The Interaction of Touch and Gravity in Plant Roots.
Open Access
- Author:
- Massa, Gioia Donna
- Graduate Program:
- Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 21, 2003
- Committee Members:
- Richard Cyr, Committee Member
Michael Allan Arthur, Committee Member
Simon Gilroy, Committee Chair/Co-Chair
Claude Walker Depamphilis, Committee Member - Keywords:
- gravitropism
thigmotropism
gravity
touch
plant
root - Abstract:
- Plant roots must sense and respond to a wide array of stimuli, such as gravity, as they grow through the soil. Studies of gravitropism have traditionally focused on reorientation of horizontally placed roots to restore vertical downward growth. In nature, however, it is much more likely that a root will be forced to reorient by growing into a rock, a hard-pan layer of soil, or some other impenetrable obstacle. I have designed a bioassay to examine the interaction of roots with obstacles preventing downward growth and have characterized the response of both primary and lateral roots of Arabidopsis to these barriers. These experiments have shown that both touch and gravity signals appear to be integrated in the root cap, and that the response to a barrier is dependant on the gravitropic set-point angle of the root. As I have attempted to unravel the relative importance of touch and gravity signaling, I have examined the requirement of gravisensing for this response using both mutants with altered gravisensing abilities, and laser ablation of the gravity sensing root cap cells. I have also modified the angle of the obstruction by presenting the roots with both concave and convex barriers. I have demonstrated that a similar response occurs in the larger roots of cress, while a somewhat different response occurs in timothy grass, a monocot with roots similar in size to Arabidopsis. I have developed, constructed, and flown Space Shuttle hardware to examine touch responses under microgravity conditions. Due to technical difficulties, however, I was unable to obtain data on plant growth from this experiment. In addition, analysis using a clinostat to randomize growth has allowed us to determine that touch down-regulates the graviresponse by reducing the curvature of gravistimulated roots. Also, touch reduces the rate of amyloplast sedimentation in the central columella cells, cells that when removed have the strongest deleterious impact on gravitropism. To understand signaling cross-talk between touch and gravisensing I characterized a microtubule-dependent increase in cytoplasmic Ca2+ upon touch in the peripheral cells, and this Ca2+ spike moved as a ‘wave’ into the columella cells of the root cap. Generation of an artificial Ca2+ increase will cause the roots to reverse the direction of growth along an obstacle in the manner of a root hitting a secondary object while traversing a barrier. Immunofluorescence showed that the microtubules in these peripheral cells are oriented differently than in columella cells or in cells that are sloughed off. In addition, touch reduced the pH alterations that occur in the root cap upon gravistimulation. I propose, therefore, that touch is sensed in the root cap via an intact, dynamic microtubule array, and this force sensing leads to the observed cytoplasmic Ca2+ transients. This Ca2+ ‘wave’ inhibits either gravity sensing or response events, and inhibition of gravitropism allows the development of a normal obstacle induced response.