Evaluating physical and nutritional stress during mycelial growth as inducers of tolerance to heat and UV-B radiation in Metarhizium anisopliae conidia
Elevated tolerance to UV-B radiation and heat may be induced in conidia produced on fungi exposed during mycelial growth to sublethal stresses other than heat or UV-B. This is due to a phenomenon referred to as ‘cross-protection’. Several mechanisms are associated with this increased conidial tolerance, one of which is the accumulation of trehalose and mannitol within conidia. In the present study, conidia of the insect-pathogenic fungus Metarhizium anisopliae var. anisopliae were produced on mycelium subjected to nutritive, heat-shock, osmotic, or oxidative stress. The tolerance levels to UV-B radiation and heat of the conidia from stressed mycelium were evaluated, and the amounts of trehalose and mannitol accumulated in conidia were quantified. Conidia produced under nutritive stress (carbon and nitrogen starvation) were two-times more heat and UV-B tolerant than conidia produced under rich (non-stress) nutrient conditions [potato–dextrose agar with yeast extract (PDAY)], and they also accumulated the highest concentrations of trehalose and mannitol. Conidia produced on heat-shock stressed PDAY cultures had higher tolerance to UV-B radiation and heat than conidia produced without heat shock; however, both the UV-B tolerance and trehalose/mannitol concentrations in conidia produced on heat-shocked mycelium were less than those of conidia produced under nutritive stress. Conidia produced under osmotic stress (sodium or potassium chloride added to PDAY) had elevated heat and UV-B tolerances similar to those of conidia produced under nutritive stress; however, they had the lowest levels of mannitol and trehalose, which indicates that accumulation of these compounds is not the only mechanism used by M. anisopliae for protection from heat and UV-B radiation. Oxidative stress from UV-A irradiation or hydrogen peroxide did not produce conidia with elevated UV-B or heat tolerances. Conidia produced under oxidative stress generated by menadione had increased or unchanged tolerances to heat or UV-B, respectively. The levels of mannitol or trehalose in conidia were similar to those in the unstressed controls. Conidial yield was reduced, in some cases severely, by nutritive and osmotic stress; whereas oxidative and heat-shock stress did not alter levels of spore production.