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THE ROLE OF PHYTOCHROMES IN ROOT TROPISMS.  M.J. Correll, K.M. Coveney, S.V. Raines, and J.Z. Kiss.  Dept of Botany, Miami Univ, Oxford, OH.

   Phytochromes are involved in many physiological responses in plants including, germination, circadian clock regulation, shade avoidance, and the positive red-light-induced phototropism in roots.  In hypocotyls, both phytochromes A and B are important for regulating blue-light-induced phototropism and gravitropism.  However, the roles of phytochrome in blue-light-induced phototropism and gravitropism in roots are largely uncharacterized.  Through the use of single, double, and triple mutants deficient in the phytochromes (A-E), we have studied the roles of these photosensitive pigments in gravitropism and blue-light-induced phototropism of Arabidopsis roots.  Preliminary results indicate that roots from phyA mutants have reduced phototropic response if the seedlings were previously grown in white light; however, roots from seedlings previously grown in the dark have a normal phototropic response.  Gravitropism and the growth rate of roots from both dark-grown and light-grown phyA seedlings are comparable to wild-type roots.  Therefore, it appears that roots from light-grown and dark-grown seedling respond differently to blue light.   Roots from the double mutant phyAB have reduced growth rates, reduced phototropic responses, and reduced gravitropism compared to roots from wild-type plants.  These results make it difficult to determine whether phytochromes A and B are involved in any of these tropisms in roots or if these results are due altered root morphology.  Studies are in progress to decipher the complex interaction between root morphology and tropisms through the use of a feedback system that maintains root orientation while the stimulus is being applied.  (Financial Support by NASA:  NCC2-1200)