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PHOSPHORYLATION SIGNALING AND THE REGULATION OF AUXIN TRANSPORT IN ARABIDOPSIS THALIANA ROOTS AND HYPOCOTYLS.  N.N. Kirpalani, A.M. Rashotte and G.K. Muday.  Dept. of Biology, Wake Forest University, Winston-Salem, NC 27109.  

     Redistribution of auxin contributes to gravitropic bending of plant tissue, however the mechanisms by which gravitropic signaling leads to altered auxin transport are not fully understood.  Protein phosphorylation and dephosphorylation have been implicated in this regulation by studies of the roots curl in NPA1 (rcn1) mutant, which has a mutation in a gene that encodes a protein phosphatase 2A regulatory subunit.  Roots of rcn1 respond more slowly to gravity stimulation than wild-type (Ws) roots.  Gravitropic bending and gradients in auxin induced gene expression were examined in the hypocotyls of rcn1 mutants transformed with the DR5- β-glucuronidase (GUS) reporter gene.   After gravity stimulation, the auxin induced DR5-GUS expression was localized to the lower side of the hypocotyl where gravitropic bending occurs.  In order to identify a kinase that may also function in the regulation of auxin transport during gravitropic bending, plants with mutations in the PINOID (PID) gene, which encodes a serine-threonine protein kinase were examined.  Roots of the pid-9 mutant have a slower response to gravity stimulation compared to wild-type (Columbia) as shown with high temporal resolution using a computerized image analysis program, Multi-ADAPT.  Furthermore, basipetal IAA transport was measured in pid-9 roots since a correlation has been suggested between basipetal auxin transport and gravity response.   The gravitropic response alterations in these plants with either reduced phosphatase or kinase activity indicate that phosphorylation signaling plays an important role in regulation of auxin transport in Arabidopsis roots and hypocotyls.  This work was supported by the National Aeronautics and Space Administration Specialized Center of Research and Training 

     (grant no. NAGW-4984).