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Chapter 7
where ethylene is thought to affect growth or morphogenesis in tissue cultures. Silver ions. Silver ions are known to overcome the action of ethylene on whole plants (Beyer, 1976a, b). Both silver thiosulphate (Ag2S2O3) and silver nitrate (AgNO3) are therefore effective in preventing ethylene action although the former is much more effective because it is readily translocated. They do not, as was once thought, appear to replace Cu(I) in the binding domain and in fact increase rates of ethylene biosynthesis in peas (Sanders et al., 1991), but may act by inhibiting the copper transporter protein RAN1. Carbon dioxide. High concentrations (5-10%) of carbon dioxide can antagonise some ethylene effects for example inhibition of epicotyl or hypocotyl extension (Burg and Burg, 1967; Beyer, 1979). The effect is competitive in vivo (Burg and Burg, 1967) but does not appear to be a result of displacement of ethylene from its receptor (Sisler, 1982; Sanders et al., 1991). On the other hand, CO2 at atmospheric concentrations appears to be necessary for some responses to ethylene (Hall et al., 1980). Chemical inhibitors. Although a wide range of chemicals [e.g. ioxynil (23) 3,5-diiodo-4-hydroxybenzoic acid (DIHB) (24) and 5-methyl-7-chloro-4ethoxycarbanylmethoxy-2,1,3-benzothia-diazole (benzothiadiazole TH6241; 25)] are known to block developmental effects of ethylene reputedly via effects on ethylene action, it is questionable whether this is so or whether the mechanisms’ effects may be indirect and/or the result of modulation of ethylene biosynthesis. However, a number of hydrocarbons do specifically inhibit ethylene action by competing with the growth regulator for binding sites on its receptor. Such substances include 2,5 norbornadiene (26), cis2-butene (27), cyclooctene (28) and methylcyclopropene (29) (Sisler, 1991; Sisler et al., 1996). The latter is particularly effective, at concentrations as low as 10-9 M (Sisler and Serek, 1997). Cytokinins. Cytokinins antagonise ethylene in many systems - for example leaf senescence probably via early events in signal transduction (see e.g. Novikova et al., 1999). B
3.2.4. Chelating agents
Chelating agents such as 8-hydroxyquinoline (22) are effective in prolonging the vase life of cut flowers (thereby overcoming ethylene-induced senescence). Their mode of action is unknown, but may be related to their ability to sequester copper ions (see below).
3.3. ETHYLENE ACTION
Five partially functionally redundant receptors for ethylene have now been identified (Bleecker, 1999) and signal transduction appears to involve protein kinases (Kieber et al., 1993; Novikova et al., 2000, Moshkov et al., 2003a), monomeric GTP-binding proteins (Novikova et al., 1997, 1999, Moshkov et al., 2003b), transcription factors (Chao et al., 1997) and other proteins of unknown function (Johnson and Ecker, 1998; Stepanova and Ecker, 2000). The receptors all contain Cu(I) in the binding domain (Rodriguez et al., 1999). The downstream effects include altered gene transcription and protein synthesis. 3.3.1. Inhibitors of ethylene action
Several chemicals and environmental factors are known to inhibit ethylene action. These may either compete with ethylene for binding domains in the receptors or act by unknown mechanisms. Inhibitors of ethylene action have been used experimentally as an alternative to biosynthesis inhibitors, in conditions
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3.3.2. Ethylene-Releasing Chemicals
Several synthetic ethylene-releasing chemicals have been discovered. The one most commonly used in plant tissue culture experiments is ethephon (2CEPA or 2-chloroethanephosphonic acid; 30). This compound is absorbed into plant tissues where it breaks down to release ethylene at cytoplasmic pH