Culture of Metarhizium robertsii on salicylic-acid supplemented medium induces increased conidial thermotolerance

Salicylic acid (SA), a cell-signaling metabolite in plants, is involved in resistance of plants to pathogens and environmental stresses; however, there is little information available on the responses of fungi to SA. Conidia of Metarhizium robertsii (ARSEF 2575) (Hypocreales: Clavicipitaceae) were produced on potato dextrose agar medium plus yeast extract (PDAY) supplemented with 1, 2, 4, or 8 mM SA (pH adjusted to 6.9) and incubated under constant-dark conditions. Then the tolerance of conidia against wet heat (45 °C, 3 h) and UV-B radiation (7.0 kJ m(-2)) was tested. For comparison, conidia were also produced on minimal medium (MM) that contained no carbon source (carbon starvation), a condition known to induce elevated conidial tolerance to heat and UV-B radiation in M. robertsii. The heat tolerance of conidia produced on PDAY containing 1, 2, or 4 mM SA were two-fold higher than that of conidia produced on PDAY alone; which is the same level of thermotolerance induced by growth on MM. Conidia produced on PDAY with 8 mM SA, however, did not exhibit increased heat tolerance. Growth on PDAY + SA did not increase conidial UV-B tolerance at any of the SA concentrations tested. The conidial yields of M. robertsii produced on PDAY with all levels of SA were somewhat reduced in comparison to the yield on PDAY alone. Nevertheless, conidial yields on PDAY + SA were 20-40 times greater than that obtained on MM alone. In conclusion, M. robertsii conidia produced on PDAY medium containing low concentrations of SA demonstrated increased tolerance to heat, but not to UV-B radiation. In comparison to PDAY alone, SA-amended PDAY afforded somewhat reduced conidial yields; however, in a mass-production situation, yield reductions would be offset by the fact that the conidia obtained would have relatively high heat tolerance.