Increase in cell wall-associated phosphatase activity in cucumber roots during calcium starvation: Binding nature and properties of the phosphatase and cell wall analysis

In Ca^2$-starved cucumber roots, about 23% of phosphatase assayed at pH 9.0 (ALPase) in the crude cell walls was solubilized with either 2 M NaCl or purified endo type polygalacturonase (endo-PG) from yeast culture broth. Coexistence of NaCl and endo-PG had little effect on further release of ALPase, and a small amount of the activity was solubilized from the NaCl-pretreated cell walls by incubation with endo-PG. Ionically bound ALPase, therefore, seemed to be localized in the fraction which was hydrolyzed by endo-PG in the crude cell walls of Ca^2$-starved cucumber roots. In the control roots, however, ALPase was not effectively solubilized by the treatment with endoPG. Ca^2$ starvation reduced the contents of rhamnose, uronic acids and galactose among non-cellulosic sugars in the cell walls, suggesting that the structure of pectic substances, possibly rhamnogalacturonan, is altered during the starvation.Activities of both ionically and covalently bound ALPases greatly increased during Ca^2$ starvation. The increased ALPase in the NaCl-solubilized fraction hydrolyzed most phosphate esters tested, whereas the enzyme from control roots only cleaved nucleoside 2'(3')-monophosphates and p-nitrophenylphosphate. Differences in the properties between both types of roots were also found when the effects of various inhibitors were tested. Profiles of ALPase-isozymes after polyacrylamide gel electrophoresis were also altered by Ca^2$ starvation.