PHOTORECEPTION AND PHOTOTROPISM IN PHYCOMYCES - ANTAGONISTIC INTERACTIONS BETWEEN FAR-UV, BLUE, AND RED-LIGHT

Phototropism of the sporangiophore of the fungus Phycomyces is mediate d by UV and blue light. Classical phototropism action spectra with max ima near 280, 370 and 450 nm indicate a flavin-like photoreceptor. Blu e light mediates positive phototropism while far-UV light mediates neg ative phototropism. To better understand the mode of interaction of fa r-UV with blue light we performed phototropism experiments in which sp orangiophores were placed for 4 h between sources of 280 and 454 nm li ght coming from opposite directions, The fluence rates of the far-UV w ere chosen such that unilateral light alone elicited 90 degrees of neg ative bending, For blue light, moderate fluence rates were applied tha t elicited about 40 degrees bending, Under conditions of bilateral irr adiation the blue light substantially reduced the far-UV elicited phot otropism. In the presence of tonic red light the antagonism between fa r-UV and blue light was greatly reduced, Red light, which by itself is phototropically ineffective, also reduced phototropic bending elicite d by either far-UV or blue light. These observations are taken as indi cations for the existence of a red light-absorbing intermediate of the blue-light receptor, Because the far-UV/blue-light antagonism disappe ared almost completely in the presence of tonic red light, the antagon ism may occur at the level of this receptor intermediate.