ROOTGRAVITROPISM: An Experimental Tool to Investigate Basic Cellular and Molecular Processes Underlying Mechanosensing and Signal Transmission in Plants

Abstract

The ability of plant organs to use gravity as a guide for growth, named gravitropism, has been recognized for over two centuries. This growth response to the environment contributes significantly to the upward growth of shoots and the downward growth of roots commonly observed throughout the plant kingdom. Root gravitropism has received a great deal of attention because there is a physical separation between the primary site for gravity sensing, located in the root cap, and the site of differential growth response, located in the elongation zones (EZs). Hence, this system allows identification and characterization of different phases of gravitropism, including gravity perception, signal transduction, signal transmission, and curvature response. Recent studies support some aspects of an old model for gravity sensing, which postulates that root-cap columellar amyloplasts constitute the susceptors for gravity perception. Such studies have also allowed the identification of several molecules that appear to function as second messengers in gravity signal transduction and of potential signal transducers. Auxin has been implicated as a probable component of the signal that carries the gravitropic information between the gravity-sensing cap and the gravity-responding EZs. This has allowed the identification and characterization of important molecular processes underlying auxin transport and response in plants. New molecular models can be elaborated to explain how the gravity signal transduction pathway might regulate the polarity of auxin transport in roots. Further studies are required to test these models, as well as to study the molecular mechanisms underlying a poorly characterized phase of gravitropism that is independent of an auxin gradient.

The ARG1-LIKE2 Gene of Arabidopsis Functions in a Gravity Signal Transduction Pathway That Is Genetically Distinct from the PGM Pathway

“…Most plant organs are characterized by a specific gravity set point angle, which defines their preferential growth vector relative to gravity (Firn and Digby, 1997). In young Arabidopsis seedlings, shoots grow upward, displaying negative gravitropism, whereas roots grow downward, toward the center of gravity (positive gravitropism; Bullen et al, 1990;Boonsirichai et al, 2002).Gravity perception by dicot organs involves primarily the sedimentation of amyloplasts within specialized cells (statocytes) located in the columella region of the root cap and in the starch sheath, which constitutes the endodermis of hypocotyls and inflorescence stems (Kiss et al, 1996;Kuznetsov and Hasenstein, 1996;Blancaflor et al, 1998; Weise et al, 2000). In shoots, sedimentable amyloplasts and the curvature response to gravistimulation occur along the elongation zone (for review, see Masson et al, 2002).…”

“…In roots, sites of gravity perception and curvature response may be physically separated (Poff and Martin, 1989). Hence, physiological signals resulting from activation of the gravity signal transduction pathway should be transported from the root cap columella to the elongation zones where the gravitropic curvature is initiated (for review, see Boonsirichai et al, 2002).Auxin is a physiological signal that has been shown to mediate the gravitropic response (for review, see Masson et al, 2002). In gravistimulated roots, auxin is redistributed asymmetrically across the root cap and transmitted to the elongation zones where it promotes a gravitropic curvature.…”

“…Most plant organs are characterized by a specific gravity set point angle, which defines their preferential growth vector relative to gravity (Firn and Digby, 1997). In young Arabidopsis seedlings, shoots grow upward, displaying negative gravitropism, whereas roots grow downward, toward the center of gravity (positive gravitropism; Bullen et al, 1990;Boonsirichai et al, 2002).…”

“…In roots, sites of gravity perception and curvature response may be physically separated (Poff and Martin, 1989). Hence, physiological signals resulting from activation of the gravity signal transduction pathway should be transported from the root cap columella to the elongation zones where the gravitropic curvature is initiated (for review, see Boonsirichai et al, 2002).…”

Interactions between Auxin Transport and the Actin Cytoskeleton in Developmental Polarity ofFucus distichusEmbryos in Response to Light and Gravity

“…In land plants, differential growth in response to unilateral light or the gravity vector is a well-characterized process that requires appropriate distribution of the plant hormone auxin (Muday, 2001;Boonsirichai et al, 2002). Auxin is transported from cell to cell in a polar fashion Friml, 2003).…”