Roles of BrlA and AbaA in Mediating Asexual and Insect Pathogenic Lifecycles of Metarhizium robertsii

Abstract

BrlA and AbaA are key activators of the central developmental pathway (CDP) that controls asexual development in Aspergillus but their roles remain insufficiently understood in hypocerealean insect pathogens. Here, regulatory roles of BrlA and AbaA orthologs in Metarhizium robertsii (Clavicipitaceae) were characterized for comparison to those elucidated previously in Beauveria bassiana (Cordycipitaceae) at phenotypic and transcriptomic levels. Time-course transcription profiles of brlA, abaA, and the other CDP activator gene wetA revealed that they were not so sequentially activated in M. robertsii as learned in Aspergillus. Aerial conidiation essential for fungal infection and dispersal, submerged blastospore production mimicking yeast-like budding proliferation in insect hemocoel, and insect pathogenicity via cuticular penetration were all abolished as a consequence of brlA or abaA disruption, which had little impact on normal hyphal growth. The disruptants were severely compromised in virulence via cuticle-bypassing infection (intrahemocoel injection) and differentially impaired in cellular tolerance to oxidative and cell wall-perturbing stresses. The ΔbrlA and ΔabaA mutant shad 255 and 233 dysregulated genes (up/down ratios: 52:203 and 101:122) respectively, including 108 genes co-dysregulated. These counts were small compared with 1513 and 2869 dysregulated genes (up/down ratios: 707:806 and 1513:1356) identified in ΔbrlA and ΔabaA mutants of B. bassiana. Results revealed not only conserved roles for BrlA and AbaA in asexual developmental control but also their indispensable roles in fungal adaptation to the insect-pathogenic lifecycle and host habitats. Intriguingly, BrlA- or AbaA-controlled gene expression networks are largely different between the two insect pathogens, in which similar phenotypes were compromised in the absence of either brlA or abaA.

Essential Role of WetA, but No Role of VosA, in Asexual Development, Conidial Maturation and Insect Pathogenicity of Metarhizium robertsii

“…Recent studies have confirmed that brlA and abaA regulate asexual development in B. bassiana ( 20 ) and M. robertsii ( 34 ), as they do in A. nidulans . The regulated processes include aerial conidiation and submerged blastospore production mimicking proliferation in vivo after hyphal invasion into insect hemocoel.…”

“…The Δ wetA transcriptome was then compared with the Δ brlA and Δ abaA transcriptomes (up/down ratios: 52:203 and 101:122, respectively), analyzed previously in M. robertsii ( 34 ). Surprisingly, the 3 transcriptomes comprised 64 co-dysregulated genes, including 44 co-downregulated, 8 co-upregulated, and 12 differentially regulated ( Fig.…”

“… bassiana, despite similar sizes of their genomes ( 4 , 6 ). Knockout mutations of brlA and Δ abaA resulted in differential regulation of 255 and 233 genes in M. robertsii ( 34 ), but 1,513 and 2,869 genes in B. bassiana ( 33 ), respectively.…”

SNARE Protein AoSec22 Orchestrates Mycelial Growth, Vacuole Assembly, Trap Formation, Stress Response, and Secondary Metabolism in Arthrobotrys oligospora

“…In F. graminearum , FgSec22 was reported to be indispensable for normal conidiation and conidial morphology [ 24 ]. In the present study, the reduced conidiation level correlated with transcriptional repression of several asexual development-required genes, particularly brlA , abaA , and wetA , crucial for the conidiation of Beauveria bassiana , Monascus ruber , and other fungi [ 52 , 53 , 54 ]. In B. bassiana , loss-of-function mutations of wetA have compromised conidiation capacity by 98% [ 55 ].…”

“…Moreover, BrlA and AbaA are highly conserved activators of asexual development in filamentous fungi [ 56 , 57 ]. BrlA and AbaA are indispensable for aerial conidiation in Metarhizium roberstii as well as in B. bassiana [ 52 , 54 ]. These results indicate that Sec22 plays a conserved role in sustaining mycelial radial growth, aerial hyphal development, and conidiation in filamentous fungi, and is also crucial for the development of the hyphal septation and nuclear division in A. oligospora .…”