Abiotic stress induced by dehydration followed by freezing enhances resistance to oomycete pathogens in the red alga Pyropia yezoensis

Red rot and Olpidiopsis blight, caused by oomycete pathogens, pose serious threats to Pyropia yezoensis cultivation, particularly during the early farming season under elevated seawater temperatures. This study demonstrates that a simple preservation treatment, moderate air-drying (~80% water loss) followed by freezing at -20°C (dehydrationthen- freezing [DTF]), can enhance pathogen resistance of gametophytic blades. DTF-treated blades showed delayed disease onset and reduced infection severity following oomycete pathogen challenge compared with untreated controls, and infection severity was lower than that observed after dehydration-only treatment under the tested conditions. Microscopic analysis showed reduced lesion development and limited pathogen-associated tissue damage. Importantly, dehydration at moderately elevated temperatures (15 and 20°C) during DTF treatment was associated with reduced infection severity, while expression analysis of heat shock protein (HSP) genes showed upregulation of several PyHSPs and an earlier response in DTF-treated blades upon pathogen challenge. These results indicate that reduced infection severity following DTF treatment was associated with altered HSP transcription and was consistent with a stress-responsive state. The findings provide a biological rationale for the long-standing frozen-net practice in East Asian aquaculture and suggest its potential relevance for improving disease management in Pyropia aquaculture.