Wound-induced ethylene synthesis in stem tissue of harvested broccoli and its effect on senescence and ethylene synthesis in broccoli florets

Broccoli (Brassica oleracea, var. italica) florets (flower buds) senesced rapidly after harvest at ambient temperatures. ACC (1-aminocyclopropane-1-carboxylic acid) synthase (ACS) was induced rapidly in the cut surface of stem tissue in the first 2 turn thick and the basal portions of curds soon after harvest, leading to an accumulation of ACC. The abundance of one ACS mRNA (BO-ACS1) increased in the first layer, the inner layer of stem (2-4 mm, second layer), and the basal portion of curds, while the transcripts for another (BO-ACS2) increased only in the first layer. Conjugated ACC (malonyl ACC, MACC) levels increased in all portions as senescence progressed. In florets, ACC synthase activity and BO-ACS1 transcripts were detected with no significant changes observed during senescence. ACC levels in florets stayed low throughout the experimental period, whereas MACC levels were much higher than those of ACC. The marked rise in ACC oxidase (ACO) activity in florets was detected almost in parallel with a significant increase in ethylene production. The abundance of ACO transcripts (BO-ACO1 and BO-ACO2) increased concurrently with the rise in ACC oxidase activity. These findings suggest that ACC and ethylene synthesized in the stem in response to wounding may have involved the enhanced activity of ACC oxidase and increased abundance of its transcripts in florets. (C) 2002 Elsevier Science B.V. All rights reserved.