EXPRESSION AND INTERNAL FEEDBACK-REGULATION OF ACC SYNTHASE AND ACC OXIDASE GENES IN RIPENING TOMATO FRUIT

We have examined whether or not a positive feedback regulation of gene expression for 1-aminocyclopropane-1-carboxylate (ACC) synthase and A CC oxidase also operates in ripening tomato (Lycopersicon esculentum) fruit during the burst of ethylene production, Two cDNA fragments for ACC synthase and one for ACC oxidase were cloned with high homology to already known genes involved in ethylene biosynthesis in ripening tom ato fruit, Accumulation of mRNAs which hybridize to these cDNA probes were induced in mature green fruit within two days by treatment with p ropylene, In the fruit ripened from the turning stage, red color devel opment, ethylene production, ACC content, and activities of ACC syntha se and ACC oxidase increased as maturity progressed, The abundance of two ACC synthase and one ACC oxidase mRNAs in the fruit increased from the turning to pink stage and were followed by a slight decline towar ds the red stage, These increases in mRNAs abundance with ripening wer e prevented to a large extent by treatment with the ethylene action in hibitor, 1-methylcyclopropene (MCP), This was most pronounced in the f ruit treated with MCP at the turning stage, in which the accumulation of ACC synthase and ACC oxidase transcripts was almost completely elim inated in the first two d, precisely the same stage at which the contr ol fruit had the greatest level of each mRNA accumulation, The inhibit ion of transcript accumulation recovered to the control level within t wo to four d, MCP also decreased ethylene biosynthetic activity, altho ugh this decrease did not reflect the reduction in the mRNAs accumulat ion, These results suggest that a strong positive feedback regulation is involved in ethylene biosynthesis at the gene transcriptional level in tomato fruit, even at the stage with a burst of ethylene productio n.