[47]

TROPISTIC RESPONSES IN ROOTS–HOW LIGHT AND GRAVITY INTERACT IN SHAPING PLANT FORM.  J.Z. Kiss¹, J.L. Mullen², M.J. Correll¹, R.E. Edelmann¹. ¹Dept.  Bot., Miami Univ., Oxford, Ohio 45056.  ²Dept. Biology, Indiana Univ., Bloomington, Indiana 47405.

   To receive adequate light and nutrients for survival, plants orient stems and stem-like organs toward light and away from the gravity vector and, conversely, orient roots into the soil, away from light toward the direction of gravity.  Therefore, both gravity and light can influence the differential growth of plant organs.  To add to the complexity of the interactions between gravity and light, each stimulus can enhance or reduce the effectiveness of the other.   On earth, the constant presence of gravity makes it difficult to determine whether plant growth and development is influenced by gravity or light alone or the combination of the two stimuli.

   In roots, gravitropism (i.e. the directed growth in response to gravity) is the predominant tropistic response, but recent studies have shown that phototropism also plays a role in the oriented growth of roots in flowering plants.    In blue or white light, roots exhibit negative phototropism that is mediated primarily by the phototropin family of photoreceptors.  In contrast, we recently have discovered a positive phototropism in response to red light in Arabidopsis roots.  Since this red-light-based response is weak relative to both gravitropism and negative phototropism, we have utilized a novel device which combines image analysis of root growth with a feedback system in order to study phototropism without the complications of a counteracting gravitational stimulus.  In positive root phototropism, sensing of red light occurs in the root itself and is not a result of a transfer of a signal resulting from light perception in the shoot.  The differential growth response occurred at the basal end of the elongation zone, in contrast to the location of gravitropic curvature, which is initiated in the distal elongation zone.   Mutants that lack the red-light-absorbing photosensitive pigment phytochrome A (phyA) or phyB were severely impaired in red-light-based phototropism while the phyD and phyE mutants were normal in this response.  Thus, as in gravitropism, phyA and phyB play a key role in mediating red-light-based phototropism in roots.  (Supported by NASA grant NCC 2-1200.)

[