Postharvest Biology and Technology 19 2000 155 – 164
Consequences on ethylene metabolism of inactivating the ethylene receptor sites in diseased non-climacteric fruit
E.D. Mullins , T.G. McCollum, R.E. McDonald
U.S. Horticultural Research Laboratory, Agricultural Research Ser6ice, U.S. Department of Agriculture, Ft Pierce, FL
34945
, USA Received 9 June 1999; accepted 16 January 2000
Abstract
Penicillium digitatum-infected grapefruit synthesize large quantities of the stress hormone ethylene. The compound 1-methylcyclopropene 1-MCP inhibits the binding of ethylene to the ethylene receptor site, the ethylene binding
protein EBP. Treating infected fruit with 1-MCP prevented infection-induced degreening, such that fumigated fruit retained their green immature color compared to yellow non-fumigated controls. However, 1-MCP treatment
significantly increased whole fruit ethylene production. In flavedo tissue of infected non-1-MCP treated fruit, 1-aminocyclopropane-1-carboxylate ACC synthase transcript accumulation, ACC synthase ACS enzyme activity,
ACC and ethylene synthesis were all significantly higher + 5 mm ahead of the lesion front than in uninfected non-1-MCP treated controls, but decreased significantly with increased sampling distance away from the lesion.
1-MCP treatment increased ethylene production in infected fruit at all three sampling distances compared to the non-fumigated samples. Even in the absence of infection, 1-MCP treatment resulted in increased ethylene synthesis.
The results suggest that, in the presence of a pathogenic stress, blocking the EBPs prevented regulatory control of the ethylene biosynthetic pathway that resulted in an uninhibited expression of the ACS stress-associated genes, increased
ACS activity and elevated ACC accumulation and ethylene production. Blocking of the EBPs with 1-MCP did not affect progression of the pathogen through the fruit. Published by Elsevier Science B.V.
Keywords
:
Ethylene; Ethylene binding protein; Grapefruit; Penicillium digitatum; 1-Methylcyclopropene www.elsevier.comlocatepostharvbio
1. Introduction
Ethylene is associated with the regulation of several plant metabolic processes and is produced
rapidly in response to pathogenic invasion Boller, 1990. Indeed increased ethylene biosynthesis is a
common feature of citrus infected with the com- mon postharvest pathogen Penicillium digitatum
Achilea et al., 1985; Dutta and Biggs, 1991.
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The mechanisms by which plants perceive and transduce the ethylene signal are topics that have
been researched intensively. Following the isola- tion of Arabidopsis ethylene insensitive mutants
Bleecker et al., 1988 and the cloning of two genes in particular involved in the pathway ETR
1 gene, Chang et al., 1993; CTR 1 gene, Kieber et al., 1993, models of the chain of events that
proceed ethylene binding have been proposed Bleecker and Schaller, 1996; Pallin et al., 1996;
Kieber, 1997; McGrath and Ecker, 1998. Subse- quent to ethylene binding with the ethylene bind-
ing protein EBP, a suspected heterotrimetric G-protein Harpham et al., 1996, they suggest a
negative regulation of the receptors His kinase activity which inactivates the activity of the CTR
1 protein kinase. It is predicted that this in turn influences through a MAP kinase cascade the
downstream products, EIN 2, EIN 3, EIN 5 and ultimately ethylene sensitivity.
Several inhibitors of ethylene synthesis have assisted in the study of the pathway Yu and
Yang, 1980; Yoshii and Imaseki, 1982; Kende, 1989; Bouquin et al., 1997; Nakatsuka et al.,
1997; Mullins et al., 1999. 1-MCP is a non-toxic compound that inhibits ethylene perception by
irreversibly binding to and hence inactivating the EBP Sisler et al., 1996. Shown to be an effective
inhibitor of ethylene action in banana Golding et al., 1998, Petunia hybrida Serek, 1995 and
Gypsophila paniculata Newman, 1998, this an- tagonist has been suggested as a possible alterna-
tive for the commercially used silver thiosulfate STS in improving product longevity Porat et
al., 1995.
Wound-induced production of ethylene in cit- rus flavedo is strongly inhibited by treatment with
ethylene Riov and Yang 1982, a phenomenon referred to as ‘autoinhibition’. Investigations into
the mechanisms by which ethylene modifies its own synthesis have focused on the action of ACS
Yoshii and Imaseki 1982; Yang and Hoffman 1984; Hyodo et al., 1985 and how autoinhibition,
by acting at the level of ACS gene expression Peck and Kende, 1998; Mullins et al., 1999,
suppresses induction of the protein Yang and Hoffman 1984; Hyodo et al., 1985 and subse-
quently inhibits the accumulation of ACC Riov and Yang 1982; Hyodo et al., 1985. If however
the first step of the pathway, binding of ethylene to the EBPs Ecker, 1995, is interceded with by
treating P. digitatum-infected fruit with 1-MCP, it is unclear what the outcome would be with re-
spect to the evolution of stress ethylene and the progression of disease through the host tissue.
The aim of this research was to provide some answers to this question.
2. Materials and methods
