Academic journal article Genetics

Regulation by Blue Light of the Fluffy Gene Encoding a Major Regulator of Conidiation in Neurospora Crassa

Academic journal article Genetics

Regulation by Blue Light of the Fluffy Gene Encoding a Major Regulator of Conidiation in Neurospora Crassa

Article excerpt

ABSTRACT

The development of asexual spores, that is, the process of conidiation, in the fungus Neurospora crassa is increased by light. The fluffy (fl) gene, encoding a major regulator of conidiation, is activated by light. We describe here a detailed characterization of the regulation by blue light of fl in vegetative hyphae. This induction requires the white collar complex (WCC) while the FLD protein acts as a dark repressor of fl transcription. We show that the WCC directly regulates fl transcription in response to blue light after transiently binding the promoter. We propose that fl is repressed by FLD in vegetative mycelia and that the repression is lost after light exposure and WCC activation. The increase in fl mRNA in vegetative mycelia after light exposure, and the corresponding increase in the amount of the regulatory FL protein, should promote the activation of the conidiation pathway. The activation by light of fl provides a simple mechanism for the activation of conidiation by blue light in Neurospora that may be at work in other fungi.

A SEXUAL sporulation in the fungus Neurospora crassa leads to the development of asexual spores called macroconidia. Other types of conidia, microconidia and arthroconidia, are produced by Neurospora (Springer 1993), but macroconidia are most abundant, and we will focus on the regulation of macroconidiation, or conidiation, from here on. Conidiation is induced by several environmental signals, including desiccation, carbon and nitrogen starvation, and exposure to blue light (Springer 1993; Davis 2000). In addition, conidiation is controlled by an endogenous circadian clock (Dunlap and Loros 2004; Tan et al. 2004; Heintzen and Liu 2007; Brunner and Káldi 2008). During conidial development, vegetative hyphae grow away from the substrate to form amass of aerial hyphae. About 4 hr after conidial induction, hyphal growth changes from apical elongation to apical budding, leading to the formation of chains of proconidia divided by minor constrictions. Budding continues in proconidial chains, and major constrictions appear ~8 hr after the induction of conidiation. Interconidial junctions are cleaved several hours later, but conidia are held together by fragile connective threads until they are dispersed by wind currents (Springer 1993).

Several genes are required for conidiation. Strains with mutations in aconidiate-2 (acon-2) or fluffyoid (fld) are blocked in the transition from filamentous to budding growth. Mutations in aconidiate-3 (acon-3) or fluffy (fl) allow the production of minor, but not major, constriction chains. Mutations in two conidial separation genes (csp-1 and csp-2) prevent the separation of cross walls to release free conidia (Springer 1993). Two developmental genes,fl(NCU08726)andcsp-1 (NCU02713), have been identified, and the corresponding proteins are putative zinc-finger transcription factors (Bailey and Ebbole 1998; Lambreghts et al. 2009).

The fl gene has been investigated in some detail. The FL protein is a 792-amino-acid polypeptide containing a Zn2Cys6 binuclear zinc cluster domain belonging to the Gal4p family (Bailey and Ebbole 1998). A mutation in fl blocks conidiation at the minor constriction stage, ~4 hr after induction (Springer and Yanofsky 1989), and fl mRNA is observed 6 hr after the initiation of conidiation when major constrictions appear in proconidial chains (Bailey and Ebbole 1998). The presence of fl mRNA in aerial hyphae where conidiation-specific genes are expressed suggests a major role for FL in conidiation and in conidial-specific gene expression (Bailey-Shrode and Ebbole 2004). However, fl is transiently induced 30 min after the induction of conidiation, suggesting an additional role for FL in the formation of aerial hyphae (Correa and Bell- Pedersen 2002). The relevance of FL as a major regulator of conidiation in Neurospora is supported by the observation of conidial development when fl expression is forced in vegetative hyphae (Bailey-Shrode and Ebbole 2004), a condition that leads to the expression of eas (Bailey-Shrode and Ebbole 2004), the gene for the hydrophobin rodlet protein that is located on the surface of matured conidia (Bell-Pedersen et al. …

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