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Pectic Polysaccharide Breakdown of Cell Walls in Cucumber Roots Grown with Calcium Starvation ¹

Division of Biochemistry, Institute for Agricultural and Biological Sciences, Okayama University, Kurashiki-shi, Okayama 710, Japan, Division of Plant Physiology, Institute for Agricultural and Biological Sciences, Okayama University, Kurashiki-shi, Okayama 710, Japan

Pectic polysaccharides from the roots of cucumber (Cucumis sativus L.) grown in liquid culture medium with or without calcium (1 mM CaCl₂) were studied after extraction successively by hot water and Na hexametaphosphate solution. The Ca²⁺ starvation-treatment caused a striking reduction in content of extracted pectic polysaccharide; from an equivalent weight of cell walls, only 33.1% of the control level was extracted from root cell walls of plants cultured under Ca²⁺ deficiency. The extracted pectic polysaccharides were fractionated into neutral and acidic polymers by a DEAE-Sephadex column. The acidic polymers, which represented more than 76% of the yield, appeared to be a major fraction of extracted pectic polysaccharides. The changes of molecular size and glycosyl residue composition of this fraction were compared for the control and Ca²⁺-deprived samples. The results indicate that Ca²⁺ deficiency caused structural changes which could involve both branching pattern and extent of contiguous galacturonosyl units in the water-solubilized pectic polysaccharides. Ca²⁺ starvation also led to a notable decrease in molecular size of the hexametaphosphate-solubilized polysaccharides and, to a lesser extent, of the water-solubilized fraction as well. In addition, polygalacturonase activity in tissue homogenates increased remarkably with the Ca²⁺ deficiency, whereas -galactosidase activity did not undergo a change. Thus, it appears that one major effect of Ca²⁺ deprivation was to stimulate polygalacturonase activity, an effect which could be involved in the control of the breakdown of pectic polysaccharides in the cell walls.