ABSTRACT
INTRODUCTION The quality of agricultural products at the time they get to the consumers strongly depends on the developmental stage at harvest, shipping and storage conditions. It is commercially advantageous that fruit and vegetables have a long shelf life and do not deteriorate immediately after harvest. In many deteriorative processes the plant hormone ethylene plays an important role; by controlling the ethylene production or sensitivity, important benefits can be obtained (Saltveit, 1999). The plant hormone ethylene Ethylene is involved in virtually all aspects of the plant life cycle, as well as in the plant’s response to many environmental stimuli. In the broadest of terms, ethylene is responsible for signaling changes during germination, growth, flower and fruit development, senescence of plant organs, programmed cell death, the onset of plant defense mechanisms and the action of other plant hormones. Biotic stress (e.g., pathogen attack) and abiotic stress conditions (e.g., wounding, hypoxia, ozone, chilling, and freezing) elicit ethylene synthesis in plants (Abeles et al., 1992; Mattoo and Suttle, 1991). The elucidation of the ethylene biosynthetic pathway and the molecular cloning of genes encoding the enzymes involved have provided insight into the regulation of ethylene biosynthesis in plants. Plants biosynthesize ethylene
via the Yang cycle, wherein methionine is converted to S-adenosylmethionine (SAM) by the enzyme SAM synthase. The conversion of SAM to 1-aminocyclopropane-1-carboxylic acid (ACC) is then catalyzed by the enzyme ACC synthase (ACS). ACC is oxidized to ethylene by ACC oxidase (ACO) (Yang and Hoffman, 1984) (Figure 1). The conversion of SAM to ACC is generally considered to be the ratelimiting step in the synthesis of ethylene (Kende, 1993). Both latter enzymes play a role in the regulation of ethylene biosynthesis and are encoded by small gene families. Ethylene biosynthesis in microorganisms In addition to plants, some microorganisms, including phytopathogenic fungi and bacteria, can synthesize ethylene themselves. Except for few fungal species, such as the slime mold Dictyostelium mucoroides (Amagai and Maeda, 1992) and Penicillium citrinum (Jia, 1999), the ACC pathway for ethylene biosynthesis has not been found operative in microorganisms. Presently, two different ethylene biosynthetic pathways have been established in microorganisms (Fukuda et al., 1993). Ethylene can be produced either from glutamic acid via 2oxoglutarate as in Penicillium digitatum (Fukuda et al., 1989a) and in Pseudomonas syringae (Nagahama et al., 1991) or from methionine via 2-keto-4-methylbutyric acid (KMBA) as in Escherichia coli (Ince and Knowles, 1986), Cryptococcus albidus (Fukuda et al., 1989b),
Colletotrichum musae (Daundasekera et al., 2003) and in Botrytis cinerea (Cristescu et al., 2002; Chague et al., 2002) (Figure 1). Additionally, KMBA has been identified as an intermediate in methionine-derived ethylene biosynthesis by microbial cultures in soil (Nazli et al., 2003). Effect of ethylene on fungal development It was reported that ethylene has different effects on various phases of fungal development in vitro. Exogenous application of ethylene stimulates conidial germination of B. cinerea, Penicillium expansum, Rhizopus stolonifer and Gloeosporium perennans (Kepczynski and Kepczynska, 1977), P. digitatum, P. italicum, Thielaviopsis paradoxa (El-Kazzaz et al., 1983), Diplodia natalonis and Phomopsis citri (Abeles, 1973). Elad (2002) showed that ethylene did not affect conidial germination and hyphal growth of B. cinerea on PDA media (potato dextrose agar), whereas on glass, tomato or bean leaf surfaces both germination rate and germ tube elongation were enhanced. A specific inhibitor of ethylene action in plants, 2,5-norbornadiene (NBD) inhibited growth of hyphae and mycelium and retarded the B. cinerea development (Kepczynska, 1989; 1993). A similar inhibitory effect was reported following application of the plant ethylene production inhibitor, aminoeth-
oxyvinylglycine (AVG) a specific inhibitor of ACC synthase (Figure 1), which reduced mycelium growth and sporulation of B. cinerea. As ethylene biosynthesis in B. cinerea does not involve ACC synthase, the target of AVG is probably some other aminotransferase and the effect may not be related to ethylene biosynthesis. Many fungi are known to remain dormant at the fruit surface until the fruit ripens, at which time the fungus infects the fruit. In some fungi ethylene was found to play a role as a signaling molecule. For instance, in Colletotrichum gloeosporioides and C. musae that attack ripe fruit, exposure to ethylene induces germination and appressorium formation. The reception of ethylene by the fungus was supposed to act through a mechanism with similarity to the receptor-mediated effects of ethylene in plants. Sensing of ethylene was blocked by the ethylene perception inhibitors, silver thiosulphate (STS) and NBD, while the ethylene analog propylene (but not methane) could substitute for ethylene. On transgenic tomato fruits, that did not produce ethylene, the fungus was unable to germinate. Upon treatment with ethylene, the spores germinated and produced multiple appressoria and infection lesions (Kolattukudy et al., 1995).
