DIFFERENTIAL EXPRESSION OF 2 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID OXIDASE GENES IN BROCCOLI AFTER HARVEST

Broccoli (Brassica oleracea L.) floral tissues rapidly differentiate a nd grow before harvest and then senesce rapidly after harvest. Associa ted with this postharvest deterioration is an increase in ethylene pro duction by florets. Two cDNA clones having high nucleotide identity to sequences encoding 1-aminocyclopropane-1-carboxylic acid (ACC) oxidas e were isolated from senescing florets. The cDNAs, ACC Ox1 and ACC Ox2 , apparently encode mRNAs from different genes. ACC Ox1 transcripts we re found at low levels in whole florets at the time of harvest and inc reased markedly in abundance after harvest. ACC Ox1 transcript abundan ce also increased in sepals after harvest and in excised yellowing lea ves. Transcripts corresponding to ACC Ox2 were found exclusively withi n the reproductive structures. These ACC Ox2 transcripts were absent a t harvest but started to increase in abundance within 2 h of harvest a nd then accumulated to high levels. Hormone treatment did not alter th e abundance of ACC Ox1 transcripts, whereas ACC Ox2 transcripts increa sed in abundance after treatment with abscisic acid and propylene. Wou nding did not affect the levels of ACC Ox1 or Ox2 transcripts after ha rvest. At harvest, individual broccoli florets were closed and remaine d unpollinated. We propose a model whereby the rapid increase in ACC O x1 and Ox2 transcript abundance after harvest contributes to increased ethylene production by florets. This ethylene may regulate aspects of postharvest senescence, in particular chlorophyll loss.