Regulators of the Asexual Life Cycle of Aspergillus nidulans

The genus Aspergillus, one of the most abundant airborne fungi, is classified into hundreds of species that affect humans, animals, and plants. Among these, Aspergillus nidulans, as a key model organism, has been extensively studied to understand the mechanisms governing growth and development, physiology, and gene regulation in fungi. A. nidulans primarily reproduces by forming millions of asexual spores known as conidia. The asexual life cycle of A. nidulans can be simply divided into growth and asexual development (conidiation). After a certain period of vegetative growth, some vegetative cells (hyphae) develop into specialized asexual structures called conidiophores. Each A. nidulans conidiophore is composed of a foot cell, stalk, vesicle, metulae, phialides, and 12,000 conidia. This vegetative-to-developmental transition requires the activity of various regulators including FLB proteins, BrlA, and AbaA. Asymmetric repetitive mitotic cell division of phialides results in the formation of immature conidia. Subsequent conidial maturation requires multiple regulators such as WetA, VosA, and VelB. Matured conidia maintain cellular integrity and long-term viability against various stresses and desiccation. Under appropriate conditions, the resting conidia germinate and form new colonies, and this process is governed by a myriad of regulators, such as CreA and SocA. To date, a plethora of regulators for each asexual developmental stage have been identified and investigated. This review summarizes our current understanding of the regulators of conidial formation, maturation, dormancy, and germination in A. nidulans.


Introduction
Filamentous fungi are eukaryotic organisms that are ubiquitous in our surrounding environments, and they have large and small effects on human life [1][2][3]. Several fungi, such as Fusarium, Aspergillus, and Penicillium, produce mycotoxins that can cause plant diseases or contaminate stored foods, leading to economic losses [4]. Some fungi can directly exert adverse clinical effects on humans [1]. Conversely, other filamentous fungi have been used as industrial cell factories for producing various proteins because of their efficient secretion and sustainable production systems [5,6]. Therefore, to suppress side effects or maximize positive effects, it is important to understand the biological characteristics of filamentous fungi.
Aspergillus is one of the filamentous fungi that comprise the maximum proportion of airborne organisms. Among the numerous species, Aspergillus flavus is a saprophytic fungus that contaminates preharvest and postharvest crops and produces potent hepatocarcinogenic secondary metabolite aflatoxins; it is known to be the second leading cause of invasive aspergillosis [7,8]. Aspergillus fumigatus is an opportunistic pathogenic fungus that causes life-threatening disease (aspergillosis) in immunocompromised individuals [9,10]. In contrast, Aspergillus niger and Aspergillus oryzae are biochemical cell factories in the fermentation and enzyme industry, which efficiently produce several enzymes and useful secondary

Conidiogenesis
Differentiated vegetative hyphae develop thick-walled foot cells and form conidiophores. An overview of the roles of regulators coordinating the formation of asexual structures and spores in A. nidulans is provided in Figure 2 and Tables 1 and 2.

Conidiogenesis
Differentiated vegetative hyphae develop thick-walled foot cells and form conidiophores. An overview of the roles of regulators coordinating the formation of asexual structures and spores in A. nidulans is provided in Figure 2 and Tables 1 and 2.

