ASGSB 2003 Annual Meeting Abstracts


[99]

AN EXAMINATION OF LONG TERM MOTOR ACTIVITY IN MAIZE ROOT GRAVITROPISM UNDER CONDITIONS OF CONSTANT STIMULUS INPUT. H. Ishikawa, E. Natori, and M.L. Evans. Depart of Plant Biology, Ohio State Univ, Columbus.

   Maize roots display tri-phasic gravitropism: early curvature (Phase I), curvature reversal (“auto-straightening,” Phase II) and slower curvature (Phase III).  Using a feedback system (ROTATO) we examined curvature when the root tip was held at the initial angle of stimulation throughout.  Under these conditions Phase I lasted about 1 h and was induced even at very small stimulus angles. The rate of Phase I (rate of ROTATO Rotation, RR) was 0.2 deg/min for small angles and 1.0 deg/min for a 90 deg stimulus.  There was no difference in RR between 80-120 deg stimulus.  Phase III began 2.5 hr after stimulus application.  However, Phase III was induced only at angles of 35 deg or more.  Above 35 deg RR increased to a maximum of 0.5 deg/min for a 90 deg stimulus (i.e.half the maximum RR of Phase I). Because Phase III continued unabated for hours, it appears that there is steady signal output from the root apex to the curving region with no adaptation.  Phase II occurred between 1 and 2.5 hr after the initial stimulus.  This matches "auto-straightening" in control roots. Like Phase I, Phase II was induced even at low stimulation angles.  Phase II was difficult to observe at angles of 70 degrees or higher, possibly because it was masked by the strong expression of Phases I and III at these angles. The fact that auto-straightening occurs even when the stimulus is constant indicates that it is inherent to the response and not simply initiated by declining angle as the root approaches vertical.  We speculate that there are two motors (A and B) driving curvature.  Phase I may represent the activity of both motors (motor A producing curvature near the tip and responding to the initial stimulus; motor B maintaining curvature once established). It may be that curvature during Phase III is slow because it is driven only by motor B.  Using software that tracks relative elemental growth rate, we are examining the location of the two motors and comparing the responses in maize and Arabidopsis.

(Supported by NASA:  NAG2-1411)

 

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