Controlled-atmosphere suppression of ACC synthase and ACC oxidase in 'Golden Delicious' apples during long-term cold storage

Preclimacteric 'Golden Delicious' apples (Malus domestica Borkh.) were stored at 0 degrees C in: air; air + 5% CO2; 2% O-2 + 98% N-2; or 2% O-2 + 5% CO2 + 93% N-2, and sampled monthly for 4 months to investigate the mechanism(s) by which reduced O-2 and/or elevated CO2 atmospheres inhibit C2H4 biosynthesis. Ethylene biosynthesis rates and in vitro ACS activity were closely correlated in all treatments, while in vitro ACO activity significantly increased over time regardless of the treatment. Only a small amount of C2H4 biosynthesis inhibition by lowered O-2 and/or elevated CO2 atmospheres could be accounted for by suppressed induction of ACO activity, Western blot analysis demonstrated that apples held for 2 months in lowered O-2 and/or elevated CO2 atmospheres had significantly reduced abundance of ACO protein, compared to fruit held in air. Northern blot analysis of ACS and ACO transcript abundance revealed that reduced O-2 and/or elevated CO2 atmospheres delay induction and reduce the abundance of both transcripts, Reduced O-2 and/or elevated CO2 atmospheres reduce C2H4 biosynthesis by delaying and suppressing expression of ACS at the transcriptional level and by reducing the abundance of active ACO protein. Chemical names used: 1-aminocyclopropane-1-carboxylic acid (ACC), ACC synthase (ACS), ACC oxidase (ACO), ethylene (C2H4), S-adenosylmethionine (AdoMet).