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HAPTOGRAVITROPISM: WHEN A ROOT ENCOUNTERS A BARRIER TO DOWNWARD GROWTH.  G.D. Massa and S. Gilroy.  Department of Biology, Penn State University. 

     In nature, roots will encounter a variety of obstacles that will cause a growing root to reorient.  Rocks, hard-pan soil layers, and even other roots will block downward growth, requiring the root to change direction until the obstacle is bypassed.  We have designed an experimental system to examine the interaction of roots with rigid obstacles, and have tested primary roots of the model plant Arabidopsis thaliana.  A vertically growing root appears to sense an obstacle, and rapidly initiate a growth response, possibly by integrating gravity and touch stimuli.  The response is characterized by the formation of a bend in the region of the root behind the tip, allowing the root tip to maintain a set angle of 136˚ as the root tracks across the surface of an obstacle.  Kinetic analysis indicates that the root growth rate remains constant before, during, and after interaction with the barrier.  To dissect the relative contributions of touch and gravity, laser ablations of groups of root cap cells were performed and these indicate that all of the cells in the root cap are required for a normal haptogravitropic response.  The pgm1-1 mutant, however, does exhibit normal haptogravitropism, indicating that reduced gravisensing is sufficient for a normal response.  Gravitropic reorientation after the root reaches the end of the barrier appears identical to reorientation from the same angle in rotated roots that did not interact with a barrier.  Other species are being examined to determine the generality of the response, and differences between species with tap root systems versus fibrous root systems appear to occur.  The haptogravitropic response of lateral roots is also examined.