Hormonal Modification of Endogenous Auxin

Much of our information on hormonal regulation has been from experiments in which hormone is applied to tissue segments. While these studies have been informative, they have made us aware of precautions necessary in applying results to functions of hormones in the intact plant. It has become clear that the effects of a particular hormone are dependent on other hormones, and that the applied hormone may trigger changes in the endogenous levels of other hormones. 

ABA inhibits the enhancement of elongation by auxin, and there is evidence that ABA can inhibit auxin biosynthesis. ABA appears to reduce lAA levels during the regeneration of a physiological tip in decapitated coleoptiles (Anker 1975), and there is evidence for ABA-enhanced auxin degradation in the presence of supraoptimal concentrations of lAA (Milborrow 1966). 

There is extensive evidence that GA increases auxin levels either by enhancement of auxin biosynthesis (Jindal and Hemberg 1976, Sastry and Muir 1965) or by retardation of auxin destruction (Kogl and Elema 1960). Nevertheless, there is convincing evidence that GA can also stimulate elongation independently of auxin (Kazama and Katsumi 1974, Cleland et al. 1968, Kaufman et al. 1969). In excised Avena stem internodes, for example, GA stimulates cell elongation, while auxin is inhibitory (Kaufman et al. 1969). Similarly, stimulation of lettuce hypocotyl elongation has been used as a bioassay for GA, while 

One of the most potent factors in the regulation of lAA biosynthesis may be lAA itself. lAA appears to suppress its own biosynthesis in decapitated coleoptiles (Anker 1973), and certain auxin-induced modifications of growth patterns have been attributed in part to effects on endogenous auxin production (Evans and Schmitt 1975, Evans et al. 1977). Red light-induced suppression of auxin production in coleoptile tips may result from a suppression of auxin movement out of the tip leading to feedback inhibition of auxin biosynthesis (Huisinga 1976). 

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