11.5.6 Ethylene

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Ethylene metabolism has been a main focus for biochemical research into fruit behaviour (see Feature essay 11.1). Following 30 years of biochemical studies, transgenic plants with modifications to the ethylene synthetic pathway have now been used to explore ways of controlling fruit ripening. The pathway of biosynthesis is as follows: methionine (a sulphur-containing amino acid also important in protein synthesis) is converted to SAM (S-adenosylmethionine) through the action of SAM synthase; SAM is converted to ACC (amino-cyclopropane carboxylate) through action of ACC synthase; ACC is converted to ethylene through action of ACC oxidase. In summary:

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Transgenic tomatoes have yielded much information about steps in the control of ripening (Gray et al. 1994). On the one hand, fruit with reduced levels of ACC oxidase arising from antisense ACC oxidase constructs (Section 11.7) developed and grew normally. The only changes were delays in ripening and over-ripening and a reduction in shrivelling associated with senescence. All ripening processes still occurred. One interesting difference related to fruit harvest. Fruit remaining on vines ripened much faster than those which were detached when mature and ripened off the plant. On the other hand, fruit with much reduced ACC synthase activity from an antisense ACC synthase construct were unable to produce ethylene during fruit ripening (the level was 0.5% of normal). These fruit were severely delayed in colouring and in onset of the respiratory climacteric, and were slow to soften. Similarly, removal of ACC by formation of an inert side-product due to bacterial ACC deaminase resulted in fruit with similar characteristics to the two previous examples. Fruit with a high level of ACC deaminase were more like fruit with reduced ACC synthase, whereas those with moderate levels of ACC deaminase resembled fruit with reduced ACC oxidase.

Such experiments imply that there are many parallel processes which accompany ethylene responses, but are not specifically dependent on ethylene. That is, ethylene does not control all the processes of ripening as once believed. Rather, ethylene interacts with gene expression at both transcriptional and translational levels, acts as a modulator of ripening and can coordinate some of the processes (see Hobson and Grierson 1993). Future research will concentrate on the nature of ethylene receptors and how they in turn modulate gene expression.

In contrast to other fruit such as banana or apple, kiwifruit produce very low quantities of ethylene, yet are very sensitive to this hormone (Beever and Hopkirk 1990; Given 1993). An absolute minimum threshold has not yet been determined, but is known to be below 0.5 µL L–1 air. This sensitivity is such that kiwifruit can be triggered to ripen by nearby ripening apples or bananas. As a consequence, the kiwifruit industry is very strict in its control of ethylene sources (such as car exhaust) in handling and packing environments, and during storage.

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