Apoptosis-like death was involved in freeze-drying-preserved fungus Mucor rouxii and can be inhibited by l-proline

Freeze-drying is one of the most effective methods to preserve fungi for an extended period. However, it is associated with a loss of viability and shortened storage time in some fungi. This study evaluated the stresses that led to the death of freeze-dried Mucor rouxii by using cell apoptotic methods. The results showed there were apoptosis-inducing stresses, such as the generation of obvious intracellular reactive oxygen species (ROS) and metacaspase activation. Moreover, nuclear condensation and a delayed cell death peak were determined after rehydration and 24 h incubation in freeze-dried M. rouxii via a propidium iodide (PI) assay, which is similar to the phenomenon of cryopreservation-induced delayed-onset cell death (CIDOCD). Then, several protective agents were tested to decrease the apoptosis-inducing stresses and to improve the viability. Finally, it was found that 1.6 mM l-proline can effectively decrease the nuclear condensation rate and increase the survival rate in freeze-dried M. rouxii. In conclusion: (1) apoptosis-inducing factors occur in freeze-dried M. rouxii. (2) ROS and activated metacaspases lead to death in freeze-dried M. rouxii. (3) l-proline increases the survival rate of freeze-dried M. rouxii.