Frost Tolerance Under Cold Flooding Involves the Presence of Specific PR Proteins and Is Linked to Snow Mould Pathogen Resistance.

Winter survival of crops is influenced by multiple environmental stress factors, including low temperature, excessive soil moisture, and pathogen pressure. With climate change, the role of flooding during cold acclimation has become increasingly important. This study examined the effect of cold flooding on frost tolerance of two winter rye (Secale cereale L.) lines differing in resistance to snow mould. Plants were subjected to cold acclimation for three weeks at 4 °C, followed by ten days of flooding at the same cold temperature. Cold flooding improved frost tolerance only in the snow mould-resistant line (343), which maintained the presence of a 50 kDa β-1,3-glucanase isoform and exhibited a flooding-induced decrease in cytosolic sucrose synthase (SuS) activity in leaves, while maintaining higher SuS activity in roots. In contrast, the less resistant line (620) lacked the 50 kDa isoform in roots and showed substantially lower SuS activity than line 343; however, leaf SuS activity increased during prolonged flooding to levels comparable with those of line 343. The 35 kDa isoform was no longer detectable in the leaves of either line after one or ten days of flooding, whereas in the roots of both lines, its accumulation was maintained after one day but was abolished after ten days of flooding. Additionally, line 620 exhibited prolonged flooding-induced upregulation of glu-8 and TLP3 genes, which, in the absence of protein accumulation, suggests post-transcriptional or translational constraints associated with prolonged stress. These findings demonstrate that higher frost tolerance under cold flooding involves the accumulation of specific pathogenesis-related (PR) proteins, maintaining a high reduced and oxidised glutathione ratio and metabolic adjustment of sucrose synthesis. Furthermore, cold acclimation alone did not differentiate frost tolerance between rye genotypes differing in snow mould resistance, whereas flooding under low-temperature conditions enhanced freezing tolerance only in the resistant line, indicating that flooding may function as a positive signal involved in frost tolerance induction.