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The Role of Phytochromes in the Phototropic and Gravitropic Responses of Arabidopsis Inflorescence Stems.  E.T. Lagedrost, M.J. Correll, and J.Z. Kiss.  Department of Botany, Miami University, Oxford, Ohio.

   Plants sense stimuli from their environment, and transduce these signals to induce a response.  Phototropism and gravitropism are the two main signaling pathways in plants.  These two pathways are similar and may, in fact, be interrelated.  Recent results from our laboratory have shown that the phytochrome genes are involved in sensing both gravity and light in the roots of seedlings.

   In order to extend our studies, in this project, we have examined the phototropic and gravitropic responses of mature flowering Arabidopsis thaliana stalks.  Within Arabidopsis, there are five phytochrome genes, PHYA - E respectively.  Phytochrome-deficient mutants were used to determine which phytochromes are responsible for either response, and whether the same phytochromes are involved in both tropisms. 

   Our current research has examined the responses of the phyA, phyB and phyAB mutants in comparison with the wild type, Landsberg erecta (Ler).  In terms of gravitropism, there is no significant difference in the gravity response of these mutants relative to the WT.  However, in the blue-light induced phototropic response, there is a significant difference between the WT and the phy mutants.  Interestingly, the phyB mutant shows the greatest degree of curvature, while the phyA mutant exhibits an inhibition.  Therefore, phyA and phyB are involved in the blue light phototropic pathway of Arabidopsis inflorescence stems, but do not play a role in the gravitropic response.  Current experiments are underway to test the phototropic and gravitropic responses of the phyD and phyE mutants. 

(Financial support provided by NASA grant NCC2-1200 and the Hughes Program of Miami University)