There is increasing interest in the potential role of cation redistribution in shoot and root gravitropism. This is especially true for calcium since this divalent cation is redistributed rapidly in shoots (1, 5, 13) and roots (3, 6, 9, 10) undergoing gravicurvature. Evidence for gravity-induced calcium redistribution in shoots was provided as early as 1959 when Bode (2) reported redistribution of calcium toward the upper side of gravistimulated sunflower hypocotyls. Interest in the potential role of calcium in gravitropism was renewed in 1983 when Lee et al. (7) showed that applying a calcium chelator to the root cap causes the primary root of maize to become nonresponsive to gravity. They also found that graviresponsiveness could be restored by replacing the chelator with calcium. As further evidence that calcium might be involved in the graviresponse of roots they reported that unilateral application of calcium to caps of vertically oriented roots causes the roots to curve toward the applied calcium. Since this effect could not be mimicked by manganese or magnesium, the response appeared to be specific for calcium. These observations led to the suggestions that calcium plays an important role in gravitropism, and that calmodulin may be involved in transduction of the gravitropic signal toward the site of the gravitropic growth response. To test more thoroughly the specificity of the response, we examined the effects of ion gradients on root curvature using a variety of cultivars and cations. MATERIAL AND METHODS Seeds of maize (Zea mays L., cvs B73 * Missouri 17, Merit, Yellow Dent, Bear Hybrid WF9 * 38MS, and Great Lakes Hybrid 422) were soaked in distilled water and germinated on paper towels wetted with distilled water. The growing conditions were as described previously (11). Seedlings were used when the primary roots were between 2 and 3 cm long (i.e. about 3 d after imbibition).Application of Cations. The cations tested in this study were: Al3 +, Ba2 + Ca2 + Cd2 +, Co2 +, Cu2 +, Fe3 +, Hg2 +, K+, La3 +, Li+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, and Zn2+. Sheets of agar (25 -25 -1.5 mm3, 1.5%, Difco Laboratories) were soaked overnight in solutions of the chloride salts of these cations plus 5 mM Mes/Tris buffer (pH 6). Since solutions of ZnCl2 and AlCl3 are acidic and precipitate at neutral pH, solutions of ZnCl2 and AlCl3 were buffered at pH 5.5 and 4.5, respectively. The agar sheets were cut into cubes (1.5 mm)3 using a cutting device consisting of a series of razor blades mounted 1.5 mm apart. Groups of five seedlings were mounted vertically in Petri dishes (9 x 2 cm) by attaching the grain with caulking compound to the back of the dish. An agar block containing the cation to be tested was applied to one side of the root tip and an agar block containing buffer (5 mM Mes/Tris) at the same pH as the cation-containing block was applied to the opposite side of the root tip. Plain agar blocks or agar blocks of different pH were used as controls. Moist filter papers were placed in the lids of the Petri dish...
Agar blocks that contacted the upper sides of tips of horizontally-oriented roots of Zea mays contain significantly less calcium (Ca) than blocks that contacted the lower sides of such roots. This gravity-induced gradient of Ca forms prior to the onset of gravicurvature, and does not form across tips of vertically-oriented roots or roots of agravitropic mutants. These results indicate that (1) Ca can be collected from mucilage of graviresponding roots, (2) gravity induces a downward movement of endogenous Ca in mucilage overlying the root tip, (3) this gravity-induced gradient of Ca does not form across tips of agravitropic roots, and (4) formation of a Ca gradient is not a consequence of gravicurvature. These results are consistent with gravity-induced movement of Ca being a trigger for subsequent redistribution of growth effectors (e.g. auxin) that induce differential growth and gravicurvature.
