Cryopreservation of Papaver somniferum Cell Suspension Cultures*

Cryopreservation of plant cell cultures - Diverse practices and protocols

Plant cell cultures can be used as biotechnological platforms for the commercial production of small-molecule active ingredients and recombinant proteins, such as biopharmaceuticals. This requires the cryopreservation of well-characterized cell lines as master cell banks from which uniform working cell banks can be derived to ensure high batch-to-batch reproducibility during production campaigns. However, the cryopreservation of plant cells is challenging due to their low viability and poor regrowth after thawing. Three approaches have been developed: slow freezing, vitrification, and encapsulation-dehydration. Typically, the protocols are iteratively adapted to accommodate the properties of different plant cell lines, taking time and resources while achieving moderate success. Since standardized processes are a prerequisite for industrial applications, this review presents an in-depth analysis of the different procedures for cryopreservation of plant suspension cell cultures, highlighting relevant parameters for effective cryopreservation and the re-establishment of vigorous plant cell cultures within weeks. The protocol variants are grouped into modules that facilitate the directed improvement of each step and allow protocol evolution by module recombination. Ultimately, such improved cryopreservation protocols will form the basis of processes that comply with good manufacturing practice and attract major biopharmaceutical companies to the benefits of plant molecular farming.

Initiation, growth and cryopreservation of plant cell suspension cultures

Methods described in this paper are confined to in vitro dedifferentiated plant cell suspension cultures, which are convenient for the large-scale production of fine chemicals in bioreactors and for the study of cellular and molecular processes, as they offer the advantages of a simplified model system for the study of plants when compared with plants themselves or differentiated plant tissue cultures. The commonly used methods of initiation of a callus from a plant and subsequent steps from callus to cell suspension culture are presented in the protocol. This is followed by three different techniques for subculturing (by weighing cells, pipetting and pouring cell suspension) and four methods for growth measurement (fresh- and dry-weight cells, dissimilation curve and cell volume after sedimentation). The advantages and disadvantages of the methods are discussed. Finally, we provide a two-step (controlled rate) freezing technique also known as the slow (equilibrium) freezing method for long-term storage, which has been applied successfully to a wide range of plant cell suspension cultures.

Effect of growth phase on survival of bromegrass suspension cells following cryopreservation and abiotic stresses

BACKGROUND AND AIMS

Cryopreservation is a practical method of preserving plant cell cultures and their genetic integrity. It has long been believed that cryopreservation of plant cell cultures is best performed with cells at the late lag or early exponential growth phase. At these stages the cells are small and non-vacuolated. This belief was based on studies using conventional slow prefreezing protocols and survival determined with fluorescein diacetate staining or 2,3,5-triphenyltetrazolium chloride assays. This classical issue was revisited here to determine the optimum growth phase for cryopreserving a bromegrass (Bromus inermis) suspension culture using more recently developed protocols and regrowth assays for determination of survival.

METHODS

Cells at different growth phases were cryopreserved using three protocols: slow prefreezing, rapid prefreezing and vitrification. Stage-dependent trends in cell osmolarity, water content and tolerance to freezing, heat and salt stresses were also determined. In all cases survival was assayed by regrowth of cells following the treatments.

KEY RESULTS

Slow prefreezing and rapid prefreezing protocols resulted in higher cell survival compared with the vitrification method. For all the protocols used, the best regrowth was obtained using cells in the late exponential or early stationary phase, whereas lowest survival was obtained for cells in the late lag or early exponential phase. Cells at the late exponential phase were characterized by high water content and high osmolarity and were most tolerant to freezing, heat and salt stresses, whereas cells at the early exponential phase, characterized by low water content and low osmolarity, were least tolerant.

CONCLUSIONS

The results are contrary to the classical concept which utilizes cells in the late lag or early exponential growth phase for cryopreservation. The optimal growth phase for cryopreservation may depend upon the species or cell culture being cryopreserved and requires re-investigation for each cell culture. Stage-dependent survival following cryopreservation was proportionally correlated with the levels of abiotic stress tolerance in bromegrass cells.