Gibberellin cytokinin

There also have been clear indications that interactions of ethylene with auxins, cytokinins, gibberellins and ABA are involved in both ethylene production and action. Generally the effects of ethylene tend to antagonize those of auxins, cytokinins and gibberellins, and tend to reinforce those of ABA, depending, however, on tissue systems involved. Reinforcement of ethylene by ABA and vice versa occurs more frequently in senescence. 

Plant hormone analysis tends to be a very difficult task due in part to the low levels that occur naturally. Over the past several years Weiler and his coworkers and a number of other groups have developed extremely sensitive immunoassays for many of the natural plant hormones ( ). These assays have greatly reduced the time and effort required to analyze indoleacetic acid, cytokinins, gibberellins, and abscisic acid. 

Effects on Cytokinin, Gibberellin, and Auxin. Desiccation causes a reduction in the cytokinin activity of the root exudate of sunflower (Itai and Vaadia 1965) as well as the leaf tissue and xylem exudate of Nicotiana rustica (Itai and Vaadia 1971). In the experiments with Nicotiana there is evidence for a reversible chemical transformation of " C-kinetin as the means of altering cyto-... 

Developing fruits are rich sources of cytokinins, which are found in tissues where rapid cell-divisions are occurring, and are considered to play an important role in the regulation of cell division in fruit.397,424 Letham424 concluded that shape of the apple at maturity probably depends upon the balance between gibberellins and cytokinins in the immature fruit, and that apple varieties differ in their response to these compounds. 

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. 

Such questions have been raised for the catabolic products of auxins, abscisic acid, conjugates of auxins, and gibberellin, and for the hormonal role of cytokinins and auxins in tRNA molecules, but are not reviewed in this article (17). A satisfactory answer to this question requires elucidation of the cellular mechanisms (discussed in this paper) that are believed to be primarily under hormonal regulation. 

Abscisic acid is a negative regulator in that it primarily antagonizes the action of cytokinins, auxins, and in particular, gibberellins. Abscisic acid decreased the activity of polymerase in radishes (52), peas (53), maize coleoptiles (54), and pear embryos (55). More detailed studies are needed before the question of ABA-induced "modification" of RNA polymerase (54) or "alterations" in the number of sites for template activity (56) can be answered. In barley aleurone cells, ABA-induced suppression of GA-induced <-amylase formation was presumed to involve the continuous synthesis of a short-lived RNA (57). 

At the preclimacteric stage of development, just prior to the rise in ethylene production the concentrations of auxins, gibberellins, and cytokinins are assumed to be very low. 

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