Temporal and Spatial Central Regulators of Conidiation
Approximately 50 years ago, the genes brlA, abaA, and wetA were morphologically investigated, which demonstrated that their appropriate temporal and spatial expression results in normal conidiophores formation in A. nidulans [36,37]. Scientists have named them central regulators of conidiation ( Figure 2). BrlA is composed of 432 amino acids having two C 2 H 2 zinc finger domains [38]. It has demonstrated that the brlA null mutant blocksin a conidial formation, exhibiting defective vesicles and loss of pigment [36]. The gene is named brlA (bristle-like) because the shape of the mutant resembles bristles. The brlA is primarily expressed in the early asexual development stage, and BrlA protein controls its own expression and that of various conidiation-related genes by binding to their promoters. The binding site is termed the BrlA-response element (BRE) and the sequence is 5 -(C/A)(G/A)AGGG (G/A)-3 in A. nidulans [39].
One of the target genes directly regulated by BrlA is abaA (abascus). The abaA deletion mutant shows rod-like, aberrant conidiophores and fails to form proper metulae and phialides at intervals in place of chains of conidia [36,37,40]. As with brlA, it is named abaA because of the morphological characteristics of the mutant [41]. During the middle stage of conidiophore development, BrlA activates abaA mRNA expression. Then, AbaA, which has ATTS/TEA DNA binding domain factor and a potential leucine zipper, regulates its own expression and controls brlAα or other developmental genes (such as wetA and vosA) by recognizing the AbaA-response element (ARE) in their promoters [42]. The ARE is 5 -CATTCY-3 , where Y is a pyrimidine.

Conidiogenesis Description Reference(s)
VapA Activation FYVE-like zinc finger protein, one of the VipC-associated protein [71] VapB Activation H3-K9 specific histone methyltransferase, one of the VipC-associated proteins [71] VipC Repression H3-K9 specific histone methyltransferase, one of the Velvet interacting proteins [71] WscA/B Activation Homolog of S. cerevisiae Wsc1 involved in the fungal cell-wall integrity signaling pathway [72] In the late stage of conidiation, wetA (wet-white) is activated by AbaA. The null strain of wetA produces normal conidiophores, but it produces colorless, immature conidia in A. nidulans [36]. The wetA mutant conidia might undergo autolysis and show reduced viability. Moreover, the deletion strains of wetA have permeable conidia, whose wall layers are less condensed than those of the wild-type strain [73]. WetA has a conserved ESC1/WetA-related DNA binding domain that binds to the WetA-response element (WRE), 5 -CCGYTTGCGGC-3 (Y = pyrimidine). The WetA, accumulated in conidia, recognizes WRE in the promoters of spore-specific genes and regulates their expression (wA, yA, vosA, and atfB) [74,75].

Upstream Developmental Activators (UDAs)
Previous studies have demonstrated the importance of UDAs in the development of A. nidulans. In 1994, the Adams group revealed that some developmental mutants formed cotton-like colonies with a "fluffy" morphology [76]. These included a mutant of fluG (fluffy locus A), and flbA~E (fluffy low brlA expression) genes. The fluG is required to produce an extracellular signal (a diorcinol-dehydroaustinol adduct) that initiates programmed asexual sporulation, and the fluG-derived signal can generate sparse conidiation and brlA expression [76]. Even the overexpression of fluG overcomes the developmental block and results in the formation of conidiophores in submerged culture [77]. In addition to brlA, the expressions of other early developmental regulatory genes (flb genes) are affected by FluG. The FluG-mediated developmental regulation is divided into two independent pathways; the activation of the Flb protein-mediated asexual development and the inhibition of FlbA-mediated vegetative growth ( Figure 2).

