Auxin and brassinosteroid differentially regulate the expression of three members of the 1-aminocyclopropane-1-carboxylate synthase gene family in mung bean (Vigna radiata L.)

Indole-3-acetic acid (IAA) markedly increased ethylene production by induci ng the expression of three 1-aminocyclopropane-1-carboxylate (ACC) synthase cDNAs (pVR-ACS1, pVR-ACS6 and pVR-ACS7) in mung bean hypocotyls. Results f rom nuclear run-on transcription assay and RNA gel blot studies revealed th at all three genes were transcriptionally active displaying unique patterns of induction by IAA and various hormones in etiolated hypocotyls. Particul arly, 24-epibrassinolide (BR), an active brassinosteroid, specifically enha nced the expression of VR-ACS7 by a distinct temporal induction mechanism c ompared to that of IAA. In addition, BR synergistically increased the IAA-i nduced VR-ACS6 and VR-ACS7 transcript levels, while it effectively abolishe d both the IAA- and kinetin-induced accumulation of VR-ACS1 mRNA. In light- grown plants, VR-ACS1 was induced by IAA in roots, and VR-ACS6 in epicotyls . IAA- and BR-treatments were not able to increase the VR-ACS7 transcript i n the light-grown tissues. These results indicate that the expression of AC C synthase multigene family is regulated by complex hormonal and developmen tal networks in a gene- and tissue-specific manner in mung bean plants. The VR-ACS7 gene was isolated, and chimeric fusion between the 2.4 kb 5'-upstr eam region and the beta-glucuronidase (GUS) reporter gene was constructed a nd introduced into Nicotiana tabacum. Analysis of transgenic tobacco plants revealed the VR-ACS7 promoter-driven GUS activity at a highly localized re gion of the hypocotyl-root junction of control seedlings, while a marked in duction of GUS activity was detected only in the hypocotyl region of the IA A-treated transgenic seedlings where rapid cell elongation occurs. Although there was a modest synergistic effect of BR on the IAA-induced GUS activit y, BR alone failed to increase the GUS activity, suggesting that induction of VR-ACS7 occurs via separate signaling pathways in response to IAA and BR . A scheme of the multiple regulatory pathways for the expression of ACC sy nthase multigene family by auxin and BR is presented.