GUARD-CELL PROTOPLASTS CONTAIN ACETYLCHOLINESTERASE ACTIVITY

Acetylcholinesterase activity has been detected in extracts of guard c ell protoplasts from Vicia faba L. and Nicotiana glauca Graham, Guard cell protoplast homogenates from V. faba exhibited 16.4 and 6.7-fold g reater specific activities for acetylthiocholine hydrolysis compared t o homogenates of mesophyll cell protoplasts or whole leaves, respectiv ely. Extracts of N. glauca guard cell protoplasts also displayed highe st specific activity for acetylthiocholine hydrolysis. Guard cell prot oplast extracts from both species displayed a distinct substrate prefe rence for acetylthiocholine. In contrast, no substrate specificity for choline ester hydrolysis was observed in extracts of mesophyll cell p rotoplasts or whole leaves. In both species, specific reversible inhib itors of mammalian acetylcholinesterase, BW284c51 and neostigmine, inh ibited 40-90% of guard cell protoplast acetylcholinesterase activity. Exogenously applied acetylcholine (1 mM) induced an 80% closure of sto mata in abaxial epidermal peels of V. faba leaves within 5 min, while only a 10-15% stomatal closure was induced by either, butyrylcholine o r propionylcholine. BW284c51, neostigmine and eserine also induced var ying degrees of stomatal closure in epidermal peels of V. faba. Result s from these studies demonstrate that guard cells have acetycholineste rase activity and suggest that acetylcholine might have a physiologica l role in stomatal movement.