Also, other physical conditions of see more the environment during mycelial growth that may not necessarily be stress conditions might improve the stress tolerance of conidia. As reported here, this is true for M. robertsii mycelia grown under continuous visible-light exposure (5.4 W m−2), which induced significantly higher (almost twofold) conidial tolerance to UVB radiation (F2, 5=24.7, P<0.0025) (Fig. 2a). The UV-B tolerance of conidia produced on PDAY under constant visible light was similar to that of conidia produced on MM (nutritive stress), which is found elsewhere (Rangel et al., 2006a, b, 2008). The mechanisms involved in inducing higher UVB tolerance in M. robertsii conidia produced

under visible light are not known; however, several BAY 80-6946 manufacturer mechanisms may be involved. For example, light is known to stimulate the production of a heat-shock protein (HSP100) in Phycomyces (Rodriguez-Romero & Corrochano, 2004), and the trehalose phosphorylase gene is photoinducible in Neurospora (Shinohara et

al., 2002). Accordingly, the synthesis of heat-shock proteins or trehalose accumulation is known to induce stress tolerance in several fungi (Iwahashi et al., 1998; Rensing et al., 1998; Fillinger et al., 2001) including Metarhizium (Rangel et al., 2008) and Beauveria (Liu et al., 2009). The survival rates of the light-grown dematiaceous fungus Wangiella dermatitidis revealed that the carotenoid-pigmented cells are considerably more resistant to UV radiation than nonpigmented ones grown in the dark (Geis & Szaniszlo, 1984). However, the pigment melanin, as well as the biosynthetic precursor of melanin (Rangel et al., 2006a, b; Fang

Dichloromethane dehalogenase et al., 2010), and carotenoids (Fang et al., 2010; Gonzales et al., 2010) have not been found in M. robertsii or Metarhizium anisopliae conidia. Therefore, these pigments are not involved in light-induced increases in the stress tolerance of M. robertsii conidia. Conidia produced on PDAY under visible light had somewhat elevated tolerance to heat (45 °C for 3 h), but not significantly different from conidia produced on PDAY under continuous dark (F2, 4=7.8, P<0.0240) (Fig. 2b). It is well known that growth under nutritive stress induces cross-protection, providing the highest tolerance to heat and other stresses as found in this study and elsewhere (Steels et al., 1994; Park et al., 1997; Rangel et al., 2008; Rangel, 2010). Light during mycelial growth did not induce as much phenotypic plasticity in heat tolerance as it did for UVB radiation for the reason that microbial growth on different environmental conditions exhibits different levels of stress tolerance (Gasch & Werner-Washburne, 2002). The growth of M. robertsii under osmotic or nutritive stress conditions decreased conidial production to approximately 20–40-fold, respectively, of that of conidia produced on PDAY medium (Rangel et al., 2008).