A Cellular Fusion Cascade Regulated by LaeA Is Required for Sclerotial Development in Aspergillus flavus

Abstract

Aspergillus flavus is a saprophytic soil fungus that poses a serious threat worldwide as it contaminates many food and feed crops with the carcinogenic mycotoxin called aflatoxin. This pathogen persists as sclerotia in the soil which enables fungal survival in harsh environmental conditions. Sclerotia formation by A. flavus depends on successful cell communication and hyphal fusion events. Loss of LaeA, a conserved developmental regulator in fungi, abolishes sclerotia formation in this species whereas overexpression (OE) of laeA results in enhanced sclerotia production. Here we demonstrate that sclerotia loss and inability to form heterokaryons in A. flavusΔlaeA is mediated by homologs of the Neurospora crassa ham (hyphal anastomosis) genes termed hamE-I in A. flavus. LaeA positively regulates ham gene expression and deletion of hamF, G, H, or I phenocopies ΔlaeA as demonstrated by heterokaryon and sclerotia loss and reduced aflatoxin synthesis and virulence of these mutants. Deletion of hamE showed a less severe phenotype. hamE-I homologs are positively regulated by the clock controlled transcription factor ADV-1 in N. crassa. Similarly, the ADV-1 homolog NosA regulates hamE-I expression in A. flavus, is required for sclerotial development and is itself positively regulated by LaeA. We speculate that a putative LaeA>NosA>fusion cascade underlies the previously described circadian clock regulation of sclerotia production in A. flavus.

The tetrameric pheromone module SteC‐MkkB‐MpkB‐SteD regulates asexual sporulation, sclerotia formation and aflatoxin production inAspergillus flavus

“…Interestingly, the deletion of hamE did not result in any significant defects in sporulation with regards to the wild type. This largely supports a previous study of the role of ham genes on fusion processes in A. flavus where loss of hamE had relatively small contributions to sporulation but a large impact on sclerotia, as found here (Zhao et al, ).…”

“…This data also propose that HamE is essential for aflatoxin production but may exert its regulatory role independently of the pheromone module. A previous study showed hamE was regulated by NosA, a transcription factor involved in sexual development in A. nidulans (Vienken & Fischer, ) via LaeA activation (Zhao et al, ).…”

The Antioxidant Gallic Acid Inhibits Aflatoxin Formation in Aspergillus flavus by Modulating Transcription Factors FarB and CreA

“…The transformation construct for creA mutant was made by double joint PCR [ 49 ] and transformed into the parental strain TJES20.1 (∆ ku70 , ∆ argB :: Aspergillus fumigatus pyrG , pyrG -). Transformation of fungal strains was carried out according to the protocol of Szewczyk et al [ 50 ], with some modifications that can be found in Zhao et al [ 51 ]. The creA mutants were confirmed by PCR and qRT-PCR ( Figure S1A,B ), and aflatoxin synthesis of creA mutants ( Figure S1C ) was analyzed by TLC, the primers for qRT-PCR can be found in Table S7 .…”

Advances in research of the structural gene characteristics of the aflatoxin biosynthetic gene cluster

“…The canonical regulatory genes within the AF gene cluster, a lR and a lS, have been extensively investigated [94]. Beyond them, some other regulatory factors, such as LaeA, Ham, NosA, FarB, CreA, etc., can also in luence a latoxin formation in A. lavus [91,95,96].…”