyls, PCIB specifically competes with lAA for lAA-binding sites in vitro [8, 10]. The delay in caulonema production by... 

Keith and Baker (1966) found that naptalam strongly inhibits the transport and function of the basipetal polar auxin, 4-(indol-3-yl)acetic acid (lAA). 

Another important factor of hormone-hormone interaction is the hormonal control of hormonal movement and polarity. Thus, various cytokinins have been reported to increase the polar movement of indole acetic acid and vice versa. Gibberellin treatment was shown also under cer- bain circumstances to increase the basipetal auxin movement. On the other hand, abscisic acid decreases the auxin movement and ethylene, according to some evidence, the gibberellin movement, perhaps through a promotion of conjugation of the auxins with aspartate and of the gibberellins with glucose (cf. Eef. 27). 

These results suggest that in the short-term collection method basal and apical receivers should be applied simultaneously. By this means, the acropetal component of the transport system is accounted for and the auxin then delivered to basal receivers probably will give a more accurate measure for the density of the basipetal auxin stream. Moreover, the calculation of the acropetal/basipetal ratio may reflect the polarity of hormone translocation. 

Any model of the mode of action for a transport system of a polarly transported hormone must account for the preferential direction of movement. For auxins, the direction of movement is predominantly basipetally in shoots and acrope-tally, i.e., toward the tip, in roots. Basipetal auxin transport has been shown to be established, to a certain degree, in hepatics (Maravolo 1976) and in embryonic axes of vascular plants (Greenwood and Goldsmith 1970, Fry and Wangermann 1976). This polarity develops gradually up to a maximum with increasing age of the plant (e.g., Jacobs 1950, Smith and Jacobs 1968, 1969), and then declines, either because of a decrease in basipetal transport... 

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. 

Gillespie B, Thimann KV (1963) Transport and distribution of auxin during tropistic response. I. The lateral migration of auxin in geotropism. Plant Physiol 38 214-225 Goldsmith MHM (1959) Characteristics of the translocation of indoleacetic acid in the coleoptile of Avena. PhD Thesis, Radcliffe Coll, Cambridge, Mass Goldsmith MHM (1966 a) Movement of indoleacetic acid in coleoptiles of Avena sativa L. II. Suspension of polarity by total inhibition of the basipetal transport. Plant Physiol 41 15-27... 

Goldsmith MHM (1966 b) Maintenance of polarity of auxin movement by basipetal transport. Plant Physiol 41 749-754... 

This basipetal polarity suggested that auxins might be the controlling substance, because auxins were the only hormones known at that time to move... 

Among known plant hormones, only auxin exhibits basipetally polar transport. Auxin transport has a striking polarity which mirrors the structural and functional axial polarity of the plant body. Thus, even though other phytohormones are known to cause morphogenic effects, a prominent (and probably dominant) role in the spatial and temporal control of plant morphogenesis must be ascribed to auxin and especially to the still obscure processes comprising the mechanisms of its basipetal transport. 

Oscillatory phenomena associated v ith polar transport of synthetic auxin have also been reported in Avena and Zea coleoptiles (Newman 1963, Hertel and Flory 1968, Shen-Miller 1973 a, b), thus the oscillations of basipetal efflux of endogenous auxin found in our experiments may reflect a more general property of the mechanism involved in polar transport of auxin. 

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