Low temperature effects on growth and actin cytoskeleton organisation in suspension cells of winter oilseed rape

Rhodamine-phalloidin staining of winter oilseed rape suspension cells revealed that the structure of actin cytoskeleton changes with the phase of cell growth. In small, 4-day-old cells, entering the exponential phase of growth, a dense and uniformly distributed cortical microfilament networks was seen. In six-day-old vacuolated cells, which reached the stationary phase of growth, the actin cytoskeleton was composed of thicker microfilament cables in irregular arrangements. In cells acclimated in cold for 7 days a dense, uniformly distributed and cortical microfilament network was still seen. The fine microfilament network was sensitive to extracellular freezing since the structures underwent depolymerization at -3 degreesC (in the presence of extracellular ice), both in non-acclimated and cold-acclimated cells. The thicker transvacuolar cables in cells of the stationary growth phase resisted freezing to -7 degreesC. Acclimation of suspensions at 2 degreesC resulted in slowing down growth of cells and in the increased freezing tolerance of cells as indicated by a decrease of LT(5)0 from -11 degreesC to -17.5 degrees or to -25 degreesC when determined 7 or 20 days after the beginning of the cold treatment, respectively. Freezing tolerance of non-acclimated cells decreased from -11 degreesC to -8 degreesC during subculture, showing a transient increase to -17 degreesC on the day 6. Results indicate that the arrangement of actin microfilaments and their sensitivity to freezing-induced depolymerization depends on the phase of cell growth rather than on cell acclimation status. Possible mechanisms involved in the freezing-induced depolymerization of actin microfilaments are discussed.