| Research Abstract |
. Expression and internal feedback regulation of the genes related to ethylene biosynthesis during development and ripening in several fruits were investigated.
In tomato fruit, the preclimacteric system 1 ethylene was possibly mediated via constitutively expressed LE-ACS1A and LE-ACS3 and negatively feedback-regulated LE-ACS6 genes with preexisting LE-ACO1 and LE-ACO4 mRNAs, At the onset of the cimacteric stage, it may shift to system 2 ethylene, with a large accumulation of LE-ACS2, LE-ACS4, LE-ACO1, and LE-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 accumulated level of NR mRNA.
Five cDNA fragments encoding ACC synthase or ACC oxidase were cloned from cucumber fruit, a nonclimacteric type. Nevertheless., these were highly homologous in nucleotide sequences to the corresponding genes in melon fruit, a climacteric type, CS-ACS1 gene was not expressed in the ripening cucumber fruit. CS-ACS1 was a wound-inducible gene and CS.ACS3 was an auxin-inducible one. In melon fruit, the difference in the ethylene-forming capability among cultivars resulted from the expression of CMe-ACS1 mRNA during natural fruit ripening and in response to exogenous ethylene.
MA-ACS1 was the gene in banana fruit related to the ripening process and was inducible by exogenous ethylene. At the onset of cimacteric, ethylene production increased greatly with a sharp peak concomitant with an increase in the accumulation of MA-ACS1 mRNA and then decreased rapidly. This sharp peak of ethylene production was accompanying a sharp peak of in vivo AGO oxidase activity. However, the level of MA-ACO1 transcripts remained high from the onset of climacteric to the fully ripe stage. This discrepancy between enzyme activity and gene expression of AGO oxidase was, at least in part, due to reduced contents of ascorbate and ferrous, cofactors of the enzyme, during ripening.
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