PLANT PHYSIOLOGY , Vol 101, Issue 2 469-476, Copyright © 1993 by American Society of Plant Biologists
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MOLECULAR BIOLOGY AND GENE REGULATION |
Competitive Inhibition of Abscisic Acid-Regulated Gene Expression by Stereoisomeric Acetylenic Analogs of Abscisic Acid
R. W. Wilen, D. B. Hays, R. M. Mandel, S. R. Abrams and M. M. Moloney
Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada, T2N 1N4 (R.W.W., D.B.H., R.M.M., M.M.M.)
The properties of two enantiomeric synthetic acetylenic abscisic acid (ABA)
analogs (PBI-51 and PBI-63) in relation to ABA-sensitive gene expression
are reported. Using microspore-derived embryos of Brassica napus as the
biological material and their responsiveness to ABA in the expression of
genes encoding storage proteins as a quantitative bioassay, we measured the
biological activity of PBI-51 and PBI-63. Assays to evaluate agonistic
activity of either compound applied individually showed a dose-dependent
increase in napin gene expression on application of PBI-63. Maximal
activity of about 40 [mu]M indicated that PBI-63 was an agonist, although
somewhat weaker than ABA. PBI-63 has a similar stereochemistry to natural
ABA at the junction of the ring and side chain. In contrast, PBI-51 showed
no agonistic effects until applied at 40 to 50 [mu]M. Even then, the
response was fairly weak. PBI-51 has the opposite stereochemistry to
natural ABA at the junction of the ring and side chain. When applied
concurrently with ABA, PBI-63 and PBI-51 had distinctly different
properties. PBI-63 (40 [mu]M) and ABA (5 [mu]M) combined gave results
similar to the application of either compound separately with high levels
of induction of napin expression. PBI-51 displayed a reversible
antagonistic effect with ABA, shifting the typical ABA dose-response curve
by a factor of 4 to 5. This antagonism was noted for the expression of two
ABA-sensitive genes, napin and oleosin. To test whether this antagonism was
at the level of ABA recognition or uptake, ABA uptake was monitored in the
presence of PBI-51 or PBI-63. Neither compound decreased ABA uptake.
Treatments with either PBI-51 or PBI-63 showed an effect on endogenous ABA
pools by permitting increases of 5- to 7-fold. It is hypothesized that this
increase occurs because of competition for ABA catabolic enzymes by both
compounds. The fact that ABA pools did not decrease in the presence of
PBI-51 suggests that PBI-51 must exert its antagonistic properties through
direct competition with ABA at a hormone-recognition site.
