Light stimulation of conidiation in Neurospora crassa: Studies with the wild-type strain and mutants wc-1, wc-2 and acon-2

Blue light influences several events in the life cycle of the ascomycete Neurospora crassa. It has been shown previously that light stimulates asexual sporulation (conidiation) in the double mutant strain, albino-2, band ( al-2 bd) ( E. Klemm and H. Ninnemann, Correlation between absorbance changes and a physiological response induced by blue light in Neurospora, Photochem. Photobiol., 28 (1978) 227–230 ; E. Klemm and H. Ninnemann, Nitrate reductase—a key enzyme in blue light-promoted conidiation and absorbance change in Neurospora, Photochem. Photobiol., 29 (1979) 629–632 ; H. Ninnemann, Photostimulation of conidiation in mutants of Neurospora crassa, J. Photochem. Photobiol. B: Biol., 9 (1991) 189–199 ; H. Ninnemann, Some aspects of blue light research during the last decade, Photochem. Photobiol., 61 (1995) 22–31 ). In this paper, we report that light is also essential for maximal conidiation by the wild-type strain. Consistent with this finding, two mutants which are defective in other light responses, wc-1 and wc-2, conidiate as efficiently as the wild-type strain in the dark, but show no incremental response to light. Light has a profound effect on conidiation in acon-2, a mutant which is temperature sensitive for conidiation. At the permissible temperature, this mutant produces approximately 500-fold more conidia in the light than in the dark. In addition, the double mutants acon-2 wc-1 and acon-2 wc-2 conidiate very poorly, even in the light. Microscopic studies have shown that the majority of aerial hyphae produced by the acon-2 wc-1 and acon-2 wc-2 double mutants at the non-permissible temperature are blocked at the same stage of development as observed for their single mutant parent acon-2, namely at the stage leading to minor constriction chain formation. Northern blot analyses examining the expression of two conidiation-specific genes are consistent with this conclusion.