Regulation phytohormones

In animal organisms regulation is mediated from cell to cell by nervous or humoral means. In plants regulation via the nervous system is precluded. There remains the possibility of regulation via the vascular system in which certain regulatory factors can be transported from cell to 

The four most important and chemically best known groups of phytohormones are certain indole derivatives, the gibberellins, cytokinins, and abscisins. In the following sections we shall first discuss each of these groups in turn and then consider them again in more detail with respect to their mechanism of action. 

Tropic bendings resulting from a dis-placement of the hormone to the lower side of the body of the plant. The shoot bends upward since its growth is enhanced by the homone and the root bends downward because its growth is inhibited (cf. optimal curves rn Fig. 194). 

Illumination from one side shifts lAA to the shaded part of the hypocotyl, leaf, stalk, or similar organ. The higher concentration that results promotes more intense growth on this side and the organ bends toward the light. 

A few indole derivatives are phytohormones. The most important of them is /3-indolylacetic acid, indole-3-acetic acid or, abbreviated, lAA (Fig. 160). It was detected in 1934 by Kogl in human urine, then in microorganisms, and, finally, also in higher plants. lAA occurs in plants either free or bound, e.g. esterifield to glucose or in peptide linkage with aspartic acid and glutamic acid. 

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Phytohormones such as ethylene, salicylic acid (SA), JA, and abscisic acid (ABA) regulate responses of plants to stresses via action referred as signaling crosstalk. Moreover, reactive oxygen species (ROS), the toxic byproducts of aerobic metabolism, play the important role of signaling molecules. Usually, the defensive responses of plants depend on the interaction (positive or negative) between phytohormone signaling pathways rather than on the independent contribution of each of them. " ... 

Like all steroids, brassinosteroids derive from a single common precursor mevalonic acid. Some phytohormones are synthesized totally or in part via the isoprenoid pathway, such as absdsic acid, gibberellins and cytokinins. The importance of this biosynthetic pathway in processes involved with cell cycle regulation and tumorigenesis in mammals is well documented. Having in mind the similarities between certain regulatory systems in plants and animals the question arose whether brassinosteroids as putative plant steroid hormones would show a specific effect on plant tumor cells. 

Phytohormones are those compounds that regulate and control the physiological phenomena of higher plants such as growth, maturation, abscission and dormancy. They are biosynthesized at a special part of a plant, then migrate to the places where they control the various physiological phenomena. Seven major phytohormones are shown in Figure 2.1. 

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