Gibberellin inhibition

They act by altering the endogenous balance of gibberellins (inhibition) and ethylene (enhancement) and are often used in mixtures. Thus, ethephon, an ethylene generator, is applied in combination with mepi-... 

Gibberellins. Although the gibberellins inhibit cell division in some instances (Setterfield 1963, Kaufman et al. 1969) more often they are stimulatory (Boeken and van Oostveldt 1977, Kohler 1977, Shininger 1975) especially in subapical tissue (Wardlaw 1965, Liu and Loy 1976). GA usually has less effect on mitotic activity in the shoot apical meristem itself (see review by Loy 1977), an indication that it may be more effective in initiating cell division than in accelerating division in cells already mitotically active. In instances where GA acts to shorten the cell cycle in the shoot apical meristem, it increases the proportion of cells in S and M, an indication that GA may act at the points of entry into S and M (Lyndon 1976). 

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. 

The most commercially successful PGRs are those that operate either through the inhibition of gibberellin biosynthesis, from mevalonic acid (Knee, 1982) or through the production of ethylene. These represent the two most valuable pathways for growth modification since they have key roles in extension growth, ripening, fruit set and dominance. 

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. 

Interestingly, several natural products, besides their role as antibiotics and phytotoxins, exhibit plant growth responses (mainly inhibition). In order to keep their identity as such, these were listed under miscellaneous natural products (Table 1). Furthermore, compounds which have structural features similar to those present in hormones (gibberellins and ABA.) and which exhibit similar growth responses are also listed under miscellaneous natural products (Table 1). 

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