CHARACTERIZATION OF 3 MEMBERS OF THE ACC SYNTHASE GENE FAMILY IN SOLANUM-TUBEROSUM L

Two genomic clones corresponding to three members of the 1-aminocyclop ropane-1-carboxylic acid (ACC) synthase gene family in potato (Solanum tuberosum L.) have been isolated and sequenced. Two highly homologous genes, ST-ACS1A and ST-ACS1B, transcribed in opposite directions were found in an 8.9 kb region. Their coding sequences are interrupted by two introns at identical positions. Their closest relative in tomato i s the LE-ACS3 gene. The third gene in potato, ST-ACS2, was found in a 4 kb region and shows a gene structure similar to that of the tomato L E-ACS4 gene and to the mung bean VR-ACS4 and VR-ACS5 genes. Based on i ts lack of significant homology to the tomato gene family and its clos eness to the VR-ACS4 and VR-ACS5 genes, we propose that LE-ACS7 repres ents an additional isoform in the tomato genome. Moreover, in a phylog enetic comparison of known ACC synthases, the ST-ACS2 isoform was grou ped in a separate lineage together with the mung bean VR-ACS4 and VR-A CS5, and the moth orchid DS-ACS1A and DS-ACS1B gene products. Expressi on of the three potato genes was studied by reverse transcription-poly merase chain reaction on total RNA. The twin genes are positively regu lated by indole-3-acetic acid in hypocotyls and expression is modulate d by wounding in the leaves. The third gene is responsive to ethylene and wounding mainly in tubers. The roles of these three genes and of o ther members of the ACC synthase gene family in vegetative processes o f potato such as tuberization, dormancy, and sprouting have yet to be determined.