Flb Protein-Mediated Asexual Development
The transmitted FluG signal activates conidiation by derepressing SfgA-related pathways. SfgA, one of the suppressors of fluG, is a Gal4-type Zn(II) 2 Cys 6 transcription factor. The sfgA null mutant exhibits hyperactive sporulation in liquid-submerged cultures, whereas overexpressed mutant exhibits the inhibition of conidiation. In other words, SfgA plays a role in the repression of asexual development [78]. Moreover, based on genetic analyses, SfgA has been demonstrated as a downstream factor of FluG but an upstream regulator of Flb proteins (flbB, flbC, flbD, and flbE, excluding flbA). As shown in Figure 2, FluG inhibits the expression of SfgA, counteracting the inhibitory effect of SfgA on Flb proteins. Coordinated by FluG and SfgA, four Flb proteins block hyphal growth and timely mediate the asexual development by positively affecting brlA expression [76]. Among these proteins, FlbB is a basic zipper-type transcription factor localized in the nucleus and apical extension (Spitzenkörper). The flbB null mutant exhibits blockage of the synthesis of an extracellular signaling compound, which is expressed in the early phases of vegetative growth for proper conidiation [79]. Further studies report that FlbB cooperates with other UDAs for regulating conidiation. First, FlbB physically interacts with FlbE and colocalizes at hyphal tips. FlbB-FlbE heterocomplex directly binds to the promoter of brlA to express brlA mRNA levels for proper asexual development. Furthermore, this complex can activate transcriptional levels of flbD for proper fungal development. FlbE is expressed throughout the life cycle of A. nidulans and has two conserved but hitherto uncharacterized domains [80]. The deletion and overexpression of flbE cause defective development, cell autolysis, and delayed brlA expression, indicating that the appropriate amount of flbE is crucial for proper fungal growth and asexual development [81]. Second, the FlbB complexes with FlbD, expressed by the FlbB-FlbE heterocomplex, which jointly bind to the brlA promoter for the activation of brlA expression [82]. FlbD is a c-Myb transcription factor, primarily found in the nucleus. Like other flb null strains, flbD absence mutant shows fluffy colonies and aconidial phenotypes, whereas the flbD-overexpressed strain causes inappropriate activation of brlA expression and the production of complex conidiophores [82,83]. Meanwhile, FlbC is not affected by other Flb proteins and acts independently for asexual development. FlbC has two C 2 H 2 zinc fingers and one activation domain. FlbC is localized in the nucleus and binds to the promoters of brlA, abaA, and vosA, regulating their expression levels. Deletion of flbC causes delayed vegetative growth and reduced conidiation. Moreover, the overexpression of flbC results in restricted hyphal growth and reduced cellular activity. These indicate that modulated flbC is essential for balancing growth and development in A. nidulans [76,84].

Inhibition of FlbA-Mediated Stimulation of Vegetative Growth
FlbA encodes 120 amino acids, having an RGS domain for the regulator of G protein signaling, which negatively regulates vegetative growth signaling [85]. Deletion of flbA results in unusual growth and abnormal conidiophores, whereas its overexpression causes hyphal tips to differentiate into spore-producing structures [76,86]. In other words, this protein plays an important role in mycelial proliferation and activation of asexual development. Commonly, RGS domain-containing proteins sense and respond to appropriate physiological and biochemical cues and function as GTPase-activating proteins in conjunction with α subunit of heterotrimeric G-proteins. As regulators of G proteins, they promote GTP hydrolysis of Gα to GDT, inactivating the G proteins ( Figure 2). In A. nidulans, FadA is one of the Gα proteins, and the fadA deletion strain exhibits reduced growth without the impairment of sporulation [87]. In the heterotrimer status affected by FlbA, inactivated FadA (GDP-bound) exists with SfaD (suppressors of the flbA-loss of function mutations, Gβ) and GpgA (G protein gamma A, Gγ) and represses vegetative growth [88]. When GTP is bound to FadA, this protein dissociates from the complex, and the separated FadA activates PkaA, which suppresses brlA levels, contributing to normal vegetative fungal growth and the inhibition of asexual development [89]. The remainders (GpgA and SfaD) separated from the complex also positively regulate hyphal proliferation and repress asexual developmental progression [90,91].

