PLANT PHYSIOLOGY , Vol 103, Issue 1 227-234, Copyright © 1993 by American Society of Plant Biologists
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MOLECULAR BIOLOGY AND GENE REGULATION |
Ethylene and Wound-Induced Gene Expression in the Preclimacteric Phase of Ripening Avocado Fruit and Mesocarp Discs
D. A. Starrett and G. G. Laties
Department of Biology, University of California, Los Angeles, Los Angeles, California 90024
Whereas intact postharvest avocado (Persea americana Mill.) fruit may take
1 or more weeks to ripen, ripening is hastened by pulsing fruit for 24 h
with ethylene or propylene and is initiated promptly by cutting slices, or
discs, of mesocarp tissue. Because the preclimacteric lag period
constitutes the extended and variable component of the ripening syndrome,
we postulated that selective gene expression during the lag period leads to
the triggering of the climacteric. Accordingly, we sought to identify genes
that are expressed gradually in the course of the lag period in intact
fruit, are turned on sooner in response to a pulse, and are induced
promptly in response to wounding (i.e. slicing). To this end, a mixed cDNA
library was constructed from mRNA from untreated fruit, pulsed fruit, and
aged slices, and the library was screened for genes induced by wounding or
by pulsing and/or wounding. The time course of induction of genes encoding
selected clones was established by probing northern blots of mRNA from
tissues variously treated over a period of time. Four previously identified
ripening-associated genes encoding cellulase, polygalacturonase (PG),
cytochrome P-450 oxidase (P-450), and ethylene-forming enzyme (EFE, or
1-aminocyclopropane-1-carboxylic acid synthase), respectively, were studied
in the same way. Whereas cellulase, PG, and EFE were ruled out as having a
role in the initiation of the climacteric, the time course of P-450
induction, as well as the response of same to pulsing and wounding met the
criteria[mdash]together with several clones from the mixed
library[mdash]for a gene potentially involved in preclimacteric events
leading to the onset of the climacteric. Further, it was established that
the continuous presence of ethylene is required for persisting induction,
and it is suggested that in selected cases wounding may exert a synergistic
effect on ethylene action.
