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Effects of Cis- and Trans-Retinoic Acid on Growth and Gravitropic Responses in Maize Roots.   Timothy J. Mulkey    Life Science Dept., Indiana State University, Terre Haute, IN  47809

   Retinoic acid (RA) is a member of the retinoid family of molecules that are related to β-carotene and vitamin A.  Retinoic acid is of interest in animal systems due to its involvement in signaling in vision, reproduction, maintenance of epithelial cells and a variety of other developmental and regulatory systems.  The possible involvement of retinoic acid in growth and gravitropic response in plants was examined by comparing the effect of cis- and trans- isomers of retinoic acid on elongation rate, hydrogen ion secretion, Ca⁺² asymmetry during gravistimulation, and protein phosphorylation in primary roots of maize.  In animal systems, cis-RA is the active form of the retinoid and trans-RA has little or no activity by comparison.  In maize roots, cis-RA is more effect than trans-RA on promoting root elongation.  After an initial transitory inhibition of root elongation, 10 μM cis-RA increases the rate of elongation by 200% over a 4 hrs period; trans-RA at 10 μM does not significantly alter elongation of primary roots of maize.  Pretreatment of roots with 10-7 M trans-RA and subsequent exposure to 1 µM and 0.1 µM IAA results in inhibition of growth similar to IAA controls.  Pretreatment of roots with 10-7 M cis-RA for 2 hours followed by to 10 nM to 0.01 nM IAA results in an initial inhibition with subsequent recovery in the rate of elongation.  Cis-RA altered the pattern of gravitropic response in intact maize roots; the response to cis-RA appears to result from alteration of elongation and modification of calcium redistribution in the apices of gravistimulated roots.  Previous data indicates that gravitropic stimulation alters protein phosphorylation patterns in gravistimulated roots.  In this study, RA alters the in vitro phosphorylation of a 63 Kd protein membrane protein and the in vitro phosphorylation of a 51 Kd protein cytoplasmic protein.  These data suggest that cis-retinoic acid promotes elongation, modifies the auxin response, alters Ca⁺² asymmetry during gravistimulation, and influences the protein phosphorylation patterns observed in roots.