Neurospora illuminates fungal photoreception

Abstract

Light not only is indispensable as an energy source for life on earth but also serves as an essential environmental cue conveying the information of daily and seasonal time to organisms across different kingdoms. Although the molecular mechanisms underlying light responses are actively explored in various light-sensitive organisms, these studies are either hindered by the complexity of the systems or an incomplete familiarity with the light signaling components involved in the scheme. Therefore, study of a simple and well-characterized model system is desirable to expand our knowledge of basic properties underlying the regulation of biological light responses. This review will briefly introduce the basic light sensing machinery in Neurospora crassa, a filamentous fungus, and then focus on the most recent advances in employing Neurospora as a model to study light signaling cascades, photoadaptation, and circadian clock-modulated effects in eukaryotic cells. Also, we will summarize the functions of a number of putative photoreceptors in Neurospora, and discuss the implications of the study of Neurospora to the field of fungal photobiology and some challenges for future studies.

Figure 1. Light responses in Neurospora crassa

In the presence of light, the White-collar complex (WCC), made up of WC-1 and WC-2, transiently binds to the early light-responsive elements (ELREs) in the promoters of early light-responsive genes to activate the first wave of light induced transcription (i.e. early light responses), including the expression of frq, encoding the clock protein FRQ, the photoreceptor VVD, the transcription factor SUB-1, and others. Depending on the specific time-of-day at which light is imposed, FRQ modulates the WCC activity in a circadian manner to determine the amplitude of the light responses. In response to light, VVD protein is highly expressed and accumulates in the nucleus to repress transcriptional activity through a physical interaction with the WCC. Meanwhile, the induction of SUB-1, essential for most late light responses, results in a second wave of light-induced gene expression, in many cases regulated through identified late light response elements (LLRE’s) on their promoters. Several secondary photoreceptors and other proteins might affect light responses in a gene-specific or developmental stage-specific manner, including CRY, PHY-2, NOP-1, and VE-1. Finally, the roles of light-inducible transcription factors, such as CSP-1, SAH-1 and VAD-3, are yet to be determined. Components capable of sensing light directly via a chromophore are shown by the wavelength color they absorbed and marked with dashed lines. Flavin moieties are shown by yellow hexagons.