## 5 Role of the Osmotic Stress-response Pathway on Fungal Development and Secondary Metabolism
flavus veA_ deletion strain, suggesting that _veA_ is involved in modulating osmotic stress-induced conidiation. On the other hand, salinity stress has been shown to have an inhibitory effect on the formation of sclerotia in fungal species, including _Sclerotinia sclerotiorum_, _Rhizoctonia solani_ and _Sclerotium rolfsii_[79]. In the mycotoxin producer _Aspergillus ochraceus_ osmotic stress had little effect on sclerotia production, [80]. In _Botrytis cinerea_ sclerotia production was also reduced as osmotic stress increased [81]. In _A. flavus_, hyperosmotic media caused a delay in sclerotial maturation ([PERSON] _et al._, manuscript in preparation). These studies suggest that under stress caused by hyperosmolarity the fungus favors investment of material and energy towards developmental programs (conidiation) that are conducive to survival _via_ dissemination to a more favorable environment versus survival in the unfavorable hyperosmotic environment. Previous studies support an association of fungal morphogenesis with secondary metabolism, including mycotoxin production [82]. Aflatoxin is one of the most potent natural carcinogenic compounds described. Surprisingly, hyperosmotic levels of NaCl, KCl or sorbitol did not affect the biosynthesis of this mycotoxin ([PERSON] _et al._, manuscript in preparation). Some osmotic stress genetic response elements have been demonstrated to be linked to toxin production in _A. nidulans_, specifically, RcoA (Tup1p homolog) affects not only fungal growth and development, but also is necessary for the production of sterigmatocystin in _A. nidulans_[83]. Additionally, osmotic stress does affect the production of other secondary metabolites in aspergillus, for example pigments. The _A. fumigatus sakA_ deletion mutant is unable to produce pigmentation as observed in the wild-type strain, indicating that secondary metabolism is affected by the HOG signaling pathway in this fungus [68]. Across fungal genera, the role of MAPK cascades in the osmotic stress response is quite diverse. In the filamentous fungus _Fusarium graminearum_, a significant relationship between osmotic stress and secondary metabolism has been observed. _F. graminearum_ is a common pathogen of grain producing crops. Among the several secondary metabolites generated by this fungus, the synthesis of trichothecene toxins and a reddish colored pigment called aurofusarin are affected by osmotic stress. [PERSON] _et al._[84] described the participation of several histidine kinases, components of the osmotic response signalling pathway, in the regulation of secondary metabolism of _F. graminearum_. These authors showed that production of trichothecenes is markedly suppressed by NaCl, without a significant effect on fungal growth. A null mutant of _FgOs1_ (encoding the osmosensor histidine kinase) produced a reduced amount of the red pigment aurofusarin however it was unaltered in its ability to produce trichothecenes. Deletion null mutants of _FgOs4_ (encoding MAPKKK), _FgOs5_ (MAPKK), and _FgOs2_ (MAPK) all showed markedly enhanced pigmentation and failed to produce trichothecenes, coinciding with a marked reduction of expression of _Tri4_ and _Tri6_ (trichothecene biosynthetic pathway and regulatory genes). In the maize pathogen, _Cochliobolus heterostrophus_, _hog1_ mutants are more pigmented than the wild-type and demonstrate smaller approssoria and reduced virulence [85]. In _cpmk1_ (_hog1_ homolog) mutants of _Cryphonectria parasitica_, there was an increase in osmosensitivity along with reduced pigmentation, conidial, and virulence on chestnut trees compared to a wild-type strain [86] In the rice pathogen, _Magnaporthe grisea_, _osm1_ (_hog1_ homolog) mutants were more sensitive to osmotic stress and showed some morphological defects. However, glycerol accumulation and approssorial turgor generation was unaltered compared to wild-type and virulence was not affected [87].