NsdC and NsdD Affect Aspergillus flavus Morphogenesis and Aflatoxin Production

Abstract

The transcription factors NsdC and NsdD are required for sexual development in Aspergillus nidulans. We now show these proteins also play a role in asexual development in the agriculturally important aflatoxin (AF)-producing fungus Aspergillus flavus. We found that both NsdC and NsdD are required for production of asexual sclerotia, normal aflatoxin biosynthesis, and conidiophore development. Conidiophores in nsdC and nsdD deletion mutants had shortened stipes and altered conidial heads compared to those of wild-type A. flavus. Our results suggest that NsdC and NsdD regulate transcription of genes required for early processes in conidiophore development preceding conidium formation. As the cultures aged, the ⌬nsdC and ⌬nsdD mutants produced a dark pigment that was not observed in the wild type. Gene expression data showed that although AflR is expressed at normal levels, a number of aflatoxin biosynthesis genes are expressed at reduced levels in both nsd mutants. Expression of aflD, aflM, and aflP was greatly reduced in nsdC mutants, and neither aflatoxin nor the proteins for these genes could be detected. Our results support previous studies showing that there is a strong association between conidiophore and sclerotium development and aflatoxin production in A. flavus.

NsdD Is a Key Repressor of Asexual Development inAspergillus nidulans

“…However, previous studies also have suggested that NsdD might be a potential repressor of conidiation (Han et al 2001;Cary et al 2012). For instance, overexpression of nsdD resulted in the near total absence of conidiation with formation of elongated aerial hyphae.…”

“…However, as enhanced expression of nsdD also resulted in elevated sexual development even under unfavorable conditions for sexual fruiting, it has been hypothesized that NsdD primarily functions in positively regulating sexual development rather than repressing conidiation. Additional studies in A. flavus demonstrated that the removal of nsdD resulted in elevated expression of brlA (Cary et al 2012). They found that both NsdC [another sexual activator and NsdD are required for production of asexual sclerotia, normal aflatoxin biosynthesis, and conidiophore development.…”

Identification of the Anti-Aflatoxinogenic Activity of Micromeria graeca and Elucidation of Its Molecular Mechanism in Aspergillus flavus

“…Similar to VeA, NsdC modulation could participate to the morphological modifications observed in hyssop-treated cultures (Figure 3). It has also been linked to aflatoxin cluster-gene expression [51] as well as being a global regulator of secondary metabolism [52]. More data is yet to be collected on the individual and possibly collaborative roles of both of these factors in the regulation of secondary metabolism.…”

<i>Aspergillus flavus</i> Secondary Metabolites: More than Just Aflatoxins

“…Such experiments include a bottom-up approach where specific genes are targeted for disruption or ectopic expression and the resulting effect on transcriptional regulation or biological processes is determined [26][27][28][29] . Other common approaches include the use of chemical treatment, environmental conditioning, host-pathogen interaction or other stimuli relevant to the investigation to identify novel genes, proteins, interactions, expression profiles, and entire processes responding to the stimuli [30][31][32][33] .…”

“…Another global regulatory gene, nsdC, mainly known for its role in development in A. nidulans 91) , was also shown to be required for AF production in A. flavus 27) . In addition to loss of AF biosynthesis, A. flavus ΔnsdC mutants also demonstrated reduced aflatrem content compared to a control strain.…”