Photoreception and Phototropism in Phycomyces: Antagonistic Interactions between Far-UV, Blue, and Red Light

Abstract— Phototropism of the sporangiophore of the fungus Phycomyces is mediated by UV and blue light. Classical phototropism action spectra with maxima near 280, 370 and 450 nm indicate a flavin-like photoreceptor. Blue light mediates positive phototropism while far-UV light mediates negative phototropism. To better understand the mode of interaction of far-UV with blue light we performed phototropism experiments in which sporangio-phores were placed for 4 h between sources of 280 and 454 nm light coming from opposite directions. The fluence rates of the far-UV were chosen such that unilateral light alone elicited 90° of negative bending. For blue light, moderate fluence rates were applied that elicited about 40° bending. Under conditions of bilateral irradiation the blue light substantially reduced the far-UV elicited phototropism. In the presence of tonic red light the antagonism between far-UV and blue light was greatly reduced. Red light, which by itself is phototropically ineffective, also reduced phototropic bending elicited by either far-UV or blue light. These observations are taken as indications for the existence of a red light-absorbing intermediate of the blue-light receptor. Because the far-UV/ blue-light antagonism disappeared almost completely in the presence of tonic red light, the antagonism may occur at the level of this receptor intermediate.