Auxins origin

Addition of oligosaccharides to combinations of auxin plus cytokinins also influences the differentiation of shoots and roots in tissue culture. Indeed, many of the pleiotropic effects originally attributed to auxins and other plant hormones may actually be mediated by oligosaccharins. 

The original proposal concerning the mode of action of auxin stated that auxin made cell walls more plastic, thus inducing a water-diffusion-pressure deficit and causing the cells to expand (7). Much evidence has since been gathered in support of this hypothesis. For example, in Avena coleoptiles it has been established that the auxin-induced increase in plasticity precedes, or coincides with, the... 

The variety of aldehyde oxidases discovered in other plants have similarities to the maize enzyme, but also have some very important differences. Enzymes contained in a cell wall fraction from barley seedlings were able to oxidize IAAld to form IAA at a pH optimum of 7 and Km of 5 pmol 1 1, which was very similar to the enzyme found in maize.113 Two aldehyde oxidases from potato have also been identified 101 they had a similar pH optimum (between 7 and 8), but preferred aliphatic aldehydes to aromatic aldehydes. Although oat and cucumber aldehyde oxidases have been shown to oxidize IAAld to produce IAA,102 114 the oat enzyme had a lower pH optimum and higher Km than the maize enzyme, and the cucumber enzyme was inhibited by synthetic auxin and activated by 2-mercaptoethanol, which was not true for the maize enzyme. The difference in the enzymes makes it difficult to envision a common evolutionary origin for the IAAld pathway in plants if these particular enzymes are involved in each case. 

Several primary auxin response genes have been isolated from arabidopsis, using homologous genes isolated previously from other species as probes (see chapter 19). The auxin inducible promoters from these original plant species have been used in screens for auxin response mutants in arabidopsis. 

Bean First Internode Bioassay. This assay was originally designed for the detection of auxin activity (37). It takes advantage of the fact that unilaterally applied auxin causes bending of the treated bean internode, which reaches a maximum rate about 20 minutes after auxin application. The sections will respond to 10 pmol of lAA and BR increase the sensitivity of the sections to auxin by one order of magnitude. 

Auxins.- The original endogenous hormone is the Indole-3-acetic acid (lAA). Some of the most used synthetic PGR with auxin activity are indole-3-butyric acid (IBA), naphthaleneacetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), etc. 

Another explanation, based on anatomy, is also possible. When the auxin is applied to a cut surface, most of it may be transported in the cortex. If so, the ethylene that it engenders would be near the stem surface, and thus be readily removed by the reduced pressure. But auxin produced by the living apex may be transported partly within the stele, so that the resulting ethylene would not be so freely exposed to the reduced pressure however, it would still inhibit the bud development since these primordia originate in the stele. 

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