Calcium localization in Hypomyces-infected squash hypocotyls and effect of calcium on pectate lyase activity and tissue maceration

Abstract

Pectate lyase from squash plants infected with Hypomyces solani f. sp. cucurbitae is dependent upon Ca for activity. As the Ca level is increased in reaction mixtures, the rate of degradation of pectate is increased. However, when the uronic acid/Ca ratio is < 2, enzymatic breakdown of pectate is inhibited. Pectate lyase activity is affected similarly when the pectate substrate is in a semi-solid agar medium. Moreover, maceration of potato tissues is stimulated by a low exogenous supply of Ca but inhibited as the level is increased. Ca is present in higher concentrations in lesions than adjacent healthy tissues, and autoradiography indicated that 45Ca accumulated in cell walls and middle lamellae in infected areas. However, Ca accumulation around lesions apparently does not greatly influence the rate of maceration by pectate lyase.