Biochemistry: The Chemical Reactions of Living Cells, Vol. 3 - D. Metzler 1980

Cell Growth, Differentiation, and Chemical Communication
Hormones
Plant Hormones

Plants possess a unique Circulatory system in which fluid is transported upward from the roots via the xylem and downward from the leaves via the phloem. This pathway enables The transport of A wide variety of substances between Cells. At the same time, there is active Transport of substances across Cell membranes and against concentration gradients. A number of compounds transported from Cell to Cell by one of these two mechanisms can be classified as HORMONES, and their number continues to grow over time. Currently, five compounds or groups of compounds are recognized as plant hormones: Auxins (ch. 14, sec. I), Gibberellins (ch. 5, sec. D; ch. 12, sec. 3.1), Cytokinins (ch. 15, sec. B.4), Abscisic acid (fig. 12-13), and Ethylene (ch. 14, sec. G.4).

Plant hormone Functions are diverse and frequently overlap, which complicates concise Structure/133.html">Discussion of the topic. Furthermore, the MOLECULAR MECHANISMS OF hormone action remain poorly understood. Auxins are the most thoroughly studied Class, with indole-3-acetic acid (fig. 14-27) as their principal representative. This compound has been shown to regulate both Cell Division and cell elongation. Auxins influence a multitude of physiological processes in plants. They are synthesized primarily in apical Meristems, from which they diffuse downward along the stem, inhibiting The Development of lateral buds. Concurrently, the hormone stimulates stem elongation, thereby ensuring the preferential growth of the plant apex. Other Hormones also affect these processes. It is well established that auxins participate in The regulation of phototropism, the tendency of plants to bend toward light. The availability of a highly sensitive assay (measuring the curvature of oat coleoptiles, Avena sativa) makes it possible to detect amounts of auxin as small as 3 pmol. Using this assay, it has been demonstrated that auxin is transported laterally from the illuminated side of the plant to the shaded side, causing the latter to elongate more rapidly.

The Molecular Mechanism of auxin action is unknown, but it is hypothesized that this substance, like other hormones, increases The rate of RNA METABOLISM/31.html">Transcription.

The Role of gibberellins lies largely in determining plant Morphology. These compounds are synthesized in mature leaves and transported downward along the stem. Studies on dwarf varieties of vegetable crops have shown that gibberellins are highly effective in stimulating RNA Synthesis; on this basis, it has been suggested that they act as Gene activators, thereby promoting RNA production. The potential role of hormones in plant ROOT geotropism is suggested by the finding that gibberellin concentrations are higher in the upper side of horizontally oriented roots than in the lower side [23]. Conversely, it has long been known that auxin concentration is relatively higher in the lower side of the root; the latter is viewed as an indicator of the inhibitory effect of auxin on root elongation (in contrast to its growth-stimulating effect on stems).

Cytokinins constitute a family of isopentenyladenosine derivatives (fig. 15-10) that may undergo further hydroxylation or Substitution at the 2-position with a methylthio group. Cytokinins appear to act at the level of gene transcription or Translation. The hormonal effects of cytokinins in plants seem unrelated to their influence on tRNA. Cytokinin solutions exert their most pronounced effect on plant Cell Differentiation (sec. B.3).

Abscisic acid has The ability to block the action of other growth promoters, notably gibberellins and cytokinins. This compound is sometimes regarded as a general gene repressor that prepares the plant for dormancy. The synthesis of abscisic acid is triggered by autumn short-day/long-night photoperiods.

It is considerably more difficult to determine the effects of ethylene on plants, as it not only accelerates fruit ripening but also promotes senescence in all plant parts.

Other important compounds with regulatory functions include Vitamins—such as thiamine, pyridoxine, and nicotinic acid—which are synthesized in the leaves and transported downward through the stem to the roots. Because these substances promote root growth, they are sometimes called root growth hormones. Much more frequently, however, they are considered essential nutrients required by all cells. There is compelling Evidence for the existence of a specific flowering hormone, and in recent years interest has grown in studying the effects of synthetic plant BIOREGULATORS, which include chalcone derivatives (Supplement 12-B) and compounds such as diethyloctylamine [24]. Mention should also be made of another vital aspect of plant growth regulation related to The Influence of light—namely, photomorphogenesis.