Root elongation auxin

While the application of relatively high concentrations of lAA retard root elongation, there may be a considerable increase in the development of branch roots and root hairs (27). As a result, the practice of dipping cuttings in auxin has become commercially important in promoting the proliferation of adventitious roots. Usually, synthetic auxins, such as indole-3-butyric acid (IBA) and naphthaleneacetic acid(NAA) which are not subject to lAA oxidase, are used. 

TAAl was also isolated from a genetic screen for weak ethylene-insensitive mutants, wei mutants, in Arabidopsis. The taalAveiS mutants have elongated roots in the presences of ACC (1-aminocyclopropane-l-carboxylate), an ethylene biosynthesis precursor, whereas in wild-type root elongation is inhibited under the same conditions. Moreover, simultaneous inactivation of TAAl and of two of its close aminotransferase related homologs, TARl and TAR2, led to developmental defects similar to those in two well-known auxin mutants [171],... 

In contrast to the auxins and gibberellins, abscisic acid (ABA) and ethylene usually retard cell elongation. With some exceptions (Takahashi 1972, Gaither et al. 1975, Malik and Mehan 1975 a, Van Staden and Bornman 1970) ABA is inhibitory to growth (Addicott and Lyon 1969, Milborrow 1974). ABA inhibits stem and root elongation and counteracts the promotive effects of other substances on these organs (Rehm and Cline 1973, Kaufman and Jones 1974,... 

Caspar et al. (1972) suggested that the inhibition of root elongation by lAA may be caused by a metabolite of lAA, most probably 3-methyleneoxindole (3-MO). However, it has since been shown that purified 3-MO does not inhibit root elongation (Evans 1976c). These data in conjunction with the rapidity (15-20 min) with which intact root growth is inhibited by low concentrations of lAA, indicate that auxin inhibition of root cell elongation is a direct effect. 

Kinetin promotes the formation of ethylene in the elongation zone of eorn roots. Although enhancement of ethylene production by kinetin is greater than by auxin, Svensson (1972) concluded on the basis of other data that ethylene does not mediate the inhibitory effect of kinetin on root elongation. A similar conclusion was reached by Dubucq (1976). 

Caspar and Xhaufflaire (1967) noted that kinetin-treated lentil roots have more lAA oxidase activity than control roots. They suggested that kinetin inhibition of root elongation may be due to a reduction in the auxin content of the root. This was supported by Darimont et al. (1971) who found that kinetin reduces the lAA content of lentil roots. 

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... 

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