Freezing tolerance of alfalfa-rhizobia symbiosis, a shared responsibility between the plant and the microbe.

Freeze-thaw episodes during winter are damaging, compromising the persistence of perennial alfalfa (Medicago sativa L.), and are expected to be more frequent due to climate change. Symbiosis with cold-tolerant rhizobia strains can improve the plant ability to survive and grow under stressful conditions. To better understand the adaptation of alfalfa-rhizobia associations to northern environment, we measured biochemical compounds typically involved in stress tolerance and/or in plant-rhizobium crosstalk in roots, crowns, and nodules of alfalfa under simulated winter and spring conditions. Two alfalfa populations differing in freezing tolerance (A-TF0 and A-TF7) were inoculated with two Sinorhizobium (Ensifer) meliloti strains (B399 and NRG34) of contrasted adaptation to cold. Plants were grown for eight weeks under controlled conditions before being cold acclimated (CA) and subsequently exposed to a sublethal freezing stress (-11ºC) and regrown 3 wks under spring conditions. After the 3-wk regrowth, the yield of the A-TF7 × NRG34 association was 35% larger than that of the A-TF0 × B399 association demonstrating a superior freezing tolerance for the former association. Both the plant population and the rhizobia strain contributed to the vigor of regrowth after freezing. Strain-induced metabolic changes were mostly detected in nodules while plant population induced metabolic changes mostly in crowns. Marked increases in stachyose and raffinose (RFO) concentrations were observed in crowns in response to CA supporting the importance of a protective role for RFO in alfalfa. Both cold-adapted partners of the symbiotic association contributed to increases in arginine concentration in nodules in response to cold acclimation and deacclimation underscoring the importance of N storage and remobilization for a successful overwintering in alfalfa. Gaining knowledge on the roles played by each member of alfalfa-rhizobia associations in the acquisition of freezing tolerance and deacclimation will help understand and improve alfalfa winter survival and spring regrowth in Canada.