Induction of 1-Aminocyclopropane-1-Carboxylate Synthase mRNA by Auxin in Mung Bean Hypocotyls and Cultured Apple Shoots

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Abstract
Auxin is known to promote ethylene production in vegetative tissues by increasing the activity of 1-aminocyclopropane-1-carboxylate (ACC) synthase; therefore, we have studied the effect of auxins on ACC synthase mRNA expression. Total RNA was isolated from auxin-incubated cultured apple (Malus sylvestris Mill.) shoots or mung bean (Vigna radiata L.) hypocotyls. These RNAs and a set of oligonucleotide primers corresponding to two conserved amino acid sequences (SNPLGTT and MSSFGLV) found in ACC synthases isolated from other species were used for polymerase chain reaction-based amplification of DNA fragments encoding the ACC synthase-active site domain. We obtained and sequenced a 290-base pair cDNA fragment (pAA1) from cultured apple shoots and a 328-base pair cDNA clone (pMBA1) from mung bean hypocotyls. Comparisons of their deduced amino acid sequences with those of previously characterized ACC synthase cDNAs indicate that both fragments are, indeed, closely related to ACC synthase cDNA. Northern blot analyses further showed that the expression of these transcripts is regulated by auxin treatment. These data indicate that auxin induces ethylene production transcriptionally by increasing the ACC synthase transcripts. The pAA1 shares 46% amino acid sequence homology with ripening-regulated apple fruit ACC synthase, indicating that ripening-regulated and auxin-regulated ACC synthases are encoded by different genes. In mung bean hypocotyls, aminooxyacetic acid, a potent inhibitor of ACC synthase activity, promoted the expression of auxin-induced ACC synthase mRNA, but cycloheximide inhibited this induction.