Changed properties of the cytoplasmic matrix associated with desiccation tolerance of dried carrot somatic embryos. An in situ fourier transform infrared spectroscopic study

Abscisic acid-pretreated carrot (Daucus carota) somatic embryos survive deh ydration upon slow drying, but fast drying leads to poor survival of the em bryos. To determine whether the acquisition of desiccation tolerance is ass ociated with changes in the physical stability of the cytoplasm, in situ Fo urier transform infrared microspectroscopy was used. Although protein denat uration temperatures were similar in the embryos after slow or fast drying, the extent of the denaturation was greater after fast drying. Slowly dried embryos are in a glassy state at room temperature, and no clearly defined glassy matrix was observed in the rapidly dried embryos. At room temperatur e the average strength of hydrogen bonding was much weaker in the rapidly d ried than in the slowly dried embryos. We interpreted the molecular packing to be "less tight" in the rapidly dried embryos. Whereas sucrose (Suc) is the major soluble carbohydrate after fast drying, upon slow drying the tris accharide umbelliferose accumulates at the expense of Sue. The possibly pro tective role of umbelliferose was tested on protein and phospholipid model systems, using Suc as a reference. Both umbelliferose and Suc form a stable glass with drying: They depress the transition temperature of dry liposoma l membranes equally well, they both prevent leakage from dry liposomes afte r rehydration, and they protect a polypeptide that is desiccation sensitive . The similar protection properties in model systems and the apparent inter changeability of both sugars in viable, dry somatic embryos suggest no spec ial role of umbelliferose in the improved physical stability of the slowly dried embryos. Also, during slow drying LEA (late-embryogenesis abundant) t ranscripts are expressed. We suggest that LEA proteins embedded in the glas sy matrix confer stability to these slowly dried embryos.