Different Regulation of β-amylase and Starch Phosphorylase by Light in Dormant and Non-Dormant Turions of Spirodela polyrhiza

Abstract

Summary The influence of single red light pulses and of continuous red light on the activity of two enzymes, i.e. β-amylase (EC 3.2.1.2) and starch phosphorylase (EC 2.4.1.1), was investigated in dormant and non-dormant turions of Spirodela polyrhiza. The activity of β-amylase was induced by continuous red light but not by red light pulses in dormant turions. In non-dormant turions, however, the activity of β-amylase following a single red light pulse was two times higher than after continuous red light. This high effectiveness of a single red light pulse, as compared with continuous red, represents a novel mode of regulation of enzyme activity by light. It is suggested that a low fluence response of phytochrome is abolished by a high irradiance response elicited by continuous red light. The light-induced increase in the activity of β-amylase is due to de novo synthesis of the enzyme as shown by Western blot analysis using polyclonal antibodies raised against β-amylase from Spirodela polyrhiza. The presence of light-activable precursor was excluded by showing that cysteine added to the extraction medium did not influence the activity of β-amylase. In contrast, the activity of starch phosphorylase was induced by continuous red light in dormant turions but decreased in non-dormant turions. During the cold after-ripening treatment turions qualitatively changed their response to light, becoming light-insensitive when partly after-ripened (21 days). In all instances, a red light pulse was ineffective in inducing starch phosphorylase. Isoforms of β-amylase and starch phosphorylase were separated by non-denaturating PAGE. No special induction by light of any specific isoform of β-amylase and starch phosphorylase was found in turions. Since both enzymatic activities and their regulation by light do not correlate with the breakdown of starch, we conclude that they are not directly involved in the regulation of starch degradation in turions.