ASGSB 2003 Annual Meeting Abstracts


TOUCH AND GRAVITY ARE INTEGRATED THROUGH A Ca2+-CALMODULIN-DEPPENDENT SYSTEM IN THE ARABIDOPSIS ROOT CAP.  S.J. Swanson, Victoria L. Kramer and S. Gilroy. Biology Dept, PennState University, University Park, PA.

   We have previously shown that roots can combine gravitropic and thigmotropic information to yield an appropriate integrated growth response. Although it appears that touch stimulation can down-regulate the gravity response of roots at the level of the root cap, the molecular mechanism for this signal integration remains poorly defined. We have therefore examined the potential role of ionic signaling in mediating this signal processing. Touch stimulation of the peripheral cells of the root cap elicited an increase in cytosolic Ca2+ that propagates from cell to cell throughout the cap. Touch stimulation of these peripheral cap cells also inhibited the transient elevation in cytosolic pH seen in the gravity sensing columella cells upon gravistimulation of the root. These observations suggest a mechanism whereby touch and gravity coordinate root growth through the interaction of pH and Ca2+ signaling events in the columella cells. Consistent with this model, artificially increasing Ca2+ in the root cap using caged Ca2+ ionophore led to an inhibition of subsequent graviresponse as well as an inhibition of gravisignaling-related pH increases in the columella. Calmodulin is a ubiquitous regulator of plant Ca2+-dependent responses that may play a role in mediating these touch-related changes. Application of 1 ÁM of the calmodulin antagonist W7 altered the gravity responsiveness of the Arabidopsis root leading to randomized curvature on the clinostat after 15 minutes pre-stimulation. Although randomized in direction, these roots showed an enhanced growth response on the clinostat, developing larger curvature than equivalent control plants. Preliminary characterization suggests the calmodulin antagonists have no effect on touch-induced Ca2+ transients in the root cap but may alter the gravity-related pH transients seen in columella cells, implying a role for calmodulin in the Ca2+-dependent touch-related events that allow touch to modify graviresponse in these cells.

(Supported by NASA: NAG2-1518; NAG-1594, NSF MCB 02-12099).


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