LOW-OXYGEN AND ELEVATED CARBON-DIOXIDE ATMOSPHERES INHIBIT ETHYLENE BIOSYNTHESIS IN PRECLIMACTERIC AND CLIMACTERIC APPLE FRUIT

Autocatalytic C2H4 biosynthesis in preclimacteric apple fruit (Malus d omestica Borkh, 'Golden Delicious') was prevented by storage in atmosp heres of 20% CO2-enriched air (17% O-2+ 63% N-2) or 0.25% O-2 (balance N-2). In preclimacteric fruit, both treatments inhibited C2H4 biosynt hesis by suppressing expression of ACC synthase (ACC-S) at the mRNA le vel. ACC oxidase (ACC-O) mRNA abundance and in vitro enzyme activity a lso were impaired by these treatments. However, the conversion of ACC to C2H4 never became the rate limiting step in C2H4 biosynthesis, C2H4 biosynthesis also was effectively inhibited in climacteric apple frui t kept in air + 20% CO2 or 0.25% O-2. Climacteric apples also exhibite d suppressed expression of ACC-S at the mRNA level, while ACC-O transc ript abundance, enzyme activity, and protein abundance mere reduced on ly slightly. ACC-S is the key regulatory enzyme of C2H4 biosynthesis a nd is the major site at which elevated CO2 and reduced O-2 atmospheres inhibit C2H4 biosynthesis, irrespective of fruit physiological maturi ty. Chemical names used: 1-aminocyclopropane-1-carboxcylic acid (ACC).