THE LIGHT-INDUCED REDUCTION OF THE GRAVITROPIC GROWTH-ORIENTATION OF SEEDLINGS OF ARABIDOPSIS-THALIANA (L) HEYNH IS A PHOTOMORPHOGENIC RESPONSE MEDIATED SYNERGISTICALLY BY THE FAR-RED-ABSORBING FORMS OF PHYTOCHROME-A AND PHYTOCHROME-B

Hypocotyls of dark-grown seedlings of AI Arabidosis thaliana exhibit a strong negative gravitropism, which is reduced by red and also by lon g-wavelength, far-red light treatments. Light treatments using phytoch rome A (phyA)- and phytochrome B (phyB)-deficient mutants showed that this response is controlled by phyB in a red/far-red reversible way, a nd by phyA in a non-reversible, very-low-fluence response. Crosses of the previously analyzed phyB-1 allele (in the ecotype Landsberg erecta background) to the ecotype Nossen wild-type (WT) background resulted in a WT-like negative gravitropism in darkness, indicating that the pr eviously described gravitropic randomization observed with phyB-1 in t he dark is likely due to a second mutation independent of that in the PHYB gene.