The fine structure and mechanism of the large mechanosensory hair of Dionaea muscipula Ellis

Abstract

The fine structure of the hinge cells resembled the fine structure of the medullary cells of the hinge region, and of plant or animal tissue capable of active ion transport. There were abundant mitochondria, vesicles and ER. The hinge cell walls possessed numerous plasmodesmata, whose possible functions were compared with those suggested for the Pacinian cerpuscles, where degree of stimulus is proportional to the area of membrane distorted. Closure of traps is a result of bending the sensory hinge, which caused the release of an action potential. Bending the hair caused swelling of organelles, and the appearance of myelin forms in hinge cells, as shown in EM photographs of the hinge region. These effects were considered on the basis of the LUCY model (1964, 1968) for altered membrane phospholipid orientation. A muscle contracting substance (MCS) was looked for, because acetyloholine has the property of contracting frog rectus muscle, occurs in vesicles in animal synapses as a chemical transmitter of excitation by action potentials, and as well is present in hairs of the plant Urtica. The MCS was indentified as LONG'S (1967) 'B' lysophosphatidic acid, resulting from water-soluble phospholipase D acting on lysolecithin. An action potential is supposed to alter membrane potential and release calcium ions; both would change the activity of phospholipase D. The results following indicated an activated phospholipase from an action potential caused bending the hair, caused increased membrane permeability, and trap closure. Application of chlor-choline-chloride to traps with hairs, suggested activation of lecithin synthesis via activated choline kinase, and reversal of this activation by gibberellic acid. Current hypotheses, such as an ATP use 'mechanoenzyme' membrane complex altering permeability, the water-filled, phospholipid lined hypothesis of Glauert (1968), and the Mitchell hypothesis (1961), were considered in relation to the results. A model is presented of Dionaea trap cell with membranes in dynamic equilibrium, a phospholipase destroying, and a choline kinase complex reconstituting the plasma membrane.