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EFFECTS OF LITHIUM IONS ON ELONGATION AND GRAVITROPIC RESPONSES OF PRIMARY ROOTS OF MAIZE.   Timothy J. Mulkey.  Life Science Dept., Indiana State Univ., Terre Haute IN  47809

Rubbing-induced ethylene and conversion of ACC (1-aminocyclopropane-1-carboxylic acid) to ethylene in relation to thigmotropism is reduced by treatment with lithium (Plt. Physiol. 72:522).  In animal systems, Li inhibits resynthesis of PIP₂ (Biochem. J. 180:655) and inhibits inositol-1-phosphate phosphatase (J. Biol. Chem. 255:10896); but plant phosphatases appear less sensitive to Li ion exposure (Plt. Physiol. 76:40).  Previous work in our laboratory has demonstrated a role for auxin-induced ethylene production in root growth and gravitropic responses and has suggested involvement of the second messenger system in auxin-associated responses of root.  This study examines the effects of Li ions on elongation and gravitropic responses of maize roots.  Li ions at concentrations greater than 0.1 mM promotes the elongation rate of primary roots of maize.  Additionally, asymmetric application of Li to graviresponding roots alters the kinetics of curvature.  Application of Li to the lower surface of the root delays positive gravicurvature or results in negative gravicurvature.  Li ions alter in vitro protein phosphorylation patterns; Li treatment results in phosphorylation patterns similar to those observed in auxin-stimulated/ethylene-inhibited tissue.  These results will be discussed in relation to auxin-ethylene interaction and protein phosphorylation in the putative second messenger system of hormone action during elongation and gravitropic responses.