High osmolarity inhibits spore germination via:
High osmolarity within sorus fluid maintains spore dormancy preventing precocious germination while spores remain in fruiting body. Sensor adenylyl cyclase G, ACG, contains extracellular CHASE domain coupled to cyclase domain, activated by elevated osmolytes raising intracellular cAMP and sustaining PKA-mediated inhibition of germination genes. Upon dispersal into hypotonic environment such as soil water film, ACG activity drops, cAMP falls, and discoidin and germination proteins become expressed. ACA mediates aggregation; ACB mediates culmination; RegA is intracellular phosphodiesterase antagonizing germination pathway downstream of ACG rather than osmosensor itself, making ACG central to environmental sensing.
Ref: Current Biology, ACG osmosensor adenylyl cyclase - high osmolarity inhibition of spore germination via PKA.