AUTHOR=Cascant-Lopez Emma , Crosthwaite Susan K. , Johnson Louise J. , Harrison Richard J. 

TITLE=No Evidence That Homologs of Key Circadian Clock Genes Direct Circadian Programs of Development or mRNA Abundance in Verticillium dahliae

JOURNAL=Frontiers in Microbiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2020.01977

DOI=10.3389/fmicb.2020.01977

ISSN=1664-302X

ABSTRACT=<p>Many organisms harbor circadian clocks that promote their adaptation to the rhythmic environment. While a broad knowledge of the molecular mechanism of circadian clocks has been gained through the fungal model <italic>Neurospora crassa</italic>, little is known about circadian clocks in other fungi. <italic>N. crassa</italic> belongs to the same class as many important plant pathogens including the vascular wilt fungus <italic>Verticillium dahliae</italic>. We identified homologs of <italic>N. crassa</italic> clock proteins in <italic>V. dahliae</italic>, which showed high conservation in key protein domains. However, no evidence for an endogenous, free-running and entrainable rhythm was observed in the daily formation of conidia and microsclerotia. In <italic>N. crassa</italic> the <italic>frequency</italic> (<italic>frq</italic>) gene encodes a central clock protein expressed rhythmically and in response to light. In contrast, expression of <italic>Vdfrq</italic> is not light-regulated. Temporal gene expression profiling over 48 h in constant darkness and temperature revealed no circadian expression of key clock genes. Furthermore, RNA-seq over a 24 h time-course revealed no robust oscillations of clock-associated transcripts in constant darkness. Comparison of gene expression between wild-type <italic>V. dahliae</italic> and a Δ<italic>Vdfrq</italic> mutant showed that genes involved in metabolism, transport and redox processes are mis-regulated in the absence of <italic>Vdfrq</italic>. In addition, <italic>Vd</italic>Δ<italic>frq</italic> mutants display growth defects and reduced pathogenicity in a strain dependent manner. Our data indicate that if a circadian clock exists in Verticillium, it is based on alternative mechanisms such as post-transcriptional interactions of VdFRQ and the WC proteins or the components of a FRQ-less oscillator. Alternatively, it could be that whilst the original functions of the clock proteins have been maintained, in this species the interactions that generate robust rhythmicity have been lost or are only triggered when specific environmental conditions are met. The presence of conserved clock genes in genomes should not be taken as definitive evidence of circadian function.</p>