A structurally divergent 1-aminocyclopropane-1-carboxylate synthase gene family coordinates abscission, wounding, and auxin responses in soybean

ACC (1-aminocyclopropane-1-carboxylate) synthase is the key enzyme in the ethylene biosynthetic pathway. In well studied species, ACC synthase is encoded by a gene family whose members are differentially expressed in response to various cues. Soybean (Glycine max) is an agriculturally important crop whose yield and quality are dependent upon multiple ethylene-regulated processes including organ shedding, yet the expression of only one ACC synthase cDNA has thus far been reported. Moreover, the molecular origin of the ethylene that accelerates organ detachment has not been established in this species, although it is presumed to activate cell separation within abscission zones. Therefore, we characterized seven novel complementary DNAs (cDNAs) encoding putative soybean ACC synthase (GMACS) isoforms. Nucleotide and amino acid sequence identities of these cDNAs and known GMCACCS1 gene varied from 40% to 99%, and 53% to 100%, respectively. Based on putative amino acid sequences analyses, they were clustered into four phylogenetic groups termed Class I-IV. Using relative reverse transcription PCR, putative GMACS gene expression was assessed in wounded and auxin-treated hypocotyls as well as in plant tissues including abscission zones. The mRNAs of all cDNAs of Class I and II began to accumulate, by 30-60 minutes post-wounding, while the members of Class III and IV exhibited no or a significantly delayed induction. Similarly, all Class I and II family members were induced by auxin treatment and their expression levels were gradually increased for approximately 3-7 hours before stabilizing or declining. In contrast, expression of cDNAs from Class III and IV were either weakly induced by IAA or not IAA-regulated. Interestingly, transcripts of seven of eight GMACS members were constantly expressed in leaflet and flower abscission zones during organ shedding. The remaining cDNA has no response in this process. The absence of differentially regulated gene expression of any GMACS during leaf and flower abscission suggests that soybean abscission is not regulated by transcriptional activation of the eight genes assessed. The mechanism by which ethylene contributes to the acceleration of organ shedding may instead involve synthesis of uncloned GMACS genes, post-transcriptional regulation of GMACS isoforms, or regulation of ACC oxidases that directly produce ethylene