Environmental exposures are an important health determinant. Public health is threatened by various external environmental risk factors, including air and water, chemical pollution, climate changes, radiation, and disease-causing microbes (Gerba, 2009; Kohler et al., 2017; Manisalidis et al., 2020; Munzel et al., 2023; Roy, 2017). Interventions or strategies to reduce exposure to toxic chemical materials or biological pathogens improve human health and quality of life. Aspergillus flavus is an opportunistic pathogenic fungus found in air, soil, and water, as well as on host plants such as corn and peanuts (Hedayati et al., 2007; Krishnan et al., 2009). With the ability to survive even high temperatures, this specie is the most prevalent cause of superficial infection and the second most frequent cause of aspergillosis (Krishnan et al., 2009; Pasqualotto, 2009). The primary mode of transmission by A. flavus to the hosts is through asexual spores (conidia).

Conidium of A. flavus ranges in size from 3 to 6 μm and is easily inhaled by humans or infect plants (Pasqualotto, 2009). This may lead to colonization, allergic reactions, and invasive or chronic pulmonary diseases, depending on the immune status of the host. Airborne conidia contaminate crops and produce aflatoxins, which are the most toxic and potent carcinogenic natural compounds (Amaike and Keller, 2011; Hedayati et al., 2007). Aflatoxin B1 (AFB1), produced by A. flavus, causes carcinogenesis by damaging DNA either by lipid peroxidation or oxidation in the liver (Zhang et al., 2015). Studies have reported that the economic loss caused by contamination with AFB1 averages over $750 million per year in Africa and ranged from $52.1 million to $1.68 billion in the United States between 2011 and 2013 (Mitchell et al., 2016; Sefater et al., 2018). Thus, understanding sporulation and spore physiology is necessary to guide the development of strategies to control A. flavus and aflatoxins.

In the genus of Aspergillus, conidiogenesis is induced by distinctive physical and physiological cues. During conidial germination, dormant conidia transition to polarized growth by remodeling cell wall constitution and activating cell cycle and cytoskeleton activity (Baltussen et al., 2020; Harris, 1999; Harris et al., 1994). Germinated spores grow vegetatively and form stalks, vesicles, metulae, phialides, and conidiophores by multiple nuclear division, multipolar germination process, and specialized cytosolic and nuclear compartments (Etxebeste et al., 2010; Fischer and Kües, 2006; Krijgsheld et al., 2013; Mirabito and Osmani, 1994). There are a variety of transcriptional regulatory factors for proper fungal development in Aspergillus spp. The central developmental regulators (BrlA, AbaA, and WetA) are indispensable for conidiophore formation (Yu, 2010). The velvet transcription factors VosA and VelB regulate spore viability, stress resistance, cell wall integrity, and germination in Aspergillus spp. (Park et al., 2015; Park and Yu, 2016). AtfA, regulated by the mitogen-activated protein kinase (MAPK) SakA, is required for response to osmotic and oxidative stresses and spore viability (Lara-Rojas et al., 2011). StuA plays key roles in fungal morphological modification, stress resistance, and cell wall biogenesis (Miller et al., 1992; Sheppard et al., 2005). However, there are uncharacterized regulators in Aspergillus spp., especially A. flavus.

Our recent comparative transcriptomic analyses and phenotypic studies tried to find novel spore-specific transcription factors in Aspergillus species and identified a spore-specific Cys2His2A, SscA, which is essential for the regulation of asexual sporulation in the model organism A. nidulans (Son et al., 2023a). Our results also showed that the homologs of SscA in A. fumigatus and A. flavus exhibited conserved roles in A. nidulans conidia (Son et al., 2023a). However, the functions of SscA have not yet been investigated in A. flavus.

In this study, we investigated the functions of SscA in the aflatoxin-producing fungus A. flavus. The ΔsscA (AFLA_084970) strain showed defective conidia formation, loss of spore viability, and increased stress sensitivity. SscA exerts aflatoxin production and pathogenicity on maize. We identified the functions of SscA in conidial wall compositions through additional transcriptomic analyses.