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Phosphorylation of Stathmin and Growth and Gravitropic Response of Roots of Zea mays L.  Timothy J. Mulkey,  Life Science Dept., Indiana State Univ., Terre Haute, IN  47809.

   Growth, development, and response to environmental stimuli are hallmarks of multicellular living systems, whether plant or animal.  On a cellular level, the response is initiated through passage of an extracellular stimulus through a signal transduction pathway to the nucleus, eliciting specific gene expression resulting in a response to the stimulus.  One such signal transduction intermediate which seems common to all systems tested thus far is the protein stathmin.  Stathmin (also known as pp17, prosolin, Op18, p17, P19, pp20, pp21, pp23, and 19-K) is a highly conserved, 19-kDa, heat-stable,  cytosolic protein which has been implicated as a relay phosphoprotein in multiple signal transduction systems.  Twelve phosphorylated and two unphosphorylated forms of stathmin have been previously identified in signal transduction systems of vertebrate tissues.  These roles are associated with key events during growth, development and differentiation.  Our data indicates that the phosphorylation state of proteins from maize roots that are in the molecular-weight range of stathmin, and identified by Northern blots as stathmin, can be alter by treatment with EDTA, calcium, indole-3-acetic acid (IAA), tetradeconoyl phorbol-13-acetate (TPA), and staurosporine.  Exposure of roots to TPA induces elongation and phosphorylation patterns of putative stathmin proteins which are similar to those observed in tissue treated with promotive concentrations of IAA.  Phosphorylation of the putative stathmin proteins are inhibited by staurosporine in maize roots which are treated with promotive concentrations of auxin; staurosporine inhibits the elongation response under these conditions.  Further characterization of the role of stathmin as a possible regulator of signal transduction events during root elongation and gravitropic response will be presented.