Ethyleneinduced Senescence

For a number of flowers, especially in the Caryophyllaceae, Leguminaceae, and Orchi-daceae, the onset of natural or pollination-induced senescence is associated with a climacteric rise in respiration triggered by an increase in ethylene production. For many others, ethylene is involved in the abscission of flowers, petals, or florets (Woltering and van Doorn, 1988). The rapidity of the ethylene response has made flowers whose senescence is coordinated by ethylene a favored system for exploring the action of ethylene and the genes involved.

1. Ethylene production during flower senescence

More than 20 years ago researchers showed that the rapid senescence of carnation flowers was associated with increased biosynthesis of ethylene (Maxie et al., 1973) from ACC (Bufler et al., 1980). The importance of ethylene in the life of carnation flowers was confirmed by examining cultivars that exhibited extended life. For example, flowers of the "Sandra" cultivar lasted twice as long in the vase as "White Sim" flowers and showed neither the normal increase in ethylene production nor a marked respiratory climacteric during their eventual senescence (Reid and Wu, 1992). The extended vase life of "Sandra" was attributed to repressed ethylene biosynthesis, since the application of exogenous ethylene hastened flower senescence.

2. Inhibitor studies

The most important inhibitors of ethylene-mediated flower senescence are inhibitors of ACC synthase [e.g. aminoethyoxyvinylglycine (AVG) and aminooxyacetic acid (AOA)], and two potent means of inhibiting ethylene action [silver ion, formulated as the anionic silver thiosulfate complex (STS) and a range of cyclopropenes].

i. AOA/AVG

Pyridoxal phosphate is a cofactor for ACC synthase, and compounds that inhibit pyridoxal phosphate-mediated reactions are effective inhibitors of ethylene synthesis. Two such compounds, AOA and AVG, have been shown to extend the vase life of flowers whose natural senescence is coordinated by ethylene (Baker et al., 1977).

ii. STS

Beyer (1976) first demonstrated the dramatic inhibition of ethylene action by Ag+, using the response of Cattleya blossoms to ethylene. However, because of the phytotoxicity of silver salts, this effect was a matter of academic interest until the report by Veen and Van der Geijn (1978) that the very stable anionic silver thiosulfate complex (STS) moved readily in the stems of cut flowers, and could be used, at low concentrations, to extend their life.

iii. Cyclopropenes

Cyclic olefins based on the cyclopropene ring have proved to be extremely effective and irreversible inhibitors of ethylene action and the most active of them, 1-methylcyclopropene (1-MCP) has been patented, licensed, and registered for use in preventing the effects of ethylene in ornamentals. We have demonstrated the action of 1-MCP in preventing flower senescence in a range of commercial crops (Serek et al., 1995).

3. Gene expression studies

Jones (Chapter 4) describes the remarkable progress that has been made in the past decade in analyzing changes in gene expression during flower senescence. The genes controlling ethy-lene biosynthesis have been cloned from a number of species including carnation, petunia and roses, and antisense and co-suppressed constructs have been successfully engineered into some of these species, resulting in the predicted phenotypes. For example, Savin et al. (1995) transformed carnations with an antisense ACC oxidase gene under the control of the constitutive MAC promoter and demonstrated extended vase life of the transgenic flowers. The Arabidopsis gene (ETR1-1) that encodes a faulty ethylene receptor has now also been cloned (Chang et al., 1993) allowing production of transgenic plants with reduced sensitivity to ethylene. Heterologous expression of the ETR1-1 gene in petunia and tomato greatly extended the life of their flowers (Wilkinson et al., 1997), and a similar effect has recently been demonstrated in transgenic carnations (Bovy et al., 1999).

4. Effects of other plant hormones

Studies over the past two decades have demonstrated that senescence control probably depends on the interaction of different growth regulators. Treatment of carnations with cytokinins (Eisinger, 1977) or gibberellins (Saks and van Staden, 1992) has been shown to extend flower life, and treatment with high concentrations of auxins to reduce it (Sacalis and Nichols, 1980), although this latter effect may simply be due to auxin-stimulated ethylene production. A similar mechanism seems to explain the stimulation of orchid flower senescence by methyl jasmonate (Porat et al., 1993). Exogenously applied abscisic acid (ABA) has been shown to accelerate flower senescence in standard roses (Halevy and Mayak, 1972), miniature roses (Muller et al., 1999) and carnations (Mayak and Dilley, 1976).

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