Enhanced Gravitropism of Roots with a Disrupted Cap Actin Cytoskeleton

Abstract

The actin cytoskeleton has been proposed to be a major player in plant gravitropism. However, understanding the role of actin in this process is far from complete. To address this problem, we conducted an analysis of the effect of Latrunculin B (Lat B), a potent actin-disrupting drug, on root gravitropism using various parameters that included detailed curvature kinetics, estimation of gravitropic sensitivity, and monitoring of curvature development after extended clinorotation. Lat B treatment resulted in a promotion of root curvature after a 90°reorientation in three plant species tested. More significantly, the sensitivity of maize (Zea mays) roots to gravity was enhanced after actin disruption, as determined from a comparison of presentation time of Lat B-treated versus untreated roots. A short 10-min gravistimulus followed by extended rotation on a 1-rpm clinostat resulted in extensive gravitropic responses, manifested as curvature that often exceeded 90°. Application of Lat B to the cap or elongation zone of maize roots resulted in the disruption of the actin cytoskeleton, which was confined to the area of localized Lat B application. Only roots with Lat B applied to the cap displayed the strong curvature responses after extended clinorotation. Our study demonstrates that disrupting the actin cytoskeleton in the cap leads to the persistence of a signal established by a previous gravistimulus. Therefore, actin could function in root gravitropism by providing a mechanism to regulate the proliferation of a gravitropic signal originating from the cap to allow the root to attain its correct orientation or set point angle.Plants respond to an array of environmental and developmental stimuli with gravity being one of the more significant cues to which plants must adapt to survive. Under the Earth's gravitational field, shoots normally grow upward to maximize light absorption for photosynthesis, whereas roots grow down for optimal water and nutrient acquisition. This directional growth response of plant organs to gravity or gravitropism has been conveniently divided into a series of events consisting of gravity sensing, signal transduction, signal transmission, and the growth response (Kiss, 2000). In roots, gravity sensing is believed to occur in the root cap (Sack, 1997), but recent evidence suggests that roots may have dual gravity sensors, one of which may be located in a region outside the cap (Wolverton et al., 2002a(Wolverton et al., , 2002b.Despite recent proposals of alternative gravitysensing sites in roots (Wolverton et al., 2002a), there is a great deal of cell biological and physiological evidence demonstrating that starch-containing amyloplasts in the columella region of the root cap are significant for gravity sensing (for review, see Kiss, 2000;Boonsirichai et al., 2002). Popularly known as the starch-statolith hypothesis, the sedimentation of amyloplasts within the columella cells is proposed to constitute one of the initial acts of gravity sensing in roots (Sack, 1997;Kiss, 2000). In shoots, sedi...

Amyloplasts and Vacuolar Membrane Dynamics in the Living Graviperceptive Cell of the Arabidopsis Inflorescence Stem

“…The actin cytoskeleton has been proposed as a major player in plant gravitropism (Yoder et al, 2001;Blancaflor, 2002;Hou et al, 2003). Recent studies, however, showed that an intact actin cytoskeleton is not required for gravity perception in inflorescence stems and roots (Yamamoto and Kiss, 2002;Hou et al, 2003) and likely acts to downregulate gravitropism by continuously resetting the gravitropic-signaling system (Hou et al, 2004). The former finding was confirmed in this study, and in addition, the movement of amyloplasts was shown to correlate well with gravity-perceptive ability.…”

“…Findings from the actin disruption experiment suggest that the saltatory movement of amyloplasts through transvacuolar strands is dependent upon the actin cytoskeleton. The actin cytoskeleton has been proposed as a major player in plant gravitropism (Yoder et al, 2001;Blancaflor, 2002;Hou et al, 2003). Recent studies, however, showed that an intact actin cytoskeleton is not required for gravity perception in inflorescence stems and roots (Yamamoto and Kiss, 2002;Hou et al, 2003) and likely acts to downregulate gravitropism by continuously resetting the gravitropic-signaling system (Hou et al, 2004).…”

Role of the Cell Wall-Sustaining System in Gravity Resistance in Plants

“…In signal perception in gravitropism, amyloplast sedimentation may induce tensional changes within the microfilament network, leading to the activation of the downstream signaling cascades responsible for final curvature (Sievers et al, 1991). It has been reported that disruption of microfilaments promotes gravitropic curvature in shoot and root organs (Yamamoto and Kiss, 2002;Hou et al, 2003). The results of our preliminary experiments suggest that microfilaments also play a role in gravity resistance (Fig.…”

Brassinosteroids Stimulate Plant Tropisms through Modulation of Polar Auxin Transport in Brassica and Arabidopsis

“…Seedlings were reoriented by 908, cultured under unilateral (top only) light, and observed with a stereoscope (SMZ 800; Nikon) every 60 min. Seedling lengths and angles throughout the tropistic stimulation were measured from digitized images using MetaMorph 5.0 software (as described previously by Hou et al [2003]). Experiments were repeated three times (n > 60) and statistically analyzed.…”