ABSTRACT
A Ca2+ channel from root-tip endomembranes of garden cress (Lepidium sativum L.) (LCC1) was characterized using the planar lipid-bilayer technique. Investigation of single-channel recordings revealed that LCC1 is voltage gated and strongly rectifying. In symmetrical 50 mM CaCl2 solutions, the single-channel conductance was 24 picosiemens. LCC1 showed a moderate selectivity for Ca2+ over K+ (9.4:1) and was permeable for a range of divalent cations (Ca2+, Ba2+, and Sr2+). In contrast to Bryonia dioica Ca2+ channel 1, a Ca2+-selective channel from the endoplasmic reticulum of touch-sensitive tendrils, LCC1 showed no bursting channel activity and had a low open probability and mean open time (2.83 ms at 50 mV). Inhibitor studies demonstrated that LCC1 is blocked by micromolar concentrations of erythrosin B (inhibitor concentration for 50% inhibition [IC50] = 1. 8 µM) and the trivalent cations La3+ (IC50 = 5 µM) and Gd3+ (IC50 = 10 µM), whereas verapamil showed no blocking effect. LCC1 may play an important role in the regulation of the cytoplasmic free Ca2+ concentration in root-tip and/or root-cap cells. The question of whether this ion channel is part of the gravitropic signal transduction pathway deserves further investigation.
ABSTRACT
The reduction of 12-oxophytodienoic acid (OPDA) to 3-oxo-2(2'[Z]-pentenyl)-cyclopentane-1-octanoic acid is catalyzed by 12-oxophytodienoate-10,11-reductase (OPR). Analysis of the isomer preference of OPR has indicated that the activity is composed of two isoenzymes exhibiting different stereoselectivities. The two isoforms of OPR have been separated, using protein extracts of Rock Harlequin (Corydalis sempervirens) as the starting material. OPRI, the enzyme reported earlier from the same species and corresponding to the cloned OPR from Arabidopsis, utilized 9R,13R-OPDA >> 9S, 13R-OPDA but not the 13S-configured isomers, whereas the new activity, OPRII, effectively reduced all four OPDA isomers, including the natural 9S,13S-OPDA (cis-[+]-OPDA). OPRII activity is characterized in detail. The enzyme's enzymatic, biochemical, and immunological properties prove that it is a close relative of OPRI. The roles of OPRI and OPRII in octadecanoid biology are discussed.