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THE PROMOTION OF GRAVITROPISM IN ARABIDOPSIS ROOTS UPON ACTIN DISRUPTION IS COUPLED WITH EXTENDED ALKALINIZATION OF THE COLUMELLA CYTOPLASM AND A PERSISTENT ASYMMETRIC AUXIN GRADIENT   G. Hou¹, V.L. Kramer^2,,Y-S Wang¹, R. Chen¹ , G. Perbal³, S. Gilroy² and E.B. Blancaflor¹.  ¹Plant Biology Division, Samuel Roberts Noble Foundation, Ardmore, OK, ²Department of Biology, Pennsylvania State University, University Park, PA, USA, ³ Universite Pierre et Marie Curie, Lab CEMV, Paris, France

   Actin has been implicated in regulating plant gravitropism; however, its precise role in this process remains uncertain. We have shown that the disruption of the actin cytoskeleton with Latrunculin B (Lat B) promotes gravitropism in maize roots (Hou et al., 2003; Plant Physiol 131: 1360-1373). These effects were most evident on a clinostat as curvature that would exceed 90 degrees despite short doses of gravistimulation. To probe further into the mechanisms underlying this enhanced gravitropic response, we extended our studies to roots of Arabidopsis. Lat B enhanced the sensitivity and response of Arabidopsis roots to gravity. The strongest promotive effect on gravitropism was elicited when roots were treated with 100 nM Lat B and a stimulation time of 15 min was sufficient to trigger maximal root curvature response during clinorotation. Immunolabeling showed that 100 nM Lat B disrupted the fine actin filament network but did not affect the thick actin bundles. Moreover, gravistimulation induced a transient increase in cytoplasmic pH in the columella cells. Although Lat B did not inhibit the alkalinization of the columella, its duration was significantly extended. Indirect visualization of auxin redistribution using the DR5:GUS reporter showed that the enhanced curvature responses of Lat B-treated roots during clinorotation was accompanied by a persistent asymmetric auxin gradient. The auxin transport inhibitor, TIBA, blocked Lat B-induced curvature and development of the auxin gradient. Our results indicate that actin is involved in regulating lateral auxin transport and gravity induced alkalinization of the root columella.  

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