Upstream Regulators of BrlA
For hyphal cells to enter the stage of asexual development, appropriate temporal and spatial control of brlA expression must be applied in A. nidulans. Here, we describe the other upstream transcription factors of BrlA involved in the asexual development of A. nidulans, in addition to the UDAs that regulate brlA expression ( Figure 2). StuA (stunted), studied in 1969, is known as a developmental modifier and is necessary for proper conidiophore formation [36]. StuA is one of the APSES transcription factors required for the transient and spatial regulation of brlA and abaA expression [92]. RgdA, named after retarded growth and development, is a putative APSES transcription factor that is important for the orderly organization of phialide formation [93]. RgdA affects brlA and abaA expression, but it is not influenced by brlA or stuA. RlmA, a S. cerevisiae ortholog of rlm1, is a MADS-box family transcription factor that is involved in proper brlA expression and phialide development, as well as in cell wall remodeling [94]. AslA, implicated in asexual differentiation with low-level conidiation, encodes a C 2 H 2 zinc finger transcription factor that is related to normal asexual development and functions as an upstream activator of brlA [95]. MtfA is denominated from master transcription factor A, which encodes a C 2 H 2 zinc finger domain. This protein has been demonstrated as a key factor for normal brlA expression, asexual development, and the production of several secondary metabolites in A. nidulans [96]. RcoA is a member of the WD repeat proteins that plays important roles in proper vegetative growth, asexual development, and carbon catabolite repression. This transcription factor also affects the expression of brlA, but not the signal transduction of flbA and fluG [97]. RtfA, encoding RNA-pol II transcription factor-like protein, regulates proper branched hyphal growth and conidiophore morphology by the accumulation of brlA transcript. Moreover, this protein affects the biosynthesis of several secondary metabolites, including sterigmatocystin and penicillin [98]. HbxA, a member of homeobox proteins, acts as an activator of asexual development and affects brlA mRNA expression. Deletion of hbxA results in aberrant asexual structures, whereas its overexpression results in enhanced production of conidiophores in liquid-submerged cultures [99].
Although Nsd (never in sexual development) proteins were previously described as transcription factors affecting the activation of asexual development, they are key negative regulators of conidiation. NsdC, with two C 2 H 2 zinc fingers and a C 2 HC motif, is not only required for vegetative growth and sexual development but also negatively regulates asexual sporulation by repressing the brlA expression [100]. Like NsdC, one of the GATA transcription factors, NsdD, also affects conidiation by binding to the brlA promoter [101].

Other Key Regulators of Asexual Development
MedA (medusa) is also investigated in 1969 with StuA as mentioned above. This protein, which does not have any conserved domain, is essential for proper conidiophore formation [36]. MedA, known as an Neurospora crassa ortholog of acon-3, primarily modulates the expression of core conidiation genes (brlA and abaA) for the timely formation of metulae and phialides [102,103]. One of the WOPR fungi-specific DNA-binding proteins, OsaA (orchestrator of sexual and asexual development), indirectly controls conidiation by repressing downstream of the velvet regulator veA, which acts as a balancer between asexual and sexual development [104].

Conidial Maturation and Dormancy
Immature spores, formed at the ends of conidiophores, mature and stay in a dormant state. Dormant conidia modify the conidial wall, tolerate external stimuli, and maintain their viability for a long duration. The functions of controllers related to the maturation and quiescent phases in A. nidulans are shown in Figure 3 and Table 3.   Catalase B, involved in oxidative/osmotic

