Starch‐degrading enzymes during the induction of CAM in Mesembryanthemum crystallinum

Abstract

ABSTRACT Mesembryanthemum crystallinum plants were irrigated with 400 mol m−3 NaCl to induce CAM and levels of leaf starch, and activities of starch‐degrading enzymes were measured. During Crassulacean acid metabolism (CAM) induction, daily starch turnover gradually became more pronounced and was three‐ to four‐fold greater than in leaves of C3 plants after 3 weeks. Activities of α‐ and β‐amylase, D‐enzyme and starch phosphorylase all increased 10‐ to 20‐fold within 3 weeks of the start of salt treatment. Activities of α‐ and β‐amylase increased more than fourfold within the first 24 h of salt treatment, which is the fastest increase in enzyme activities so far measured during the induction of CAM with salt solution in intact plants of this species. Most enzyme activities were partially chloroplastic; however, the principal starch‐degrading activity was constituted by an extra‐chloroplastic β‐amylase. CAM starch‐phosphorylase activity, which was mainly chloroplastic, exhibited a two‐ to three‐fold diurnal change in parallel with starch content. CAM induction in M. crystallinum is clearly associated with greater starch turnover and enhanced starch‐degrading enzyme activities, which as catalysts of the initial reaction to release carbon for synthesis of phosphoenolpyruvate (PEP) appear highly significant for the functioning of the CAM pathway. The diurnal rhythm of phosphorylase activity may be of particular significance.