Regulation of Pathogenesis and Symptom Expression in Diseased Plants by Ethylene

Abstract

An enhanced production of ethylene is associated with the development of symptoms in plants infected with fungi, bacteria, viruses and viroids. Symptoms such as inhibition of growth, epinasty, abscission of leaves and organs, and chlorosis and premature senescence are part of the physiological responses of plants to various types of stress; these are similarly accompanied by stimulated ethylene production, suggesting that such symptoms in diseased plants arise as a result of the stress imposed by infection. This has led to the general conclusion that the evolution of ethylene from diseased tissues most probably is of host origin arising as a result of cellular damage. However, some pathogenic fungi and bacteria are themselves capable of producing ethylene in. vivo. Such ethylene may act as a phytotoxin or modify the plant’s response, enabling the pathogen to overcome host defense mechanisms. In tulip bulbs infected with Fusarium oxysporum f. sp. tulipae the fungus produces high levels of ethylene, resulting in storage diseases and shoot stunting and bud blasting upon planting. In the absence of ethylene, a fungitoxic compound, tulipalin A, is formed in the bulb scales after lifting. However, ethylene inhibits the synthesis of tuliposide, the precursor of tulipalin, permitting the fungus to penetrate the bulb (22). It has not been established, however, whether the ethylene produced by Fusarium-infected tulip bulbs is derived entirely from the fungus, or is, in part, synthesized by the bulb itself in response to the ethylene produced by the pathogen.