Auxin structure

Peracetylated derivatives the plant hormone-related 2-(indol-3-yl)-ethyl and 2-phenylethyl P-D-xylopyranosides 1 and 2 respectively 2-(indol-3-yl)ethyl P-D-galactopyranoside tetraacetate, related to the plant hormone auxin. Structural analysis was also assisted by NOE NMR experiments peracetates of (2S)-butyl 6-0-(P-D-apiofuranosyl)-P-D-glucopyranoside glycosides isolated from cassava Manihot esculentd)P ... 

Abscisin II is a plant hormone which accelerates (in interaction with other factors) the abscission of young fruit of cotton. It can accelerate leaf senescence and abscission, inhibit flowering, and induce dormancy. It has no activity as an auxin or a gibberellin but counteracts the action of these hormones. Abscisin II was isolated from the acid fraction of an acetone extract by chromatographic procedures guided by an abscission bioassay. Its structure was determined from elemental analysis, mass spectrum, and infrared, ultraviolet, and nuclear magnetic resonance spectra. Comparisons of these with relevant spectra of isophorone and sorbic acid derivatives confirmed that abscisin II is 3-methyl-5-(1-hydroxy-4-oxo-2, 6, 6-trimethyl-2-cyclohexen-l-yl)-c s, trans-2, 4-pen-tadienoic acid. This carbon skeleton is shown to be unique among the known sesquiterpenes. 

Scheme 1. Molecular structure of the plant growth hormone auxin (indoleacetic acid, IAA). Extremely small amounts (nanomolar) can be detected by the auxin standard test 5 mm long segments of pea shoots elongate faster in the presence of exogenous auxin, which can be taken as a sensitive assay...

FIGURE 1.5 Structures of auxin and abscisic acid derivatives of cytokinin. 

None of these had pronounced herbicidal activity but rather PGR activity. Compound % for example, has been shown to be an auxin transport inhibitor, a property probably shared by the other members of this class also. A structure activity analysis for this group of compounds has been reported by Katekar ( ) ... 

Another example of the use of the MTD and MTD approaches can be found in a series of optically active o-phenoxypropionic acids with auxin-like activity, partly published in ( ). The R-stereo isomers are much more active than the S-analoges. Both series were analyzed by Lien et al. (10) and a correlation with ir, a and the Van der Waals volume was found. The Pfeiffer rule is explained in terms of different structural requirements for the substituents as measured by and van der Waals volume. Analysing the series using STERIMOL delivered equations containing too many parameters. In Table III the equations are given as a result of... 

Phenylalanine, tyrosine, and tryptophan are converted to a variety of important compounds in plants. The rigid polymer lignin, derived from phenylalanine and tyrosine, is second only to cellulose in abundance in plant tissues. The structure of the lignin polymer is complex and not well understood. Tryptophan is also the precursor of the plant growth hormone indole-3-acetate, or auxin (Fig. 22-28a), which has been implicated in the regulation of a wide range of biological processes in plants. 

On the basis of these observations, it was tentatively concluded (3) that auxin-treated pea tissue elaborates two cellulases which are physically so distinct that it is unlikely that one could have derived from the other. Of course, if two forms of cellulase arise from genetically determined differences in protein structure, it would be legitimate to refer to them as isozymes (21). But in the absence of proof that the pea or any other plant cellulases are under separate genetic control, we will continue to refer to them as multiple forms. 

We stress these assays because, while leading to the discovery of IAA, they imposed structure-activity requirements precluding study of the IAA conjugates--and possibly the discovery of other auxins. For a substance to be active in the assays required that they 1) permeate membranes in a cut tissue surface, 2) be transported to the growing zone, and 3) promote growth in that zone. Hopefully these three requirements may someday be studied independently. 

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