The ability to respond to injury is a biological process shared by organisms of different kingdoms that can even result in complete regeneration of a part or structure that was lost. Due to their immobility, multicellular fungi are prey to various predators and are therefore constantly exposed to mechanical damage.
Nevertheless, our current knowledge of how fungi respond to injury is scarce. Here it was showed that activation of injury responses and hyphal regeneration in the filamentous fungus Trichoderma atroviride relies on the detection of two danger or alarm signals.
As an early response to injury, we detected a transient increase in cytosolic free calcium ([Ca2+]c) that was promoted by extracellular ATP, and which is likely regulated by a mechanism of calcium-induced calcium-release.
Genes related to cell signaling
In addition, we demonstrate that the mitogen-activated protein kinase Tmk1 plays a key role in hyphal regeneration. Calcium- and Tmk1-mediated signaling cascades activated major transcriptional changes early following injury, including induction of a set of regeneration-associated genes related to cell signaling, stress responses, transcription regulation, ribosome biogenesis/translation, replication, and DNA repair.
Interestingly, we uncovered the activation of a putative fungal innate immune response, including the involvement of HET domain genes, known to participate in programmed cell death.
The work shows that fungi and animals share danger-signals, signaling cascades, and the activation of the expression of genes related to immunity after injury, which is likely the result of convergent evolution.
The researchers have previously shown that T. atroviride responds to mycelial injury by rapidly regenerating its hyphae and, developing conidia in a Nox-dependent manner. We had also shown that eATP induces conidiation and triggers activation of Tmk1 and Tmk3 and that the latter is activated in a Nox1-NoxR dependent fashion.
Further, mutants in either tmk1 or tmk3 were affected in injury-induced conidiation, which is the outcome of the process. Intriguingly, depletion of extracellular Ca2+ blocked injury induced conidiation but allowed activation of both MAPKs.
In this regard, Ca2+ released from a damaged cell may be detected by neighboring cells as a signal molecule or serve as a second messenger liberated from intracellular pools and/or be transported across the plasma membrane upon detection of DAMPs.
However, it was unclear whether the regeneration and conidiation processes were mechanistically linked and how all these elements were interconnected to regulate the response to damage.
In addition to HET domain protein-encoding genes, 14 genes were found which participate in either cell death or the innate immune system in animals. Among them a caspase, a putative phosphatidylserine-specific receptor, a PITSLRE protein kinase, and the activation of apoptosis signal-regulating kinase 1, all of which play major roles in apoptosis.