Differential expression and internal feedback regulation of 1-aminocyclopropane-1-carboxylate synthase, 1-aminocyclopropane-1-carboxylate oxidase, and ethylene receptor genes in tomato fruit during development and ripening

We investigated the feedback regulation of ethylene biosynthesis in tomato (Lycopersicon esculentum) fruit with respect to the transition from system 1 to system 2 ethylene production. The abundance of LE-ACS2, LE-ACS 1, and NR mRNAs increased in the ripening fruit concomitant with a burst in ethyle ne production. These increases in mRNAs with ripening were prevented to a l arge extent by treatment with 1-methylcyclopropene (MCP), an ethylene actio n inhibitor. Transcripts for the LE-ACS6 gene, which accumulated in preclim acteric fruit but not in untreated ripening fruit, did accumulate in ripeni ng fruit treated with MCP. Treatment of young fruit with propylene prevente d the accumulation of transcripts for this gene. LE-ACS1A, LE-ACS3, and TAE 1 genes were expressed constitutively in the fruit throughout development a nd ripening irrespective of whether the fruit was treated with MCP or propy lene. The transcripts for LE-ACO1 and LE-ACO4 genes already existed in prec limacteric fruit and increased greatly when ripening commenced. These incre ases in LE-ACO mRNA with ripening were also prevented by treatment with MCP . The results suggest that in tomato fruit the preclimacteric system 1 ethy lene is possibly mediated via constitutively expressed LE-ACS1A and LE-ACS3 and negatively feedback-regulated LE-ACS6 genes with preexisting LE-ACO1 a nd LE-ACO4 mRNAs. At the onset of the climacteric stage, it shifts to syste m 2 ethylene, with a large accumulation of LE-ACS2, LE-ACS4, LE-ACO1, and L E-ACO4 mRNAs as a result of a positive feedback regulation. This transition from system 1 to system 2 ethylene production might be related to the accu mulated level of NR mRNA.