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1-aminocyclopropane-1-carboxylate synthase is a pyridoxal phosphate-dependent enzyme that converts S-adenosylmethionine into 1-aminocyclopropane-1-carboxylate, commonly abbreviated ACC, with methylthioadenosine as the other principal product. ACC is a precursor in the plant pathway producing the gaseous signal ethene, usually called ethylene in plant physiology. The synthase makes the precursor; a subsequent enzyme catalyzes the ethylene-forming step.^[ref]

The precursor/enzyme distinction helps interpret a perturbation. Reducing synthase activity can reduce pathway output even if the downstream machinery remains able to respond. In a 1991 tomato experiment, expression of Antisense RNA against the synthase inhibited fruit ripening. Applying ethylene or propylene reversed the effect, supporting a role for the pathway's signal in triggering ripening rather than merely accompanying it.^[ref]

For an original pathway example, write substrate $\to$ ACC $\to$ signal. Blocking the first arrow and restoring the response by supplying the final signal places the block upstream of that response. It does not by itself prove that every intermediate remains unchanged or that this enzyme controls all developmental responses.

The enzyme's name also does not imply an unlimited constant catalytic rate. Biochemical work showed substrate-dependent inactivation, so reaction progress and the amount of active enzyme can change together during an assay.^[ref]

Antisense RNA is RNA complementary to a specified sense transcript or part of it, allowing antiparallel base pairing with that target. Sense and antisense are relational labels: the target and strand orientation must be specified. Antisense molecules can occur naturally or be produced experimentally, and complementary pairing can influence transcript processing, stability or access to translation machinery.^[ref]

For an original sequence example, a target segment $5^{\prime }\text{-}\mathrm{AUGCC}\text{-}{3}^{\prime }$ pairs with $3^{\prime }\text{-}\mathrm{UACGG}\text{-}{5}^{\prime }$. Written in the usual $5^{\prime }$-to-$3^{\prime }$ direction, the antisense sequence is $5^{\prime }\text{-}\mathrm{GGCAU}\text{-}{3}^{\prime }$. Merely replacing each target letter by its complement without reversing the written direction gives a different sequence.

A tomato experiment expressed Antisense RNA directed against 1-aminocyclopropane-1-carboxylate synthase, a precursor-forming enzyme in ethylene biosynthesis. Ripening was inhibited, and supplying ethylene or propylene reversed the effect. The intervention and rescue linked altered expression to a physiological signal pathway, rather than treating ripening and ethylene production as a simple correlation.^[ref]

Complementarity alone is not a quantitative prediction of suppression. Target accessibility, expression levels and the cellular machinery determine the outcome. An Antisense RNA construct, an observed change in target protein and an observed phenotype therefore provide different pieces of evidence.

P. W. Oeller investigated molecular control of plant development, including the role of Ethylene in fruit ripening. A 1991 study with colleagues used Antisense RNA to reduce production of 1-aminocyclopropane-1-carboxylate synthase, an enzyme in Ethylene biosynthesis, in tomato plants.^[ref]

The modified tomatoes showed inhibited ripening, and supplying Ethylene externally reversed the inhibition. This paired intervention and rescue connected a molecular perturbation to a developmental process: reducing the signal delayed the process, while restoring the signal restored it. The result supplied causal evidence for Ethylene control in this experimental system, beyond a correlation between rising Ethylene and maturing fruit.^[ref]

Ripening is the coordinated set of developmental changes that brings a fruit toward its mature dispersal or eating state. Depending on the fruit, these changes can include altered pigmentation, softening, changes in sugars and acids, and the production of volatile compounds. They need not all occur at the same rate, so colour alone is an incomplete measure of ripening.

A particularly clear experiment established a role for Ethylene in tomato ripening. In 1991, P. W. Oeller and colleagues expressed Antisense RNA directed against 1-aminocyclopropane-1-carboxylate synthase, an enzyme required for Ethylene production. Ripening was inhibited. Supplying Ethylene from outside the plant reversed that inhibition.^[ref]

The intervention and rescue distinguish a regulating signal from a substance that merely accumulates while ripening happens. Suppressing production delays the process, while restoring the signal restores it. This establishes a causal role for Ethylene in that tomato system; the precise control of ripening must still be investigated for other fruits.