Directed Growth o f Fungal Hyphae in an Electric Field A Biophysical Analysis

Galvanotropism, Dose-Response Curve, Mycelial Growth The galvanotropic response of the mycelial fungus Neurospora crassa is investigated. The angle distribution function of growing hyphae is described by a generating function which contains two non-trivial terms; one for directional growth and one for bidirectional growth. The following results were obtained, (i) Germ tubes grew towards the anode, (ii) The cellular response was linear for small sized cells and weaker electric fields. The galvanotropic constant, Kgrow, which describes the linear response, was small for short germ tubes or hyphae (A^grow 1 = _ 20 V-cm“1 for /0 = 10 |am) and large for longer cells (A^grow1 = _ 1 -7 V cm "1 for 10 = 100 |im). The growth coefficient K P o f (-18 .5 mV) describes the response independent of cell size. The linear response is explained by the field-induced distribution of charged mem­ brane-bound proteins essential for galvanotropism. (iii) For E > E0 3.8 V -cm “1, the linear response is inhibited (inhibition coefficient K x = 1.11). The inhibition is explained by field-in­ duced changes of the membrane potential, (iv) The galvanotropic response o f longer hyphae was bidirectional. The cells grew on average perpendicular to the applied field. The bidirec­ tional response is proportional to Er with a bidirectional growth coefficient K2 o f -(0.25 V)“2. The bidirectional growth is explained by the inhibition of the directed growth process. The transition from anode-directed to bidirectional growth was a function of the applied electric field as well as of the tube length (directed growth for E2-l0< 4 A V2-cm“ ' and bidirectional growth for E?-l{)> 4.4 V2• cm“1).