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???TROPISM: WHEN A PRIMARY ROOT ENCOUNTERS A BARRIER TO DOWNWARD GROWTH. G.D. Massa and S. Gilroy. Dept of Biology, Penn State University. 

In nature, roots may encounter various obstacles as they grow through soil. To examine this interaction of roots with rigid barriers, experimental protocols have been designed and tested using Arabidopsis thaliana. A growing root appears to sense an obstacle, possibly by integrating gravity and touch stimuli, and a growth response is initiated. The response requires reorientation of the growing root tip until unobstructed vertical growth can resume. As the root interacts with the barrier, different regions of the root coordinate, letting the tip maintain a set angle while bend formation allows elongation to occur parallel to the obstacle surface. We have studied the response kinetics of Arabidopsis roots growing under these conditions, and have found that root growth rate is unaltered by interaction with a barrier.  The angle of the root tip, however, changes from 90° (vertical) to a set angle of approximately 140° as the root tracks across the surface. In addition, other species were examined and their responses to barriers support conclusions drawn from Arabidopsis. Cellular localization of sensing was examined using laser ablation, which allowed the selective removal of peripheral cap and columella cells, and demonstrated the importance of both cell types for a normal ???tropic response. Using Arabidopsis plants transformed with ion-sensitive GFP’s we are examining the fluorescence patterns during ???tropism and are comparing these with the effects of isolated gravity and touch stimuli. Suggestions for naming the ???tropic response will be solicited. 

This research was supported by NSF.