Auxin action

Enhancement of Gibberellin and Auxin Action by Alkyl Lipides... 

Napier, R.M. Venis, M.A. (1993). Tansley Review No. 79. Auxin action and auxin binding proteins. New Phytol. 129, 167-201. 

Stimulatory effects of auxins on the activity of polymerases of lentil roots have been reported (45). Hardin s model (46), in which factors that "modify" RNA polymerases are released as a result of the auxin-plasma membrane interaction, has not been confirmed, but the finding that auxin action involves regulation... 

Ray, P. M. Specificity of auxin-binding sites on maize cole-optile membranes as possible receptor sites for auxin action. Plant Physiol., 1977, 60, 585-591. 

Amid an ever clearer picture of auxin action, the auxin receptor remained a conspicuous omission from signaling models. Early biochemical efforts to isolate auxin receptors identified AUXIN-BINDING PROTEIN1 (ABP1).184,185 However, the majority of ABP1 localizes to the endoplasmic reticulum where the pH is too high for auxin binding. Furthermore, embryo-lethal null mutants and the lack of weak mutant alleles complicated detailed analysis. 

The First Internode Bioassay (15) is based on the curvature of the first internode section after insertion of a paper disc containing auxin to one side of the morphologically basal part of the internode. When it is used for the testing of BRst, the paper discs containing tested BRst are applied to the internode 1 h prior to auxin application. BRst stimulate the lag-phase of auxin action and its activity is expressed as a difference in internode curvature between BRst + IAA tested sections and controls treated with auxin alone. 

BRs do not affect the peroxidase and polyphenol oxidase activities (this is in contrast with auxin action), a fact indicating the synthesis of specific proteins by BRs. 

Since the recognition of the auxin action of phenoxy acids extensive research has been undertaken to elucidate the relationships between the biological activity and chemical structure of this group of compounds (Audus, 1953 Fawcett et al., 1956 Wain, 1958 Fawcett et al., 1959 1960 Melnikov, 1971 Wain, 1975). 

The statement of Koepfli et al. with respect to the unsaturated ring still holds true. All herbicides with auxin action that have been put on the market contain a phenyl ring. 

The alpha-hydrogen theory has been set up on the basis of a series of investigations of compounds with auxin action. According to this theory, phenoxy-, naphthoxy-, naphtho- and fluorenecarboxylic acid derivatives are active only if there is a hydrogen atom at the carbon atom adjacent to the carboxyl group (Osborne and Wain, 1949 Wain, 1953 Fawcett et al 1955b). 

The importance of alpha-hydrogen is also illustrated by the dithiocarbamates with auxin action, discovered by Van der Kerk et al. (1955). 

These observations suggest that molecules with auxin action interact with an asymmetric component of the cell responsible for growth. 

Considerable biological activity can be expected when a three-point contact is established between the molecule with auxin action and the groupings on the receptor surface this is possible only in the case of the active stereoisomer. The inactive isomer can establish only a two-point contact, as shown by the following scheme ... 

The receptor theory of auxin action has been further developed by Thimann (1969). Studying the auxin action by investigation of the electron structure of... 

The activity of the compounds substituted in the 2,4,6 position at the same time disproves the earlier theory that a precondition of auxin action was a free ortho position. 

Of the tetrachlorophenoxyacetic acids, only 2,3,4,5-tetrachlorophenoxyacetic acid is active, while the 2,3,4,6- and 2,3,5,6-isomers are inactive. Pentachlorophe-noxyacetic acid has fungicidal activity, but no auxin action. 

An ortho substituted methoxy group reduces auxin activity, but does not cancel it in compounds of otherwise strong activity. The meta methoxy group strengthens the auxin action in phenoxyacetic acids, but causes decreased activity in otherwise strongly active compounds. The para methoxy group enhances activity in phenoxyacetic acids, but reduces it in phenoxypropionic acids. 

The characteristic growth-regulating auxin action of phenoxy herbicides brings about changes in water and mineral balance, in vitamin and oil content, in respiration and photosynthesis, nitrogen and phosphorus metabolism and in enzyme function (Wort, 1964b). 

There is experimental evidence that herbicides with auxin action also interfere at the gene level in the specific protein synthesis of plants. According to the investigations of Hanson and Slife (1969) and Key (1969), this probably occurs by the abnormal enhancement of messenger RNA production. 

It can be considered that one bioactive function of herbicides with auxin action is the fundamental disturbance of vital protein synthesis however, the primary rapid action, manifested a few minutes after treatment, cannot be explained by the disturbance of protein synthesis. Similarly, the contact efifect produced at higher application rates of the herbicides cannot be explained by interference with gene function. 

Earlier experiments of Evans and Ray (1969) with substances inhibiting protein synthesis also seem to disprove the theory that disturbance of protein synthesis is a fundamental determinant of biological action. Rubery and Northcote (1970) proved by in vivo experiments that herbicides with auxin action stimulate cell wall synthesis. The investigations carried out on cell-free plasma membranes isolated from onion stalk treated with 2,4-D, furnished further proof of the stimulation of polysaccharide synthesis. 

D, 2,4-DP, 2,4,5-T and 2,4,5-TP, applying them alone and simultaneously with the parent acid. Of the 21 acid amides investigated 13 exhibited an antiauxin action, antagonising the auxin activity of the respective phenoxyalcanoic acid. At the same time, the compounds also had their own auxin action, showing on the one hand that amides too have an auxin action, and on the other hand that they are metabolised in plants by the enzymatic pathway to the respective acids and are thus the precursors of the acids. 

The investigations of Qarke and Wain (1%3) established that N-phenylglycines substituted on the ring have a considerable auxin action. It was to be expected that anilides obtained by substitution of the imide nitrogen of N-phenylglycines would exhibit a herbicidal action, primarily against grass weeds. 

Auxins are synthesized in apical buds (at the tip) of growing shoots. They stimulate growth of the main shoot and inhibit lateral shoot development. A class of auxin-binding membrane proteins may represent auxin receptors. Auxin action involves pumping protons out of the cell, possibly in conjunction with a membrane ATPase. 

The effect of 1 nM XXFG is not restricted to interfering in auxin action. Thus, 1 nM XXFG partially inhibits... 

A variety of second messengers have been implicated in auxin signaling including phosphatidylinositol, calcium, and the products of phospholipase A (PLA) enzyme. Since most of these reports have been in the literature for some time, they will not be discussed here (see [47] for review). Recent reports have provided additional evidence for a role for PLA in auxin action. Earlier studies by Andre and Scherer [48] showed that auxin treatment resulted in an increase in PLA activity in a number of plant species suggesting that a product of this enzyme functions as a signaling molecule. Consistent with this possibility, inhibitors of mammalian PLA inhibit auxin-induced hypocotyl elongation in... 

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