Primary roots of the mutant 'Ageotropic' cultivar of Zea mays are nonresponsive to gravity. Their root caps secrete little or no mucilage and touch the root only at the extreme apex. A gap separates the cap and root at the periphery of the cap. Applying mucilage from normal roots or substances with a consistency similar to that of mucilage to tips of mutant roots causes these roots to become strongly graviresponsive. Gravicurvature stops when these substances are removed. Caps of some mutants secrete small amounts of mucilage and are graviresponsive. These results indicate that (a) the lack of graviresponsiveness in the mutant results from disrupting the transport pathway between the cap and root, (b) movement of the growth-modifying signal from the cap to the root occurs via an apoplastic pathway, and (c) gravity (18). This indicates that the lesion for gravitropism is downstream from gravity-detection, perhaps in transmitting the signal from the cap to the root. We studied this by examining the nature of the cap/root interface of mutant roots. MATERIALS AND METHODSAlmost all models for root gravitropism involve a similar sequence of events. Detection of gravity occurs in the root cap and probably involves sedimentation of amyloplasts in columella cells (2,5,15). This change in cellular structure somehow generates a chemical and/or electrical signal in the cap (2,4,5). This signal moves from the root cap to the elongating zone of the root, which is 2 to 6 mm behind the cap. There, the signal preferentially inhibits cellular elongation along the lower side of the root, thus causing the root to curve down (4,5,15).Although we cannot identify the gravitropic signal, much evidence indicates that it moves from the cap to the root before gravicurvature. For example, decapped roots are nonresponsive to gravity (7,23,24), and removing half of the cap of vertically oriented roots induces curvature toward the remaining half-cap (6). Inserting barriers into one side of a root between the root cap and elongating zone causes curvature away from the barrier (26). Taken together, these results suggest that root gravitropism involves movement of a growth-modifying signal from the root cap to the root.The objective of this research was to use mutants to characterize the pathway by which the gravitropic signal moves from the root cap to the root in primary roots of Zea mays.'
We launched imbibed seeds and seedlings of Zea mays into outer space aboard the space shuttle Columbia to determine the influence of microgravity on 1) root-cap regeneration, and 2) the distribution of amyloplasts and endoplasmic reticulum (ER) in the putative statocytes (i.e., columella cells) of roots. Decapped roots grown on Earth completely regenerated their caps within 4.8 days after decapping, while those grown in microgravity did not regenerate caps. In Earth-grown seedlings, the ER was localized primarily along the periphery of columella cells, and amyloplasts sedimented in response to gravity to the lower sides of the cells. Seeds germinated on Earth and subsequently launched into outer space had a distribution of ER in columella cells similar to that of Earth-grown controls, but amyloplasts were distributed throughout the cells. Seeds germinated in outer space were characterized by the presence of spherical and ellipsoidal masses of ER and randomly distributed amyloplasts in their columella cells. These results indicate that 1) gravity is necessary for regeneration of the root cap, 2) columella cells can maintain their characteristic distribution of ER in microgravity only if they are exposed previously to gravity, and 3) gravity is necessary to distribute the ER in columella cells of this cultivar of Z. mays.
The patterns and rates of organelle redistribution in columella (i.e., putative statocyte) cells of agravitropic agt mutants of Zea mays are not significantly different from those of columella cells in graviresponsive roots. Graviresponsive roots of Z. mays are characterized by a strongly polar movement of 45Ca2+ across the root tip from the upper to the lower side. Horizontally-oriented roots of agt mutants exhibit only a minimal polar transport of 45Ca2+. Exogenously-induced asymmetries of Ca result in curvature of agt roots toward the Ca source. A similar curvature can be induced by a Ca asymmetry in normally nongraviresponsive (i.e., lateral) roots of Phaseolus vulgaris. Similarly, root curvature can be induced by placing the roots perpendicular to an electric field. This electrotropism increased with 1) currents between 8-35 mA, and 2) time between 1-9 hr when the current is constant. Electrotropism is reduced significantly by treating roots with triiodobenzoic acid (TIBA), an inhibitor of auxin transport. These results suggest that 1) if graviperception occurs via the sedimentation of amyloplasts in columella cells, then nongraviresponsive roots apparently sense gravity as do graviresponsive roots, 2) exogenously-induced asymmetries of a gravitropic effector (i.e., Ca) can induce curvature of normally nongraviresponsive roots, 3) the gravity-induced downward movement of exogenously-applied 45Ca2+ across tips of graviresponsive roots does not occur in nongraviresponsive roots, 4) placing roots in an electrical field (i.e., one favoring the movement of ions such as Ca2+) induces root curvature, and 5) electrically-induced curvature is apparently dependent on auxin transport. These results are discussed relative to a model to account for the lack of graviresponsiveness by these roots.
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