Calcium Requirement for the Induction of Hydrotropism and Enhancement of Calcium-Induced Curvature by Water Stress in Primary Roots of Pea, Pisum sativum L.

Abstract

Positive hydrotropic curvature in the roots of the agravitropic pea (Pisum sativum L.) mutant, ageotropum, occurred when the root cap was exposed to a gradient of water potential by an asymmetric application of agar containing sorbitol [Takano et al. (1995) Planta 197: 410]. As previously reported [Takahashi and Suge (1991) Physiol. Plant. 82: 24], in this study the hydrotropic response due to unilateral application of sorbitol to the root cap was totally inhibited by pretreatment with ethyleneglycol-bis-(beta-amino-ethylether) N,N,N',N'-tetraacetic acid (EGTA). However, hydrotropic response of the EGTA-treated roots was recovered only when EGTA was replaced by a 10 mM calcium (CaCl2) solution prior to hydrostimulation. A calcium channel blocker, lanthanum (LaCl3), also inhibited hydrotropic curvature of ageotropum roots, whereas the hydrotropic response was affected by neither nifedipine nor verapamil. Application of calcium ionophore, A23187, resulted in a significant promotion of hydrotropic curvature. Furthermore, ageotropum roots curved away from a calcium source when an agar block containing 10 mM calcium was asymmetrically applied to the root cap. This calcium-induced curvature was found to be accelerated by water stress and significantly inhibited by LaCl3. While the calcium-induced curvature commenced within 1 h after application, hydrotropic curvature became visible 3 to 4 h after an exposure to a gradient of water potential. These results indicate that apoplastic calcium and its influx through the plasmamembrane are involved in the induction of hydrotropism in roots. A gradient of water potential in the root cap may cause a physiological change that is mediated by calcium, which ultimately leads to the curvature in the elongation region associated with the hydrotropic response.