Poster: Tropisms
Abs #
658: Integrating tropisms: ionic signaling and the cytoskeleton mediates the interaction of touch and gravity in the root cap.
Roots must sense and respond to a variety of environmental stimuli as they grow through the soil. Obstacles such as rocks will impede the general downwardly directed gravitropic growth of the root system and so these soil features must be sensed and the information processed for an appropriate alteration in gravitropic growth to avoid the obstruction. We have shown that primary and lateral roots of Arabidopsis sense and respond to mechanical barriers in their path by eliciting a differential growth response to circumvent the barrier. This response is characterized by the formation of bends in the central and later in the distal elongation zones. Thus, the root tip maintains a fixed angle to the obstacle as the root tracks across its surface allowing the very apex of the root cap to sample the extent of the obstacle. This tropic response is under control of the root?s inherent gravitational set-point angle but also reflects touch stimulation of the root cap down-regulating the gravitropic response. However, the molecular basis for such interaction of touch and gravity sensing systems remains poorly defined. We show that touch elicits a self-propagating wave of Ca2+ initiated in the surface cells that encounter the mechanical stimulus. The generation, but not propagation, of this wave is dependent on an intact, dynamic cytoskeleton. The Ca2+ increase propagates to the gravity sensing columella cells where it suppresses events such as amyloplast motility and cytosolic pH changes associated with gravitropic signaling. These results suggest that the Ca2+-dependent touch signaling system may interact with a pH-dependent gravity sensing apparatus to integrate the touch and gravity signal to an appropriate tropic growth response. This work is supported by NASA and NSF.
