Altered expression of acetylcholinesterase gene in rice results in enhancement or suppression of shoot gravitropism

Abstract

Acetylcholinesterase (AChE), an acetylcholine-hydrolyzing enzyme, exists widely in plants, although its role in plant signal transduction is still unclear. We have hypothesized that the plant AChE regulates asymmetric distribution of hormones and substrates due to gravity stimulus, based on indirect pharmacological experiments using an AChE inhibitor. As a direct evidence for this hypothesis, our recent study has shown that AChE overexpression causes an enhanced gravitropic response in rice seedlings and suggested that the function of the rice AChE relates to the promotion of shoot gravitropism in the seedlings. Here, we report that AChE suppression inhibited shoot gravitropism in rice seedlings, as supportive evidence demonstrating the role of AChE as a positive regulator of shoot gravitropic response in plants.Abbreviations: ACh, Acetylcholine; ASCh, Acetylthiocholine; AChE, Acetylcholinesterase; DTNB, 5,5-dithiobis-2-nitrobenzoic acid; UBQ, Ubiquitin KEYWORDS Acetylcholine; acetylcholinesterase; ACh; AChE; gravitropism; rice (Oryza sativa)Acetylcholine (ACh) is well-known as the neurotransmitter in the mammalian central and peripheral cholinergic nervous systems.1 ACh has been also identified in a number of non-neuronal animal tissues and in fungi, bacteria, and plants.2,3 ACh may play a universal role as an elementary molecular agent of irritability in animal, plant and microbial cells.4 Although plants lack a nervous system, both ACh and AChE have been widely recognized in plants. 5 The function of the plant ACh and AChE is still unclear, despite more than 30 years of research. To understand those functions, we have been studying about physiological roles of plant AChE under gravitropic stimulus. Indeed plants under gravitropic stimulus showed asymmetric distribution of hormones and AChE activity. 6,7 When a plant is moved from the vertical orientation to a horizontal position, the stem shows curvature as it returns to a vertical position by response acting against gravistimulus. On the other hands, maize seedlings that were treated first with neostigmine bromide (AChE inhibitor) did not respond to the gravistimulus.7 Furthermore, after the stimulus, the AChE activity was distributed asymmetrically at the interface between the stele and the cortex of the gravistimulated maize seedlings. 6 The gravitropic bending occurs because of an asymmetric distribution of the growth hormone auxin (indole-3-acetic acid, IAA). 8 We previously observed that gravistimulation leads to an asymmetric distribution of both IAA-inositol synthase and the AChE activities in maize and rice shoots. 9,10 In addition, its asymmetric distribution of both enzyme activities in maize and rice shoots was also inhibited by the treatment with neostigmine bromide.9,10 The neostigmine bromide is an AChE inhibitor having no inhibitory effect on IAA metabolism in plants. 11 These findings suggest that AChE would play an important role in the gravitropic response of plants. To directly confirm our previous reports, the functionality of pla...

New insights on neurotransmitters signaling mechanisms in plants

“…The signaling effects of ACh hydrolyzing enzyme acetylcholine esterase (AChE] have been investigated in response to rice shoot gravitropic response. 77 AChE has been stated to be a positive regulator of shoot gravitropism in rice seedlings. Gravity sensing in rice and maize seedlings is accompanied by asymmetric distribution of AChE activity in the regions of stele and cortex.…”