NsdD Is a Key Repressor of Asexual Development inAspergillus nidulans

Abstract

Asexual development (conidiation) of the filamentous fungus Aspergillus nidulans occurs via balanced activities of multiple positive and negative regulators. For instance, FluG (+) and SfgA (2) govern upstream regulation of the developmental switch, and BrlA (+) and VosA (2) control the progression and completion of conidiation. To identify negative regulators of conidiation downstream of FluG-SfgA, we carried out multicopy genetic screens using sfgA deletion strains. After visually screening .100,000 colonies, we isolated 61 transformants exhibiting reduced conidiation. Responsible genes were identified as AN3152 (nsdD), AN7507, AN2009, AN1652, AN5833, and AN9141. Importantly, nsdD, a key activator of sexual reproduction, was present in 10 independent transformants. Furthermore, deletion, overexpression, and double-mutant analyses of individual genes have led to the conclusion that, of the six genes, only nsdD functions in the FluG-activated conidiation pathway. The deletion of nsdD bypassed the need for fluG and flbA$flbE, but not brlA or abaA, in conidiation, and partially restored production of the mycotoxin sterigmatocystin (ST) in the DfluG, DflbA, and DflbB mutants, suggesting that NsdD is positioned between FLBs and BrlA in A. nidulans. Nullifying nsdD caused formation of conidiophores in liquid submerged cultures, where wild-type strains do not develop. Moreover, the removal of both nsdD and vosA resulted in even more abundant development of conidiophores in liquid submerged cultures and high-level accumulation of brlA messenger (m)RNA even at 16 hr of vegetative growth. Collectively, NsdD is a key negative regulator of conidiation and likely exerts its repressive role via downregulating brlA.T HE filamentous ascomycete Aspergillus nidulans has served as an excellent model system for studying cell biology, asexual development (conidiation), and secondary metabolism (Timberlake 1990;Martinelli 1994;Yu and Keller 2005). The A. nidulans asexual reproductive cycle can be divided into two distinct phases: growth and development. The growth phase involves germination of an asexually derived spore called a conidium and formation of an undifferentiated network of interconnected hyphal cells that form the mycelium. After a certain vegetative growth period, under appropriate conditions, some of the hyphal cells stop normal growth and begin development by forming complex structures called conidiophores that bear multiple chains of conidia (Adams et al. 1988;Park and Yu 2012a).A key and essential step for conidiophore development in Aspergillus is the activation of brlA, which encodes a C 2 H 2 zinc-finger transcription factor (TF) ( Figure 1A) (Adams et al. 1988;Chang and Timberlake 1993). Further genetic and biochemical studies identified the additional key regulators abaA and wetA that function during the middle and late stages of conidiation, respectively ( Figure 1A) (Sewall et al. 1990;Andrianopoulos and Timberlake 1991;Marshall and Timberlake 1991). These three genes have been proposed to define a c...

The GATA‐type IVb zinc‐finger transcription factor SsNsd1 regulates asexual–sexual development and appressoria formation in Sclerotinia sclerotiorum

“…In addition, nearly all late light response genes are misregulated in the loss‐of‐function sub‐1 mutant in a white collar complex‐dependent manner (Chen et al ., ). This role in N. crassa is also consistent with the reported function of the A. nidulans NsdD orthologue as a positive regulator of sexual development and component of the endogenous regulatory circuit including brlA , rosA , nosA and nsdB functioning to balance asexual and sexual development (Han et al ., ; Lee et al ., ). Given the positive roles of Sub‐1 orthologues in light‐responsive sexual development, we hypothesized that loss‐of‐function Δ Ssnsd1 mutants would exhibit phenotypes associated with light‐dependent apothecium development.…”

Homeobox proteins are essential for fungal differentiation and secondary metabolism in Aspergillus nidulans

“…2). Previous genetic analyses have demonstrated that the overexpression of hbxD suppresses fungal growth and asexual development, suggesting that HbxD is required for the proper development 50 . The orthologs of HbxD have been studied in other fungi, such as M. oryzae and C. albicans.…”

Beyond Asexual Development: Modifications in the Gene Expression Profile Caused by the Absence of the Aspergillus nidulans Transcription Factor FlbB

“…A recent report provided experimental evidence of a functional link between the UDA pathway and the positive regulator of sexual development NsdD (Lee et al 2014). Because nsdD deletion bypassed the need of UDAs (but not CDPs) for conidia production, the authors proposed that NsdD also down-regulated brlA expression to inhibit the asexual program.…”

“…Graphs were obtained with Microsoft Excel, and the average pixel intensity for each band was calculated with Image Gauge (version 4.0; Fujifilm, Valhalla, NY). Lee et al (2014) have recently proposed that Flbs induce conidiation by simultaneously suppressing the brlA downregulation caused by NsdD, a positive regulator of cleistothecia production (Han et al 2001). To analyze whether the genetic relationship between Flbs and NsdD could be established through UrdA, we performed new Northern blot experiments.…”