Stress-induced pyruvate accumulation contributes to cross protection in a fungus

It is commonly observed that microorganisms subjected to a mild stress develop tolerance not only to higher doses of the same stress but also to other stresses - a phenomenon called cross protection. The mechanisms for cross protection have not been fully revealed. Here, we report that heat shock induced cross protection against UV, oxidative and osmotic/salt stress conditions in the cosmopolitan fungus Metarhizium robertsii. Similarly, oxidative and osmotic/salt stresses also induced cross protection against multiple other stresses. We found that oxidative and osmotic/salt stresses produce an accumulation of pyruvate that scavenges stress-induced reactive oxygen species and promotes fungal growth. Thus, stress-induced pyruvate accumulation contributes to cross protection. RNA-seq and qRT-PCR analyses showed that UV, osmotic/salt and oxidative stress conditions decrease the expression level of pyruvate consumption genes in the trichloroacetic acid cycle and fermentation pathways leading to pyruvate accumulation. Our work presents a novel mechanism for cross protection in microorganisms.