The Velvet Family
Members of the velvet family are known as essential regulators of fungal growth, development, and secondary metabolism in ascomycetes and basidiomycetes [18]. They commonly share a "velvet" DNA-binding domain that is composed of 150 amino acids [119]. There are four velvet proteins, VeA, VelB, VelC, and VosA, in A. nidulans, which interact with each other or non-velvet regulators. Through the interaction of various combinations, A. nidulans can control development and conidiation in a temporal and spatial manner. Among the velvet proteins, VelB and VosA play pivotal roles in the maturation and dormancy of asexual spores (Figure 3). VelB (velvet-like protein B) acts as a positive regulator of asexual development and mediates spore viability, trehalose biosynthesis, and conidial pigmentation [120]. VosA (viability of spores A) also regulates tolerance to several stresses and the long-term viability of conidia and trehalose biogenesis [14]. Moreover, the VosA and VelB form a heterocomplex in conidia and play important roles in conidial maturation, cell wall composition, and spore germination. One example is that the VosA-VelB complex directly binds to the promoter of fksA and controls β-glucan biosynthesis in asexual spores [121]. This heterocomplex also modulates the expression of spore-specific structural and regulatory genes during conidiogenesis. Representatively, VadA (VosA/VelB-activated developmental gene A) is known as a gene affected by the VosA-VelB complex in conidia.
VadA functions in conidial trehalose and β-glucan biogenesis, stress tolerance, spore viability, and germination in A. nidulans [122,123]. VadJ, regulated by the VosA-VelB complex, is one of the highly conserved sensor histidine kinases in A. nidulans. VadJ is required for the proper formation of asexual spores and maintenance of conidial viability [124]. Another Vad protein, VadZ, is a GAL4-like Zn(II) 2 Cys 6 transcription factor that is essential for conidiogenesis and spore longevity [125].
In contrast, VidA (VosA/VelB-inhibited developmental gene A), repressed by VosA and VelB, has two C 2 H 2 zinc finger domains at the C-terminus. This protein is involved in conidial trehalose and β-glucan biogenesis in A. nidulans [126]. There exists another Vid gene, vidD, which does not contain any known domains, but VidD is essential for normal fungal development, trehalose biosynthesis, and conidial long-term viability in A. nidulans [127].

Transcription Factors Involved in Conidial Maturation and Dormancy
McrA (Multicluster regulator A) is one of the putative GAL4-like Zn(II) 2 Cys 6 transcription factors and is highly expressed in late asexual development. This protein affects proper conidiation by modulating brlA expression through the life cycle. Furthermore, McrA is known as a direct target of the VosA-VelB heterodimer and modulates long-term spore viability, trehalose and β-glucan biogenesis, and proper conidial pigmentation [128]. One of the VosA-controlled regulatory genes, sclB (sclerotia-like), contains a Zn(II) 2 Cys 6 zinc cluster fungal-type DNA binding domain. Unlike VosA, which represses the premature induction of brlA, SclB induces the early activators of asexual development (FlbC, FlbD, and BrlA) and influences conidiogenesis. In addition, SclB plays important roles in conidial viability and tolerance to oxidative stress [129]. Another Zn cluster family member, zcfA encodes a Zn(II) 2 Cys 6 zinc finger protein and is known as a putative VosA target gene in conidia. The deletion of zcfA results in increased asexual spore formation and induced mRNA levels of brlA. Phenotypic analysis of ∆zcfA conidia shows that ZcfA plays a key role in conidial viability, trehalose biogenesis, and thermal stress resistance [130]. HbxB, one of the homeobox family members, is highly expressed in asexual spores and modulates the production of asexual spores and conidial stress resistance to thermal, oxidative, and UV stresses [99]. As a transcription factor, HbxB affects the transcriptomic levels of various genes, regulating trehalose biosynthesis, and β-glucan degradation in conidia [131]. CsgA is a GAL4-like Zn(II) 2 Cys 6 transcription factor specifically expressed in A. nidulans conidia. The deletion of csgA results in an increased number of conidia, and csgA-deleted conidia exhibit augmented trehalose contents and increased tolerance to thermal, oxidative, and UV stresses compared to WT conidia. In addition, CsgA is required for normal conidial viability and germination [132].

