Original article Acetylcholinesterase activity in Lycopersicon esculentum and its phytochrome mutants Justyna Wis ´niewska *, Andrzej Tretyn Department of Biotechnology, Institute of General and Molecular Biology, Nicolaus Copernicus University, Gagarina 9, Torun ´ 87-100, Poland Received 13 December 2002; accepted 10 March 2003 Abstract Using the radiometric method, the activity of acetylcholinesterase (AChE, E.C. 3.1.1.7) was studied in seedlings of wild type (WT) and of phytochrome mutants of tomato (Lycopersicon esculentum Mill.). The activity of this enzyme was inhibited by an excess of substrate and by two well-known inhibitors of animal AChE, eserine and neostigmine. The activity of AChE was found in all etiolated organs as well as in light-grown seedlings. Under both conditions, the highest level of the enzyme activity was detected in cotyledons and the lowest one in root tissue. The enzyme activity was phytochrome-controlled. In WT etiolated seedlings red (R) light decreased AChE activity, whereas far red (FR) light abolished the red light effect. Furthermore, in light-grown WT seedlings the level of the enzyme activity was about twice higher than in etiolated plants. However, in the aurea phytochrome mutant of tomato, deficient in biosynthesis of a phytochrome chromophore, light had no effect on the AChE activity. In case of hp, fri and tri mutant seedlings, R and FR affected the AChE activity in a different way. Based on our results, we suggest that the type I of phytochrome is involved in the regulation of AChE activity. The type II of this photoreceptor influences the rate of the AChE synthesis de novo. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Acetylcholine; Acetylcholinesterase; Lycopersicon esculentum; Phytochrome; Photomorphogenic mutants 1. Introduction The presence of acetylcholine (ACh), an animal neu- rotransmitter and a member of the so-called cholinergic sys- tem, has been found in tissues of many species of plants [6,8,31,34,35,40]. In these organisms, the activity of choline acetyltransferase (ChAT, E.C. 2.3.1.6) [2,31,35] and acetyl- cholinesterase (AChE, E.C. 3.1.1.7) [4,14,18], enzymes that take part in the synthesis and degradation of ACh, have also been described. There is also evidence indicating a presence of ACh receptors (AChR) in plant cells, whose mechanism of action seems to be similar to that of AChR in animal cells [17,36]. It is postulated that plants possess a cholinergic system similar to that functioning in animal tissues [36]. In spite of more than 30 years of investigation, the mecha- nism of action of the plant cholinergic system still remains unclear [35]. The best-known phenomenon is the effect of red (R) and far red (FR) light on ACh levels in etiolated plants tissues. It has been shown that R and FR, absorbed by the photomorphogenic pigment phytochrome, increase or de- crease ACh content in plants, respectively. It is postulated that phytochrome can regulate the activity of enzymes that take part in the synthesis and degradation of ACh [8,15,40]. Until now there has been only a limited number of papers concerning the properties of plant ChAT and AChE and the regulation of their activity by light [6,35]. In the present paper, we describe the properties of AChE from tomato seedlings. We also show that the activity of this enzyme is under phytochrome control. Using different phytochrome mutants, we have been able to show which type of this photoreceptor is responsible for the regulation of AChE activity. 2. Results The influence of substrate concentration on the AChE activity is shown in Fig. 1. ACh was used at different concen- trations, ranging from 0.1 to 6 mM. At lower concentrations Abbreviations: ACh, acetylcholine; AChE, acetylcholinesterase; au, aurea; ChAT, choline acetyltransferase; FR, far red light; fri, far red light insensitive; HIR, far red light insensitive; hp–1, high pigment–1; LFR, low fluence response; phyA, type I of phytochrome; phyB1; B2; E; F, type II of tomato phytochromes; R, red light; tri, temporarily red light insensitive. * Corresponding author. E-mail address: jwisniew@biol.uni.torun.pl (J. Wis ´niewska). Plant Physiology and Biochemistry 41 (2003) 711–717 www.elsevier.com/locate/plaphy © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S0981-9428(03)00111-6