The re-establishment of desiccation tolerance in germinated radicles of Medicago truncatula Gaertn. seeds

Germinated seeds of Medicago truncatula Gaertn. with a protruded radicle length of 2.7 mm did not survive drying below 0.2 g H2O g(-1) dw, as indicated by vital stain assays and the absence of growth resumption after rehydration. The re-establishment of desiccation tolerance was achieved using an osmotic treatment with polyethylene glycol (PEG), combined with a cold treatment. The ability to regain desiccation tolerance after germination was restricted to a period of growth characterized by radicle lengths between 1 and 3 mm. After PEG treatment of germinated seeds with 2.7 mm long radicles at -1.7 MPa at 10degreesC for 3 d and subsequent drying to 0.04 g H2O g(-1) dw, 90% survived and developed into normal seedlings after rehydration. Desiccation tolerance could also be re-established in excised radicles, demonstrating that cotyledons were not essential for this process. Upon PEG incubation, sucrose accumulated rapidly prior to the re-establishment of desiccation tolerance in germinated radicles, regardless of the presence of cotyledons. Induction of MtDHN (a dehydrin) gene expression was correlated with the re-establishment of desiccation tolerance. Furthermore, the PEG-induced expression of MtDHN was repressed when fluridone was added to the PEG solution.