Auxin other plant hormones

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 experimental results on BR behavior described above are summarized in Table I in comparison with those of IAA and GA. BR elicits physiological effects that are very similar to those of other plant hormones, in particular, auxin and GA. However, the behavior of BR is unique and different from those of the others. The facts that BR modifies plant growth by itself or in association with other hormones, that its effective concentration is very low (lOnM), that it has different modes of action, and that it occurs widely in the plant kingdom strongly indicate that BR is a new plant hormone which is probably essential for plant growth and development. 

Other plant hormones, such as auxins and GAs. Finally, selected compounds were assayed using a cress hypocotyl elongation test. It has been demonstrated that cress is very sensitive to an internal deficiency of BRs and is therefore a useful species for evaluating BR biosynthesis inhibitors [7-8],... 

O. have been shown to inhibit flowering, promote vegetative growth and control organ development in plant tissue culture. Effective concentrations of O. are 100-1000-fold lower than those of other plant hormones, such as auxins, cytokinins and gibberellins. 

Inhibition of shoot branching or bud outgrowth by SLs has been reviewed extensively, and thus, we will not go into details here. Other plant hormones, in particular auxin and cytokinin, also influence shoot branching directly or indirectly through their effects on SL production and/or signaling [51, 82-84]. 

The characteristic feature of auxins is their ability to promote cell elongation in coleoptile and stem tissue. For a detailed discussion of the auxins the reader is referred to Audus (1972). The emphasis here will be on the interaction of auxin with other plant hormones in the regulation of stem and coleoptile cell elongation. Coleoptiles are included here rather than in the section on leaf cells, since the hormonal sensitivity of coleoptiles is more stem-like than leaf-like. 

A majority of the studies on transport of auxin (and other plant hormones) published during the years of research since the first papers of the Utrecht School (Went 1928, van der Weij 1932, 1934, Dolk 1930, 1936) have been designed to estimate the parameters in the transport equation. Using varying experimental conditions and many kinds of plant parts, and presupposing a uniform auxin stream, usually only two parameters, intensity and velocity, were determined in order to calculate the third, density, of the equation. Whether such calculations are justified, will be critically discussed in Section 3.3.3. 

Herbicidal 2,4-D selectively kills broadleaf weeds and is the most widely used of all herbicides worldwide on wheat, corn, rice, and other cereal grass crops. It is a synthetic auxin, or plant hormone that is absorbed through leaves, is transferred throughout the plant, and causes unsustainable growth that results in plant death. Fortunately, given its widespread use and human exposure, it has very low acute toxicity. It is possibly carcinogenic, a status shared with many other pesticides, and is a suspected endocrine disruptor (see Chapter 2, Section 2.16). 

Herbicides may be selective, as for broad-leaved weeds in cereal crops, or unselective, essentially for land clearance. 2,4-Dichlorophenoxyacetic acid (2,4-D), 24.6, was commercialized in the 1940s it is inexpensive, is easy to make, and kills broad-leaved weeds in cereal crops. Worldwide, it is the most widely used herbicide. It is a synthetic auxin, or plant hormone, acting only on dicots and not monocots. There is a whole family of related compounds, prepared from chloroacetic acid (or other halo acids) and various chlorinated phenols. Agent Orange, used as a defoliant in Vietnam in the 1970s, was a 1 1 mixture of 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). The main concerns about its use relate to the possibility that dioxins are formed as contaminants in its manufacture. A few countries ban its use for control of weeds in domestic lawns. 

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. 

Next to the amount of P, the chemical form of this nutrient (Lambers et al. 2002 Shu et al. 2005 Shane et al. 2008) and the availability of other nutrients, especially nitrogen, potassium, and iron (Shane and Lambers 2005) affects the formation of cluster roots. It seems to be regulated by several plant hormones. Thus, application of auxin led to the production of cluster roots in white lupin at P concentrations that normally suppress cluster roots (Gilbert etal. 2000 Neumann et al. 2000). Cytokinines might also play a role, as kinetin applied to the growth medium of P-deficient white lupin inhibited the formation of cluster roots (Neumann et al. 2000). 

Even though most phenolics are not hormones themselves, they may affect plant growth by interaction with one or other of the major kinds of plant hormones, such as the auxins. Physiological studies have suggested... 

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