Characterization of the Promoter of the Mung Bean Auxin-Inducible ACC Synthase Gene, Vr-ACS6

Abstract

Auxin-induced ethylene production in mung bean hypocotyls is further modified by other plant hormones. Cytokinin synergistically stimulates, and abscisic acid and ethylene suppress auxin-induced ethylene production. We have isolated an ACC synthase gene, Vr-ACS6, the expression of which is specific to auxin. Transgenic tobacco plants were prepared with a fusion gene in which the GUS gene was placed under the Vr-ACS6 promoter (1.5 kb), and seedlings obtained from Tl seeds were treated with combinations of auxin and cytokinin, abscisic acid or ethylene, then GUS activity was assayed. The expression of GUS activity was specific to auxin (I A A, 2,4-D or NAA) and the degree of expression was dependent on concentrations of IAA, with a detectable GUS activity as low as 0.1 μM IAA. IAA-induced GUS expression was stimulated by simultaneous presence of cytokinin, and suppressed by ABA and ethylene. These interactions were also confirmed by GUS staining of treated seedlings of both green and etiolated transgenic plants. In etiolated seedlings, GUS stain was observed in the elongation zone of hypocotyls, whereas in green seedlings, the stain was observed in shoot tips and cotyledons. These interactions were found to affect the rate of ethylene production in mung bean hypocotyls. The results indicate that the 1.5 kb 5′-untranscribed region of Vr-ACS6 gene contains all these elements necessary for the hormone interactions.