Mitogen-Activated Protein Kinase (MAPK) Cascades
During maturation, the cellular composition of immature spores is altered by several regulators, and consequently, they possess the ability to withstand various external stresses. Representatively, the histidine-to-aspartate (His-Asp) phosphorelay systems actively function so that spores can survive for a long time even under extreme environmental conditions. In A. nidulans, NikA is a histidine-specific protein kinase (HK) that first recognizes stimuli. NikA plays important roles in proper conidial reproduction and conidial resistance to certain fungicides and osmotic stress. In the presence of stimuli, NikA responds and activates downstream stress-related regulators [133]. Upon activation by NikA, YpdA transmits the signals to the response regulator, SskA. The deletion of sskA results in defective asexual development, conidial viability, and sensitivity against cold and oxidative stresses (H 2 O 2 ). SskA phosphorylates SskB (MAPKKK), which subsequently phosphorylates PbsB (MAPKK) and the ortholog of Hog1 (MAPK). There are two homologs of S. cerevisiae Hog1 in A. nidulans and other Aspergilli: SakA (stress-activated MAP kinase) and MpkC. Both of these homologs physically interact with the upstream regulator PbsB, but they have the same or different functions in A. nidulans [134]. As a member of the Hog1/Sty1/p38 family, SakA is a well-known key regulator of spore stress tolerance in filamentous fungi. Similarly, in A. nidulans, the deletion mutant of sakA exhibits defective conidial production and ∆sakA is sensitive to thermal, oxidative, and cell wall stresses. Conversely, the deletion of mpkC results in increased conidial production and resistance to oxidative stress. However, both Hog1 homologs, SakA and MpkC, are essential for the long-term survival of conidia [135]. Another MAPK, MpkB, is phosphorylated by other MAPK components (pheromone MAPK pathway), and activated MpkB is also involved in fungal development. MpkB, regulated by VosA, is pivotal for spore viability and inhibits conidial germination. MpkB also influences fungal autolysis [136]. The other MAPK, MpkC, is activated by the fungal cell wall integrity signal (CWIS). Phosphorylated MpkC modulates proper conidial germination as well as conidial cell wall integrity by upregulating genes related to α-, β-1,3-glucans and chitin biosynthesis [137].

Other Transcription Factors Related to Stress Tolerance in Conidia
SrrA is one of the response regulators and components of a stress-sensing phosphorelay system in A. nidulans. Like SskA, SrrA is activated by YpdA. However, unlike SskA, SrrA directly translocates into the nucleus. As a specific transcription factor, SrrA affects conidial resistance against oxidative stress by regulating antioxidant genes such as catB. Furthermore, SrrA plays important roles in conidial formation and spore viability [138]. AtfA, an ortholog of Schizosaccharomyces pombe atf1, is a member of the activating transcription factor/cAMP-responsive element-binding protein (ATF/CREB) family. AtfA, containing a bZIP domain, permanently exists in the nucleus and responds to oxidative stress during spore development. When SakA accumulates in the conidial nucleus by oxidative stress, it physically interacts with AtfA, regulating the expression of antioxidant-related genes in conidia. Consequently, AtfA plays pivotal roles in the conidial antioxidant response (tBOOH and H 2 O 2 ) and long-term viability [139,140]. Recent research shows that AtfA interacts with another bZIP transcription factor, AtfB, for coordinating asexual development and stress tolerance [140]. Although AtfA appears to be more important than AtfB in the oxidative stress defense system of A. nidulans spores, AtfB is also essential for tolerating thermal and oxidative stresses. Another transcription factor related to oxidative response is NapA, which is an S. cerevisiae Yap1 functional homolog and contains a bZIP domain. Similar to the functions of AtfA, NapA is involved in oxidant detoxification (menadione and H 2 O 2 ). When stimulated by oxidants, NapA protects by positively regulating both nonenzymatic (e.g., glutathione and thioredoxin) and enzymatic (e.g., catalases and superoxide dismutases) pathways in conidia. Moreover, NapA regulates asexual development and carbon utilization [141]. RsrA (regulator of stress response) is known as a C 2 H 2 zinc finger transcription factor that is required for fungal growth and sporulation. RsrA directly represses antioxidant genes such as glrA, trxA, and catB as well as NapA in the presence of reactive oxygen species such as tBOOH and H 2 O 2 [142].

