Auxin analogues

Dicamba (3,6-dichloro-2-methoxybenzoic acid) is primarily used as a postemergence broadleaf herbicide, which interferes with normal plant auxin function, subsequently causing uncontrolled growth and the inhibition of the phototropic and geotropic function. Cumulative response results in plant death. The success of auxinic analogues such as Dicamba and 2,4-dichloropheno-xyacetic acid in weed control has led to widespread manufacturing and use. Estimated U.S. production for Dicamba was 5 million kg in 1990 [391]. 

Figure 3. Development of BS and BI cellulase activity in apices of pea seedlings. Intact seedlings were sprayed with the auxin analogue 2,4-D and decapitated seedlings were painted with the natural auxin IAA with or without an inhibitor of DNA synthesis, FUdR. All treatments resulted in massive swelling at the pea apex because of cell expansion cell divisions also occurred, but not in the presence of FUdR (6). Cellulases were extracted as described in Figure 1 and assayed in unpurified form.

Tuber dormancy studies can be separated into three general types based upon the tuber material used and whether the dormancy requirement is fulfilled naturally or via an artificial induction of cell division. Each method has distinct advantages and disadvantages. The tuber material includes intact tubers, tuber slices in which cell division is induced using an auxin analogue in an aqueous... 

Numerous azo dyes derived from 3-amino-2,l-benzisothiazole have been claimed.138 The University of Parma group has continued to develop auxin analogues from derivatives of 2,l-benzisothiazole-3-carboxylic acid,139... 

The synthetic auxin analogues have a much stronger effect on broadleaved plants than on grasses, which gives a high degree of valuable selectivity, but made them useless for their intended application to destroy the wheat harvest in Germany. 

In all work on auxin analogues, the carboxy group can be replaced by other electron-attracting groups (—CN, —NO2, —SO3H) with only moderate loss of biological action. For a summary of connexions between structure and action in this series, see Koepfli, Thimann and Went, 1938 Veldstra, 1963). The action of auxins requires sequential receptors (p. 41). 

TABLE 5 Inhibition by Auxin and Auxin-analogues of formation in vivo in illuminated Buckwheat Bypocotyls... 

The lAA-forming activity present in crude buffer extracts could unfortunately not be stabilized to permit isolation of the respective enzyme(s) [2]. It could, however, be attributed to a true oxidase (or a set of oxidases) which was at least group specific for aromatic aldehydes. The assay system did not permit a check for product inhibition by lAA. PAA and the auxin analogue 2,4-D did, however, inhibit indoleacetaldehyde oxidation. 

There are, however, specific tryptophol oxidases (at least two) in cucumber seedlings [ 1,25,42]. These oxidases are inhibited by indoleacetaldehyde, lAA, and synthetic auxin analogues. So IAA could regulate its own biosynthesis, at least from tryptophol. 

The formulation of 2,l-benzisothiazolin-3-one as ketonic tautomers (see Vol. 3, p. 558) has been confirmed by the preparation of its N- and O-alkylated derivatives, by unambiguous routes, and examination of their u.v. spectra. The same conclusions have been reached about 5-chloro-2,l-benzisothiazolin-3-one. Other nucleophilic displacements of the 3-halogen in 3-chloro- and 3,5-dichloro-2,1-benzisothiazole have been reported the auxin analogue (72 R = CHgCOaH) was obtained in the course of this work. The results of a comparative study of the rates of A -methylation are included under isothiazoles (p. 344). 

Of the bicyclic acids, i-naphthaleneacetic acid (NAA, IX) is the best known structural analogue of indoleacetic acid, also resembling the natural auxin in many aspects of its actions. The fact that the isomeric 2-naphthalene-acetic acid (X) is very weakly active only, forms one of the most intriguing questions on structure-activity relationships, especially when compared with the inverse relations found with the naphthoxyacetic acids (c/. Section 4). 

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