SclR, a Basic Helix-Loop-Helix Transcription Factor, Regulates Hyphal Morphology and Promotes Sclerotial Formation in Aspergillus oryzae

Abstract

Most known basic-region helix-loop-helix (bHLH) proteins belong to a superfamily of transcription factors often involved in the control of growth and differentiation. Therefore, inappropriate expression of genes encoding bHLH proteins is frequently associated with developmental dysfunction. In our previously reported study, a novel bHLH protein-encoding gene (AO090011000215) of Aspergillus oryzae was identified. The genedisrupted strain was found to produce dense conidia, but sparse sclerotia, relative to the parent strain. Here, to further analyze its function, we generated an overexpressing strain using the A. oryzae amyB gene promoter. Genetic overexpression led to a large number of initial hyphal aggregations and then the formation of mature sclerotia; it was therefore designated sclR (sclerotium regulator). At the same time, the sclR-overexpressing strain also displayed both delayed and decreased conidiation. Scanning electron microscopy indicated that the aerial hyphae of the sclR-overexpressing strain were extremely branched and intertwined with each other. In the generation of the SclR-enhanced green fluorescent protein (EGFP) expression strain, the SclR-EGFP protein fusion was conditionally detected in the nuclei. In addition, the loss of sclR function led to rapid protein degradation and cell lysis in dextrin-polypeptone-yeast extract liquid medium. Taken together, these observations indicate that SclR plays an important role in hyphal morphology, asexual conidiospore formation, and the promotion of sclerotial production, even retaining normal cell function, at least in submerged liquid culture.

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

“…Our results show that UrdA does not control nsdD expression, suggesting that sexual and asexual developmental pathways are coordinated at different levels ( Figure 9). Jin et al (2011a) indicated that the A. oryzae ortholog of UrdA, EcdR, is required for early asexual development and proposed that it was repressed by its interaction with SclR, an additional HLHtype transcription factor that promotes sclerotial formation (Jin et al 2011b). However, expression of the putative ortholog of sclR, An7170, is not significantly altered when flbB is deleted.…”

Presence and Functionality of Mating Type Genes in the Supposedly Asexual Filamentous Fungus Aspergillus oryzae

“…This might involve the manipulation of culture conditions, given that sclerotial formation is dependent on environmental conditions (14) and given the exacting demands of some aspergilli for sexual reproduction (14,47), and/or the use of genetic manipulation techniques to induce sclerotial formation. At present, only one A. oryzae gene, encoding the bHLH transcription factor SclR (for sclerotium regulator), involved in the production of sclerotia has been identified (14,30,31). Deletion of sclR in certain strains observed to form sclerotia was found to result in sparse sclerotial production, while overexpression of sclR led to Ͼ5-fold production of sclerotia with increased formation of branched aerial hy-phae (31).…”

“…At present, only one A. oryzae gene, encoding the bHLH transcription factor SclR (for sclerotium regulator), involved in the production of sclerotia has been identified (14,30,31). Deletion of sclR in certain strains observed to form sclerotia was found to result in sparse sclerotial production, while overexpression of sclR led to Ͼ5-fold production of sclerotia with increased formation of branched aerial hy-phae (31). In related work, deletion of a zinc finger calcineurin response gene, crzA, was found to result in the production of mainly immature sclerotia in A. parasiticus (8), so a homolog might also be required for the maturation of sclerotia in A. oryzae.…”

Deletion of the Aspergillus flavus Orthologue of A. nidulans fluG Reduces Conidiation and Promotes Production of Sclerotia but Does Not Abolish Aflatoxin Biosynthesis

“…The relationship between regulatory factors controlling the formation of sclerotia and conidiation is not well understood. SclR, the sclerotial pathway-specific helix-loop-helix transcription factor, was reported to promote sclerotial formation and to regulate hyphal morphology in Aspergillus oryzae (23). Deletion of sclR yields strains with sparse sclerotia but produces dense conidia.…”