Conidial Germination
Under appropriate conditions, the resting conidia break the quiescent state and modify their morphology. Through the alteration of cell wall composition and molecular organization, they swell and produce the germ tube (polarized growth). The germinated spores can expand their habitats such as humans, animals, and plants. Therefore, it is important to understand the regulatory mechanisms related to conidial germination. Although there are several coordinators mediating conidial germination ( Figure 4 and Table 4), we focus on the genetic regulators and transcription factors.
ify their morphology. Through the alteration of cell wall composition and molecular o ganization, they swell and produce the germ tube (polarized growth). The germinat spores can expand their habitats such as humans, animals, and plants. Therefore, it is i portant to understand the regulatory mechanisms related to conidial germination. A hough there are several coordinators mediating conidial germination (Figure 4 and Tab 4), we focus on the genetic regulators and transcription factors.  alA Activation Activation Fungal thaumatin-like proteins, named after calcoflour hypersensitivity [49,147] aM Activation Calmodulin involved in calcium-calcineurin signaling [148] etA Activation Fungal thaumatin-like proteins, named after conidial-enriched transcripts [147] hiA Activation Chitinase involved in fungal cell-wall integrity signaling (CWIS) [149] kA/B Activation Ca 2+ /caM-dependent protein kinase involved in calciumcalcineurin signaling [150] nF/J Activation Conidiation-specific gene Conidial sensitive to polyol [151] otA Activation Activation NDR protein kinase, homolog of S. cerevisiae Cbk1 [152,153]

Transcription Factors Involved in Germination
CreA is a Cys 2 His 2 transcription factor for carbon catabolite repression (CCR). In the presence of glucose as a carbon source, CreA directly or indirectly represses genes encoding enzymes (cellulases, xylanases) for degrading alternative carbon sources. However, in the absence of glucose, CreA is ubiquitinated and targeted by the proteasomes. As a result of the degradation of CreA, enzymes for alternative carbon sources are biosynthesized in A. nidulans [179]. In addition to CCR, the ∆creA strain shows defective spore germination [180]. SocA, a Zn(II) 2 Cys 6 transcription factor, was first discovered by mutagenesis and isolation of FLIP (Fluffy in Phosphate) mutant. The null mutant of socA shows deficient colony growth and abnormal morphogenesis as well as an altered germination pattern [62].

Other Regulators Related to Germination
The cAMP-PKA pathway (PKA pathway) is closely related to A. nidulans conidial germination. When GanB, which is Gα forming a heterotrimer with SfaD (Gβ) and GpgA (Gγ) mentioned earlier, separates from the complex and then activates CyaA, the activated CyaA as the adenylate cyclase produces cAMP from ATP, which attaches to PkaR and promotes conidial germination [169]. In addition, PkaB, a secondary protein kinase A catalytic subunit that is separated from PkaR by cAMP binding, promotes conidial germination and spore resistance to oxidative stress and inhibits conidiation while remaining alone [181]. Meanwhile, the dissociated GanB and SfaD-GpgA dimers are reunited by RgsA (regulator of G-protein signaling family), which indirectly affects asexual development and inhibits vegetative growth. RgsA plays a vital role in proper conidial germination and tolerance to thermal and oxidative stresses [182].

Conclusions
The genus of Aspergillus is one of the most abundant filamentous fungi in the air. They proliferate by producing a number of asexual spores, which constitute the major reproductive mode. Floating at short and long distances, conidia take root in the appropriate environment or host and grow by changing their morphology. They undergo asexual development and produce conidiophores. The matured conidia on the conidiophores stay in the resting phase and prepare for the next generation. These processes are coordinated by several genetic regulators or signaling pathways, which have been investigated by researchers. In this review, we summarized the key genetic regulators and their roles in each stage of asexual reproduction in the model organism A. nidulans. This information will help us gain a better understanding of the organizational and systematic developmental process and may help prevent the development of pathogenic spores or maximize the production of desired ones. Nevertheless, scientific studies must be continuously and deeply scrutinized as there are still several unexplored regulators.