Guard cell protoplasts contain acetylcholinesterase activity

Abstract

Acetylcholinesterase activity has been detected in extracts of guard cell protoplasts from Vicia faba L. and Nicotiana glauca Graham. Guard cell protoplast homogenates from V. faba exhibited 16.4 and 6.7-fold greater specific activities for acetylthiocholine hydrolysis compared to homogenates of mesophyll cell protoplasts or whole leaves, respectively. Extracts of N. glauca guard cell protoplasts also displayed highest specific activity for acetylthiocholine hydrolysis. Guard cell protoplast extracts from both species displayed a distinct substrate preference for acetylthiocholine. In contrast, no substrate specificity for choline ester hydrolysis was observed in extracts of mesophyll cell protoplasts or whole leaves. In both species, specific reversible inhibitors of mammalian acetylcholinesterase, BW284c51 and neostigmine, inhibited 40–90% of guard cell protoplast acetylcholinesterase activity. Exogenously applied acetylcholine (1 mM) induced an 80% closure of stomata in abaxial epidermal peels of V. faba leaves within 5 min, while only a 10–15% stomatal closure was induced by either butyrylcholine or propionylcholine. BW284c51, neostigmine and eserine also induced varying degrees of stomatal closure in epidermal peels of V. faba. Results from these studies demonstrate that guard cells have acetycholinesterase activity and suggest that acetylcholine might have a physiological role in stomatal movement.