Auxin antagonists

Kawano T, Kawano N, Hosoya H, Lapeyrie F. Fungal auxin antagonist hypaphorine competitively inhibits indole-3-acetic acid-dependent superoxide generation by horseradish peroxidase. Biochem Biophys Res Commun 2001 288 546-51. 

These herbicides are auxin antagonists. They inhibit growth in the roots and stems of sensitive grassy weeds. Chlorophyll contents and photosynthetic activity are reduced in sensitive plants. The translocation of the photosynthesis in the roots is also reduced, and the accumulation of sugars increases in the stem (Chow and La Barge, 1978). 

Auxin antagonists inhibitors of the effects of auxins on growth and development. The action of A. a. can be at least partly reversed by auxins. The term is used independently of the mechanism of inhibition competitive inhibitors are called antiauxins. The A. a. include many structurally different kinds of compounds, including some synthetic auxins, e.g. phenylacetic acid and phenylbutyric acid. 

The problem of selectivity of detection has been a major aspect of plant hormone analysis since the early days of bioassay, as shown by the plethora of studies on naturally occurring auxin synergists and auxin antagonists . Two complementary approaches have been taken to improve the selectivity of detection. The first approach has been the application of high resolution techniques of separation science in order to purify the phytohormone away from interfering compounds. The second approach has been the use of selective detectors following chromatographic purification. The quality of an analytical method is dependent on the interplay of these two factors, resolution and detector selectivity. 

It was interesting in this connection to find that -isopropyl-PAA (VII e) is an auxin antagonists. This holds also for the w-butyl-, tert-h xiy - and M-amyl derivative (racemates, VII f,g,h). Apparently when a certain size of the ix-substituent is exceeded, normal interaction at the active site is prevented cf. also the discussion in Section 12). That rather subtle factors play a part here is evident if one compares the effects of the n- and isopropyl derivatives, and this is more apparent still when analyzing the activities of alkylidenephenylacetic acids. Whereas the activity of double bond is co-planar with the benzene nucleus (U.V.-spectra), one is inclined to make a comparison with the active 2-substituted i-naphthoic acids (Section 7). The picture is not complete, however, as the ethylidene- and M-propyUdene-phenylacetic acids (VIII b,c) were found to be practically inactive, for which no explanation is at hand so far. 

Illustrative in this respect are the cases referred to already in the preceding section, in which of the (+) and (—) forms of an optically active compound one is an auxin and the other an auxin antagonist as e.g. the naphthoxy-2-propionic acids (LXI, LXII). This implies that in the action of a racemate the components may be counteracting each other. 

Isolated nicotonic acetylcholine as well as plant receptors have been employed in receptrodes for the assay of acetylcholine and its antagonists, and auxin and toxin, respectively (Rechnitz, 1987 Thompson et al., 1986). 

Other processes involve an antagonistic action of auxin vs. cytokinin as well. For example, studies with transgenic plants containing genes for enhanced synthesis of either auxin or cytokinin has shown that both apical dominance and xylem development depend on the relative amounts of these hormones [32]. Enhanced auxin increases apical dominance and xylem formation, while enhanced endogenous cytokinin promotes the outgrowth of lateral buds, leading to a more branched plant, and decreased xylem development. 

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