Calcium Antagonist TMB-8 Inhibits Cell Wall Formation and Growth in Pea

The effects on auxin-stimulated growth and cell wall formation of 8-(N, N-diethylamino)-octyl-3, 4, 5-trimethoxybenzoate.HCI (TMB-8), an intracellular Ca2+ antagonist, were investigated in abraded stem segments from aetiolated seedlings of Pisum sativum L. cv. Alaska. Incubation of segments at pH 6.0 with 200 mmol m−3 TMB-8 resulted in a 50% inhibition of auxin-stimulated growth. Added Ca2+ did not restore normal auxin-stimulated growth, presumably because of its well-known stiffening effect on the cell wall. In segments incubated at a pH (7–2) which prevented elongation, auxin promoted the incorporation of [3H]glucose into the cell wall relative to total uptake of label. TMB-8 abolished about 60% of the total incorporation of label into cell walls in the presence of auxin, but was not effective in the absence of auxin. Exogenous CaCl2 reversed the inhibitory effect of TMB-8 on relative cell wall incorporation in a parabolic manner, with a 50% reversal at about 100 mmol m−3 and complete reversal at 1.0 mol m−3 Ca2+. Other ions tested (Mg2+, Mn2+, Cu2+, Zn2+) were without substantial effect at concentrations of 0.5 mol m−3. Both apparent uptake of Ca2+ and consequent reversal of TMB-8 inhibition of cell wall incorporation were blocked by the Ca2+ channel blockers verapamil and La3+. The data provide further evidence that auxin-stimulated growth is dependent upon continued cell wall incorporation, and suggest that a Ca2+ messenger system may be involved in the promotory actions of auxin on cell wall synthesis and long-term growth.