Post harvest diseases fruits and vegetables - Xavier University ...

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Novel Approaches for Enhancing Host Resistance 265
the resulting produce had a longer shelf life with no effect on fruit
texture or color development.
C. MANIPULATION OF ETHYLENE BIOSYNTHESIS AND
GENETIC RESISTANCE IN TOMATOES
The importance of ethylene ('the ripening hormone') in accelerating
ripening and senescence in harvested fruits, and the considerable
shortening of shelf life through its effects (see the chapter, Factors
Affecting Disease Development - the Effects of Ethylene) led to the
search for ways to suppress its influence in a reversible manner. Among
the solutions offered to achieve this goal were the prevention of ethylene
production by the plant tissue or the construction of a mutant plant
whose fruits would not ripen until treated with ethylene (Theologis,
1992).
Elucidation of the pathway for ethylene synthesis in higher plants by
Yang and Hoffman (1984) has been a major contribution, not only to the
understanding of the biochemistry of this process, but also to the
possibility of manipulating it to suppress or prevent ethylene production.
An efficient way to prevent ethylene synthesis in tomato fruits was the
inhibition of 1-aminocyclopropane-l-carboxylic acid synthase (ACC
synthase), a key enzyme in the biosynthesis of ethylene, by an
ACC-synthase antisense transgene. This function led to an almost
complete inhibition of the ethylene precursor, ACC synthase, and to the
production of mutant tomato plants with non-ripening fruits (Oeller et
al., 1991). Using tomato plants in which synthesis of an ethylene-forming
enzyme had been inhibited by an antisense gene, Picton et al. (1993)
showed that the degree of inhibition of ripening was dependent upon the
stage of development at which the fruits were detached from the plant:
the effects were much more pronounced when fruits were detached from
the vine before the onset of color change. Application of exogenous
ethylene to such fruits only partially restored fruit ripening: it failed to
increase lycopene accumulation to the level in the normal ripening fruits,
and the ethylene-treated fruits demonstrated persistent resistance to
over-ripening and shriveling.
Another approach to the manipulation of ethylene biosynthesis was
reported by Klee et al. (1991), who introduced a bacterial gene encoding
an ACC-metabolizing enzyme into tomato plants. As a result of this
procedure, inhibition of the accumulation of ACC and, consequently, a
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