Simplified cryopreservation of sweet potato [ Ipomoea batatas (L.) Lam.] by optimizing conditions for osmoprotection

Critical Role of Regrowth Conditions in Post-Cryopreservation of In Vitro Plant Germplasm

Cryopreservation is an effective option for the long-term conservation of plant genetic resources, including vegetatively propagated crops and ornamental plants, elite tree genotypes, threatened plant species with non-orthodox seeds or limited seed availability, as well as cell and root cultures useful for biotechnology. With increasing success, an arsenal of cryopreservation methods has been developed and applied to many species and material types. However, severe damage to plant material accumulating during the multi-step cryopreservation procedure often causes reduced survival and low regrowth, even when the optimized protocol is applied. The conditions at the recovery stage play a vital role in supporting material regrowth after cryopreservation and, when optimized, may shift the life-and-death balance toward a positive outcome. In this contribution, we provide an overview of the five main strategies available at the recovery stage to improve post-cryopreservation survival of in vitro plant materials and their further proliferation and development. In particular, we discuss the modification of the recovery medium composition (iron- and ammonium-free), exogenous additives to cope with oxidative stress and absorb toxic chemicals, and the modulation of medium osmotic potential. Special attention is paid to plant growth regulators used at various steps of the recovery process to induce the desired morphological response in cryopreserved tissues. Given studies on electron transport and energy provision in rewarmed materials, we discuss the effects of light-and-dark conditions and light quality. We hope that this summary provides a helpful guideline and a set of references for choosing the recovery conditions for plant species that have not been cryopreserved. We also propose that step-wise recovery may be most effective for materials sensitive to cryopreservation-induced osmotic and chemical stresses.

Development of a fast and user-friendly cryopreservation protocol for sweet potato genetic resources

Sweet potato (Ipomoea batatas) is one of the ten most important staple crops and provides a livelihood for many people around the globe. To adapt to ever-changing circumstances farmers and breeders need to have access to a broad diversity of germplasm. This study focuses on the development of a cryopreservation protocol that allows the long term storage of different sweet potato cultivars. For this, a droplet vitrification protocol was optimized, comparing several parameters; preculture method (0.3 M sucrose vs no preculture); meristem position (axillary vs apical); plant age (3 to 9 weeks); regeneration medium (MS + 2.22 µM BA, Hirai and MS); and length of loading solution treatment (20 to 360 min). Two months after cryopreservation, the regeneration rates of the meristems were compared, which resulted in significant differences for the preculture method, meristem position and loading solution. With these new insights an optimized droplet vitrification protocol was developed with the following parameters: use of 3-9 week old axillary meristems, no preculture phase, 20 min LS treatment, 30 min PVS2 treatment, exposure to liquid nitrogen by droplet vitrification, warming treatment in RS for 15 min, 1 day 0.3 M sucrose recuperation culture, 1 month MS + 2.22 µM BA followed by 1 month of MS cultures. This protocol was subsequently tested on 10 representative accessions resulting in a post cryopreservation regeneration rate of more than 40% for 70% of the tested cultivars, showing that this protocol could be implemented for a large portion of existing sweet potato collections.

Effects of trimethylamine oxide (TMAO) and loading duration on the shoot tip cryopreservation of loquat (Eriobotrya japonica)

Two cryoprotectant solutions, including trimethylamine oxide (TMAO) and dimethyl sulfoxide (DMSO), and several loading durations were used to evaluate the cryopreservation of the shoot tip of Eriobotrya plants. The best results for regrowth (59.91%) were obtained from 10% TMAO compared to 10% DMSO as cryoprotectant, although nonsignificant differences were found for survival between the two cryoprotectants. We detected pronounced effects of loading duration on survival and regrowth rates of shoot tips. The maximum regrowth (56.36%) was observed at 9 h of loading duration. The cryoprotectants and loading durations greatly affected the regrowth of Eriobotrya shoot tips, and TMAO could be introduced as a nontoxic and efficient cryoprotectant. These results could lay a foundation for the cryopreservation of Eriobotrya.

Cryopreservation of shoot tips of recalcitrant and tropical species: Advances and strategies

There is a pressing need for practical and successful conservation efforts to establish long-term germplasm collections of recalcitrant and tropical species, given the challenge and threat that these plants are facing. Cryopreservation is the only way of conserving some of these species, especially those with temperature or desiccation sensitive (recalcitrant) seeds. This review covers reports on cryopreservation studies of shoot tips (apical and axillary) of tropical and subtropical plants. Since many of these species have recalcitrant seeds, the cryopreservation successes, failures and problems involved with these seeds are also discussed. The methodologies, important factors and steps involved in successful cryopreservation protocols are analyzed. Finally strategies are suggested to develop a successful cryopreservation protocol for new plant species, in particular those with tropical recalcitrant seeds.

Elimination of two viruses which interact synergistically from sweetpotato by shoot tip culture and cryotherapy

Sweet potato chlorotic stunt virus (SPCSV; Closteroviridae) and Sweet potato feathery mottle virus (SPFMV; Potyviridae) interact synergistically and cause severe diseases in co-infected sweetpotato plants (Ipomoea batatas). Sweetpotato is propagated vegetatively and virus-free planting materials are pivotal for sustainable production. Using cryotherapy, SPCSV and SPCSV were eliminated from all treated single-virus-infected and co-infected shoot tips irrespective of size (0.5-1.5mm including 2-4 leaf primordia). While shoot tip culture also eliminated SPCSV, elimination of SPFMV failed in 90-93% of the largest shoot tips (1.5mm) using this technique. Virus distribution to different leaf primordia and tissues within leaf primordia in the shoot apex and petioles was not altered by co-infection of the viruses in the fully virus-susceptible sweetpotato genotype used. SPFMV was immunolocalized to all types of tissues and up to the fourth-youngest leaf primordium. In contrast, SPCSV was detected only in the phloem and up to the fifth leaf primordium. Because only cells in the apical dome of the meristem and the two first leaf primordia survived cryotherapy, all data taken together could explain the results of virus elimination. The simple and efficient cryotherapy protocol developed for virus elimination can also be used for preparation of sweetpotato materials for long-term preservation.

Cryopreservation of in vitro-grown shoot tips of raspberry (Rubus idaeus L.) by encapsulation-vitrification and encapsulation-dehydration

The first efficient cryopreservation procedure for in vitro-grown shoot tips of raspberry (Rubus idaeus L.) has been developed based on encapsulation-vitrification (EnVi) and encapsulation-dehydration (EnDe). EnVi resulted in higher survival (85%) and regrowth (75%) of cryopreserved shoot tips than EnDe (65 and 50%, respectively). In both cryogenic procedures, shoots regenerated from cryopreserved shoot tips without intermediary callus formation. Histological studies showed that a much larger number of meristematic cells survived following EnVi than EnDe. The EnVi procedure was applied to seven raspberry genotypes with an average survival and regrowth of 71 and 68%, respectively. Regenerated plants showed normal morphology. Results here indicate EnVi as a simple and efficient method for long-term preservation of R. idaeus germplasm.