The role of three calcineurin subunits and a related transcription factor (Crz1) in conidiation, multistress tolerance and virulence in Beauveria bassiana

Abstract

The eukaryotic calcineurin (CN) pathway comprising catalytic A (CnA) and regulatory B subunits (CnB) is crucial for many biological processes but functionally unexplored in entomopathogenic fungi. Here, we characterise three CN subunits (CnA1, CnA2 and CnB) and a downstream CN-responsive zinc finger transcription factor (Crz1) in Beauveria bassiana. CN-mediated phosphatase activity decreased by 16-38 % in all deletion mutants compared with wild type. Growth and conidiation were most defective in ΔcnB, which showed a large proportion of abnormally branched germlings but were less defective in ΔcnA1 and ΔcnA2. Conidiation defects also occurred in Δcrz1, uniquely accompanied with slower germination. Compared with wild type, the four deletion mutants became, to varying degrees, more sensitive to Ca(2+), Mn(2+), Zn(2+), Mg(2+), two oxidants, three cell wall stressors, carbendazim, heat shock and ultraviolet (UV)-B irradiation. They were also less virulent to Spodoptera litura larvae. Only ΔcnB and Δcrz1 were less tolerant to high osmolarity. The altered phenotypes of the deletion mutants were associated with lower intracellular mannitol and trehalose levels, reduced overall activity of superoxide dismutases and catalases, altered cell wall composition and down-regulation of numerous phenotype-influencing genes. Additionally, the transcription of six cascaded genes in two stress-responsive mitogen-activated protein kinase (MAPK) pathways and the phosphorylation of hallmarking Hog1 and Slt2 were largely down-regulated in all the deletion mutants under osmotic and cell wall stresses, respectively. All the changes were restored by gene complementation. Taken together, three calcineurin subunits and Crz1 play vital, but variable, roles in B. bassiana responses to environmental stresses during development and host signals during infection.

Disruption of calcineurin catalytic subunit (cnaA) in Epichloë festucae induces symbiotic defects and intrahyphal hyphae formation

“…For example, in A. fumigatus , calcineurin has been shown to control β‐glucan and chitin levels through the down‐regulation of β‐1,3‐glucan synthase catalytic subunit and chitin synthase‐encoding genes (Cramer et al ., ; Fortwendel et al ., ; Juvvadi et al ., ). Similar down‐regulation of glucan synthase‐encoding genes as a result of calcineurin pathway impairment has also been observed in M. oryzae , C. albicans and Beauveria bassiana (Choi et al ., ; Li et al ., ; Sanglard et al ., ). In contrast, the expression of U. hordei and C. neoformans glucan synthase‐encoding genes was increased in a calcineurin‐dependent manner (Cervantes‐Chavez et al ., ; Kraus et al ., ).…”

“…Although the genomes of most fungi contain a single cnaA copy, we found two cnaA paralogues in E. festucae E2368 and several other Epichloë strains. The presence of multiple cnaA copies has also been reported for fungi of the Clavicipitaceae family, Beauveria bassiana and Cordyceps militaris , which contain two paralogues each (Li et al ., ), whereas the genomes of fungi in the Zygomycetes family, Mucor circinelloides and Rhizopus delemar , contain three paralogues each (Lee et al ., ). These cnaA genes are often flanked with the same repetitive sequences, which might explain the occurrence of multiple cnaA genes through individual gene duplication events (Lee et al ., ).…”

Components of the Calcium-Calcineurin Signaling Pathway in Fungal Cells and Their Potential as Antifungal Targets

“…Calcineurin (CN) is a Ca 2ϩ /calmodulin (CaM)-activated protein phosphatase that is highly conserved from fungi to mammals (66) and transmits Ca 2ϩ signals to elicit downstream responses mainly by regulating various transcriptional factors, such as CRZ1 and PRZ1 (19,67,68). CN consists of two subunits, a catalytic subunit A (encoded by CNA1 and CNA2) (66,69) and a regulatory subunit B (encoded by CNB1) (70).…”

Transcriptomic insights into the alternative splicing‐mediated adaptation of the entomopathogenic fungus Beauveria bassiana to host niches: autophagy‐related gene 8 as an example

“…During survival in the host haemocoel, the fungal pathogen encounters superoxide released by insect haemocytes (Bergin et al ., ). Among AS‐associated genes, the B. bassiana Cna1 gene, a catalytic subunit of calcineurin, is crucial for fungal resistance to oxidative stress (Li et al ., ). No direct evidence indicates the contribution of B. bassiana AP1‐like transcription factor to fungal oxidation tolerance, but AP‐1 factor is regarded as an important oxidative stress sensor in fungi (Rodrigues‐Pousada et al ., ).…”