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Updated: Feb 2, 2026
Quantitative Analysis of Aspergillus nidulans Growth Rate using Live Microscopy and Open-Source Software
Published on: July 24, 2021
SakA and MpkC Stress MAPKs Show Opposite and Common Functions During Stress Responses and Development in Aspergillus
Verónica Garrido-Bazán1,2, Rafael Jaimes-Arroyo1, Olivia Sánchez1
1Departamento de Biología Celular y del Desarrollo, Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, Mexico City, Mexico.
Abstract:
Stress activated MAP kinases (SAPKs) of the Hog1/Sty1/p38 family are specialized in transducing stress signals. In contrast to what is seen in animal cells, very few fungal species contain more than one SAPK. Aspergillus nidulans and other Aspergilli contain two SAPKs called SakA/HogA and MpkC. We have shown that SakA is essential for conidia to maintain their viability and to survive high H2O2 concentrations. H2O2 induces SakA nuclear accumulation and its interaction with transcription factor AtfA. Although SakA and MpkC show physical interaction, little is known about MpkC functions. Here we show that ΔmpkC mutants are not sensitive to oxidative stress but in fact MpkC inactivation partially restores the oxidative stress resistance of ΔsakA mutants. ΔmpkC mutants display about twofold increase in the production of fully viable conidia. The inactivation of the SakA upstream MAPKK PbsB or the simultaneous elimination of sakA and mpkC result in virtually identical phenotypes, including decreased radial growth, a drastic reduction of conidiation and a sharp, progressive loss of conidial viability. SakA and to a minor extent MpkC also regulate cell-wall integrity. Given the roles of MpkC in conidiation and oxidative stress sensitivity, we used a functional MpkC::GFP fusion to determine MpkC nuclear localization as an in vivo indicator of MpkC activation during asexual development and stress. MpkC is mostly localized in the cytoplasm of intact conidia, accumulates in nuclei during the first 2 h of germination and then becomes progressively excluded from nuclei in growing hyphae. In the conidiophore, MpkC nuclear accumulation increases in vesicles, metulae and phialides and decreases in older conidia. Oxidative and osmotic stresses induce MpkC nuclear accumulation in both germinating conidia and hyphae. In all these cases, MpkC nuclear accumulation is largely dependent on the MAPKK PbsB. Our results indicate that SakA and MpkC play major, distinct and sometimes opposing roles in conidiation and conidiospore physiology, as well as common roles in response to stress. We propose that two SAPKs are necessary to delay (MpkC) or fully stop (SakA) mitosis during conidiogenesis and the terminal differentiation of conidia, in the highly prolific phialoconidiation process characteristic of the Aspergilli.
Insights
Two stress-activated protein kinases (SAPKs), SakA and MpkC, in Aspergillus play distinct roles in fungal development and stress response. SakA is crucial for oxidative stress survival, while MpkC influences conidia production and partially opposes SakA's stress response.
Area of Science:
- Mycology
- Cellular Biology
- Biochemistry
Background:
- Stress-activated protein kinases (SAPKs) are vital for signal transduction in response to environmental stressors.
- Fungal species, unlike animal cells, typically possess only one SAPK, with exceptions like Aspergillus species harboring two: SakA and MpkC.
- SakA is known to be essential for conidial viability and survival under high hydrogen peroxide (H2O2) conditions, involving nuclear accumulation and interaction with transcription factor AtfA.
Purpose of the Study:
- To elucidate the functions of MpkC, the second SAPK in Aspergillus, particularly its role in conidiation and stress response.
- To investigate the interplay between SakA and MpkC, and their combined effects on fungal physiology.
- To determine the subcellular localization and activation dynamics of MpkC during asexual development and stress exposure.
Main Methods:
- Comparative analysis of wild-type and mutant strains (ΔmpkC, ΔsakA, ΔsakA/ΔmpkC) to assess phenotypes related to oxidative stress, growth, and conidiation.
- Utilizing a functional MpkC::GFP fusion to track MpkC's nuclear localization as a proxy for its activation.
- Induction of stress conditions (oxidative and osmotic) to observe MpkC's dynamic response in different cellular compartments.
Main Results:
- MpkC inactivation did not confer sensitivity to oxidative stress; instead, it partially rescued the oxidative stress resistance of ΔsakA mutants.
- ΔmpkC mutants exhibited a twofold increase in the production of viable conidia.
- Simultaneous inactivation of sakA and mpkC, or inactivation of the upstream MAPKK PbsB, led to severe defects in growth, conidiation, and conidial viability.
- MpkC nuclear accumulation, dependent on PbsB, was observed during conidia germination, in the conidiophore, and in response to oxidative and osmotic stresses.
- Both SakA and MpkC contribute to cell-wall integrity, with SakA having a major role and MpkC a minor one.
Conclusions:
- SakA and MpkC have distinct, sometimes opposing, roles in Aspergillus conidiation and conidiospore physiology, alongside shared functions in stress response.
- The study proposes that the two SAPKs, MpkC and SakA, are necessary to regulate mitosis during conidiogenesis, with MpkC potentially delaying and SakA halting it.
- These findings highlight the complex regulatory network involving multiple SAPKs in orchestrating fungal development and stress adaptation.
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