Cell elongation auxin

The N-arylalanine ester herbicides such as benzoylprop-ethyl and flam-prop-methyl and difenzoquat (Figure 2.26) prevent cell elongation in certain grass weeds, allowing the crop to overtop them. The weeds are thus outcompeted and die. The exact mode of action is not certain but it is proposed that these compounds interfere with the site of action of the auxins. 

Both compounds are rapidly translocated herbicides that selectively kill broad-leaved weeds. They mimic the natural auxin, indol-3-ylacetic acid, which is responsible for promoting cell elongation and hence cause unrestrained growth. This uses up all the available nutrients, leading to the death of the plant. A review of the pyridinecarboxylic acids has been published (B-75MI10702). 

Rayle, D.L. Cleland, R.E. (1992). The acid growth theory of auxin-induced cell elongation is alive and well. Plant Physiol. 99,1271-1274. 

Ethylene as a stimulator of growth and development. The most observed actions of ethylene on growing plants involves growth inhibition, or acceleration of senescence. These actions are especially evident in the antagonism or opposition of ethylene to auxins, gibberellins and cytokinins (27), as already outlined above. Actually ethylene stimulates growth in many types of cells, especially in water plants (Table II). When ethylene acts to stimulate cell elongation, as in water plants, auxins and CC>2 enhance the ethylene effect (38,39). This interaction is the reverse of that observed on land plants wherein ethylene opposes the effects of auxin, GA3 and cytokinins. 

Matthyssee and Phillips (20) isolated two nuclear proteins, from tobacco cells, that bound specifically to 2,4-D. Receptor proteins for auxins, kinetins, and GA have been found (21). Sub-cellular fractions from bean leaves were recently shown to bind abscisic acid (22). Preliminary experiments (22) indicated that maximum ABA binding activity coincides with the activities of membrane-bound Mg -dependent, K+-stimulated ATPase and glucan synthetase. Table I of Biswas and Roy (21) lists hormone receptor proteins reported in plant tissue. For a protein to qualify as a receptor molecule, it should have a high stereo-specific binding capacity (Kd 10 6 to 10 SM) for its particular hormone. In com coleoptiles, both IAA and NAA are equally effective in inducing cell elongations fractions of plasma membrane and endoplasmic reticular membrane contain receptor proteins with Kd values of 10 M to 10 M for auxins (5, 18). When one considers procedural... 

Plant hormones rarely act alone hormones interact to produce a final effect. According to Gaspar et al., Some responses of plants to auxins may be caused by increased ethylene synthesis in response to auxin treatment. At high ethylene concentrations, microtubule and microfibril orientation are altered, which results in decreased cell elongation and increased cell expansion. The role of ethylene is hard to understand because it effects vary with developmental stage and because low concentrations can promote (or sometimes inhibit) a process, whereas higher levels have the opposite effect [22]. 

Auxins are defined as organic substances that promote cell elongation when applied in low concentrations to plant tissue segments in a bioassay. By this definition, there are several other native auxins that have been reported to occur in plants in addition to the most often studied auxin, IAA. These include the halogen-substituted 4-C1-IAA,23 as well as phenylacetic acid and indole-3-butyric acid.24 All native auxins are found in planta as both free acids and conjugated forms through ester or amide linkages. IAA, the auxin most extensively studied, will be the focus of this chapter. 

Auxin-like responses, exudates, and initiation of adventitious roots affected. Cell elongation and division promoted or inhibited. Geotropic response lost. 

The plasma membranes of growing plant cells select to incorporate sugars, amino adds, ions and other low molecular weight compounds from the apoplastic space, and then, the cells have a certain level of osmotic pressure. The difference between their osmotic pressure and their wall pressure (=turgor pressure) is due to motive power (suction force) to suck water from the apoplastic space (Figure 1). The plant hormone auxin, which decreases the wall pressure in a growing plant cell, therefore induces cell elongation or expansion. 

The cell wall defines the shapes of plant cells and restricts their elongation. Auxin-Induced loosening of the cell wall permits elongation. 

There is some evidence to suggest that / -glucanases may play a role in the expansion of yeast cells in the absence of cell duplication. Yanagishima (61) and Shimoda et al. (62) discovered that a plant hormone, auxin (indole-3-acetic acid), induced cell elongation in auxin-... 

While it was clear in the 1870s that transportable chemical signals exist in plants, solid evidence for specific hormones required another half century. Fitting [12], who first introduced the term hormone into plant physiology, showed that orchid pollinia contain some factor that causes swelling of orchid ovaries. He was not, however, able to isolate or identify the substance. Then in 1926, Went isolated a substance from coleoptile tips which caused coleoptile cell elongation he called this substance auxin [13]. After some unfortunate false starts, the identity of the main natural auxin was established as indole-... 

There are several ways to alter effective levels of putative hormones. The first is to excise a plant tissue that is incapable of synthesizing the hormone itself, and allow the tissue to become depleted of the hormone. If this causes cessation of a particular response, and upon readdition of the compound the response is restored, there is reason to believe that the compound is a hormone controlling that process. For example, excision of sections of coleoptiles results in a marked decline in growth rate [13]. Since auxin can restore the growth rate, while none of the other hormones can substitute for auxin, the evidence that coleoptile cell elongation is regulated by auxin is strong. 

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