Carbon Source Dependence and Photostimulation of Conidiation in Hypocrea atroviridis

Abstract

Hypocrea atroviridis is frequently used as a photomorphogenetic model due to its ability to conidiate upon exposure to light. Light is thereby believed to be the primary trigger for spore formation. In contrast, we show here that conidiation is primarily carbon source dependent and that illumination plays a catalytic role; of a total of 95 tested carbon sources, only a small set of carbohydrates, polyols, and sugar acids allowed conidiation in darkness, and on most of them, conidiation was significantly more strongly expressed in light. In addition, there are also a number of carbon sources on which H. atroviridis conidiates in darkness, but light does not further stimulate the process. Yet on another small set of carbon sources (L-sorbitol, D-fucose, D-and L-arabinose, and erythritol), H. atroviridis shows better sporulation in darkness than in light. No sporulation was observed on organic acids and amino acids. Mutants with deletions in the two blue-light receptor proteins BLR-1 and BLR-2 generally showed weaker conidiation on a smaller number of carbon sources than did the parental strain, yet they clearly sporulated on 15 and 27 of the 95 carbon sources tested, respectively. Of the carbon sources supporting sporulation, only 11 supported the conidiation of both mutants, suggesting that the BLR-1 and BLR-2 receptors are variously involved in the carbon source-dependent regulation of spore formation. The addition of cyclic AMP, which has been reported to lead to conidiation in darkness, both positively and negatively affected sporulation and resulted in different effects in the parental strain and the two ⌬blr mutants. Our data show that the carbon source is the prime determinant for conidiation and that it influences the organism's regulation of conidiation by means of BLR-1 and BLR-2 and their cross talk with cyclic AMP.

Reproduction without sex: conidiation in the filamentous fungus Trichoderma

“…Conidiation rates varied depending on the carbon source, and the level of variation in dark-grown cultures was comparable with that observed in response to a single light exposure. In contrast, conidiation is not observed in T. atroviride when amino acids or alcohols are the sole carbon source, and although light-induced conidiation correlates with conidiation in the dark, light appears to inhibit conidiation on D-arabinose and Dgluconic acid (Friedl et al, 2008). The T. viride study employed a single light exposure to dark-grown cultures to evaluate light-induced conidiation, whereas alternating light/dark conditions were used to evaluate the influence of light on conidiation in T. atroviride.…”

“…In T. atroviride dark-grown cultures, sudden glucose deprivation triggers a conidial ring at the perimeter of the colony, whereas this conidiation response is impaired in Dblr-1 and Dblr-2 mutants, suggesting a light-independent role for BLR-1/BLR-2 in carbon deprivation-induced conidiation Friedl et al, 2008). Photoconidiation is also carbon source-dependent, since carbon sources that do not allow carbon deprivation-induced conidiation in the dark also do not allow photoconidiation (Friedl et al, 2008).…”

“…From the literature, it is clear that the carbon and nitrogen status and the C : N ratio, in addition to the ambient pH, are the main nutritional factors influencing conidiation in Trichoderma (Aube & Gagnon, 1969;Bastos, 2001;Brian & Hemming, 1950;Friedl et al, 2008;Gao et al, 2007;Jackson et al, 1991;Kredics et al, 2004;Lewis & Papavizas, 1983;Monga, 2001;Steyaert et al, 2010b, c).…”

“…The source of carbon has been suggested to be the primary determinant for both conidiation in the dark and light-induced conidiation in T. viride and T. atroviride (Chovanec et al, 2001;Friedl et al, 2008). Chovanec et al (2001) observed conidia in T. viride cultures grown on 30 out of 32 carbon sources, including polysaccharides, amino acids and alcohols.…”

“…Photoconidiation is also carbon source-dependent, since carbon sources that do not allow carbon deprivation-induced conidiation in the dark also do not allow photoconidiation (Friedl et al, 2008). It is possible that BLR binds to one or more ligands, responding to the availability of specific carbon sources, or that BLR responds to the intracellular balance between NAD + /NADP + and NADH/NADPH (the redox state) .…”