Controlling Ethylene Responses in Horticultural Crops at the Receptor Level

Ethylene is a plant hormone that controls many plant responses, such as growth, senescence, ripening, abscission and seed germination. Recently, 1-methy- cyclopropene (1-MCP), was shown to bind to ethylene receptor for a certain period of time and prevent ethylene action. The objectives of this research were to synthesize analogues of 1-MCP and test their potency to block the ethylene receptor and inhibit ethylene action. During the course of this project, procedures for synthesis and shipment of the cyclopropene compounds were developed as well assay procedures for each compound were worked out. Thirteen new compounds were synthesized. All of them are structural analogues of 1-MCP, with substitution in the 1-position and a side chain containing 2 to 10 carbons. After preliminary studies, nine promising compounds were selected for in-depth study. The potency of the compounds to inhibit ethylene action was tested on a wide scope of systems like: climacteric fruits (banana, avocado and tomato), the triple response (etiolated peas), and leaf abscission (citrus). As the putative inhibitors are suspected to compete for the site of binding and a competitive type of inhibition could be considered, a high concentration of ethylene (300 m1.L-1) was used to induce ripening and other physiological processes. The tests were conducted under extreme conditions which hasten ripening like treatment and storage at 22 to 25oC. There were fluctuations in the responses as related to the concentrations of the inhibitors. Some required much higher concentration to exert the same effect, while some, when applied at the same concentration, blocked the receptor for a longer period of time than the others. Some fruits and other plant organs responded differently to the same inhibitor, indicating differences in characteristics and availability of the ethylene receptors in the various tissues. The potency of the putative inhibitors was found to be greatly affected by their molecular structural and size. In addition, it was found that treatment with the inhibitor should be given before the onset of ethylene action In the case of fruit, treatment should be carried out before the pre-climacteric stage. Simultaneous treatment with ethylene and the inhibitors reduced the inhibitors' effect. The relationship between ethylene and the inhibitors is of a non-competitive nature. All the fruits treated with the putative inhibitors resumed normal ripening after recovery from the inhibition. This fact is of great importance when considering the inhibitors for practical use. The advantage of using inhibitors of ethylene action over inhibitors of ethylene production lies in the ability of the inhibitors of ethylene action to protect the tissue against both endogenous and exogenous ethylene, thus providing better overall protection. Our findings indicate that 1-MCP and its structural analogues are potent inhibitors of ethylene action capable of providing good protection against endogenous and exogenous ethylene. The fact that the compounds are in a gas phase and are non-phytotoxic, odorless and effective at minute concentrations, renders them promising candidates for commercial use. However, the development of water-soluble inhibitors will expand the potential use of the inhibitors in agriculture.