Comparative studies on tolerance of Medicago truncatula and Medicago falcata to freezing

Medicago falcata is a legume species that exhibits great capacity of tolerance to abiotic stresses. To elucidate the mechanism underlying tolerance of M. falcata to freezing, we compared the characteristics of M. falcata in response to cold acclimation and freezing with those of the legume model plant Medicago truncatula. M. falcata seedlings were more tolerant to freezing than M. truncatula, as evidenced by a lower value of EL(50) (temperature at which 50% electrolyte leakage after freezing) and greater survival rate for M. falcata than M. truncatula. Cold acclimation led to greater reduction in EL(50) for M. falcata than M. truncatula. Sucrose was the most abundant sugar in both M. falcta and M. truncatula, and a greater accumulation of sucrose and Pro in M. falcata than in M. truncatula during cold acclimation was observed. Cold acclimation induced small amounts of raffinose and stachyose in M. falcata, but not in M. truncatula. The activities of sucrose phosphate synthase and sucrose synthase were greater in M. falcata than in M. truncatula. In contrast, the activity of acid invertase was higher in M. truncatula than in M. falcata. There was an increase in transcript of CRT binding factor (CBF) upon exposure to low temperature in the two species. The low temperature-induced increase in transcript of CBF2 was much higher in M. truncatula than in M. falcata, while transcript of CBF3 in M. falcata was greater than that in M. truncatula. There were sustained increases in transcripts of cold acclimation specific (CAS), a downstream target of CBF, during cold acclimation and the increases were greater in M. falcata than in M. truncatula. These results demonstrate that accumulation of greater amounts of soluble sugars coupled with higher CBF3 and CAS transcript levels in M. falcata may play a role in conferring greater tolerance of M. falcata to freezing than that of M. truncatula.