Collections of common potato maintained in the field genebanks suffer significant losses due to the impact of extreme environmental factors, diseases and pests. The solution of the problem of safe long-term preservation of common potato accessions is to create doublet in vitro and cryo-collections. Cryogenic collections are stored at ultra-low temperatures in cryobanks. Several methods of potato cryoconservation are known, of which the droplet vitrification method developed by B. Panis with colleagues in 2005 is the most widely used in genebanks. This paper provides a detailed description of the modified method of droplet vitrification, which is used for cryopreservation of apexes (shoot tips) of potato in vitro plants at the N.I. Vavilov All-Russian Institute of Plant Genetic Resources (VIR). The method modified at VIR includes the main steps of the original droplet-vitrification method developed by B. Panis and colleagues: 1) preparation of plant material, 2) isolation of shoot tips, 3) treatment of explants with cryoprotector solutions, 4) freezing/immersion in liquid nitrogen, 5) thawing, 6) post-cryogenic recovery and evaluation of viability and regeneration capacity. The modifications of stages 1, 2 and 6 proposed at VIR lead to a significant reduction in the duration of cryopreservation experiments in comparison with the original method of B. Panis. This paper presents the results of cryopreservation of modern potato cultivars and South American landraces which were obtained using the method of droplet vitrification as modified at VIR. The majority (76.7 %) of the studied accessions of cultivated potato were characterized by high rates of postcryogenic recovery (40–95 %) and 23.3 % of the samples had the values of postcryogenic regeneration from 20 to 39 %, which corresponds to the minimal permissible values for long-term storage in a cryobank. Currently the modified droplet-vitrification method is used for further expanding of the VIR potato cryocollection.
Cryopreservation provides long-term storage of the gene pool of potato varieties in cryobanks at extremely low temperatures. Currently, droplet vitrification is the most widely used method for cryopreservation of potato varieties, which is constantly improving to increase the regeneration rates of the stored plant material. Different modifications of this method are used in the world’s leading potato genebanks. This paper presents the results of studying the effect of cultivation conditions after plunging into liquid nitrogen and thawing of shoots tips and axillary buds of in vitro plants on their postcryogenic recovery. The droplet-vitrification method modified at VIR was used for cryopreservation. The factor “prolonged dark incubation of explants” did not have a significant effect on the frequency of post-cryogenic regeneration of the studied varieties except for one variety (Krepysh), for which a significant increase in the regeneration rate was observed for the shoot tips cultivated in the darkness compared to the cultivation under the photoperiod 16/8 hours (light/darkness). The frequency of post-cryogenic regeneration of shoot tips was higher than that of the axillary buds for all varieties; however, these differences were significant (p < 0.05) only in two cases: for the variety Udacha (a photoperiod of 16/8 hours) and for the variety Krepysh (the dark incubation). The results of two-factor analysis of variance indicate that there is no effect of interaction of factor 1 (prolonged dark incubation) and factor 2 (explant type) on the ability of varieties to post-cryogenic recovery. Taking into account the obtained results, the further cryopreservation of an extended subset of 9 varieties was carried out using shoot tips, which, after freezing-thawing, were cultivated under the photoperiod of 16/8 hours. The frequency of post-cryogenic regeneration of these varieties varied from 30 to 60 %. A significant effect of genotype on postcryogenic recovery has been established. The ability of varieties to regenerate shoots after freezing and thawing was not related to the values of morphogenic indices of in vitro plants. The age of the meriklons (2–4 years) did not significantly affect either the morphogenic indices or the frequency of post-cryogenic regeneration.
Cryobanks use plant cryocollections for long-term preservation of crops which cannot be preserved in seed collections. These are vegetatively propagated crops, accessions of species which form either a small amount of seeds, or recalcitrant seeds. Shoot tips (apexes) of in vitro plants are used for cryopreservation for most berry crops, therefore maintenance of in vitro collections is very important. The VIR in vitro collection includes 150 accessions of Rubus L. species, 85 of them are raspberry cultivars, 59 of which were bred in Russia. These cultivars reflect a wide ecogeographic diversity. Among them, there are raspberry cultivars created at the end of the 19th – first half of the 20th centuries, including cultivars bred by I.V. Michurin and by the pioneer of northern horticulture V.V. Spirin. More than half of national raspberry varieties (33) are listed in the State Register for Selection Achievements Admitted for Usage. Raspberry cultivars from Russian breeding programs have a very limited representation in foreign genebanks. The first aim of the present work was cryopreservation of mostly folk and old Russian raspberry cultivars received by VIR from 1925 till 1950 and their transfer into the cryobank. The second aim of the work was to monitor post-cryogenic regeneration of raspberry cultivars transferred to the cryobank earlier. A modified protocol of the droplet vitrification method by “DV-biotech” was used for cryopreservation of shoot tips of in vitro plants of 10 raspberry cultivars (7 of which are folk and old Russian ones) from the VIR in vitro collection. Post-cryogenic regeneration was evaluated for 17 raspberry cultivars preserved in the cryobank from one to five years. Ten raspberry cultivars (900 apexes) with an average mean post-cryoregenic regeneration value of 38.2±3.0% determined in control tests, were placed in the cryobank for long-term storage. A statistically significant effect of the genotype on the viability of explants after cryopreservation was noted, while the post-cryogenic regeneration was genotype insensitive. Additionally, levels of post-cryogenic regeneration were evaluated for 17 raspberry cultivars (296 apexes) preserved in the cryobank from one to five years. Post-cryogenic regeneration within the 20-70% range was displayed by four raspberry cultivars preserved in the cryobank for one year, and for 8 cultivars conserved there from three to five years post-cryogenic regeneration was within the 10-50% range. According to the results of monitoring, regeneration displayed by 12 raspberry cultivars was within the 10-70% range, which can be considered as a reliable rate of apex preservation in liquid nitrogen vapors in the VIR cryobank. Monitoring of the post-cryogenic regeneration of the raspberry accessions preserved in the VIR cryobank and cryopreservation of new raspberry cultivars will be continued.
Gusar', 'Evpatij', 'Solnečnyj', 'Tango')* had low post-cryogenic regenerative capacity from 20 to 30%; the regeneration rate exceeded 30% in 11 accessions, and 8 cultivars ('Grand', 'Zlatka', 'Lina', 'Safo', 'Siverskij', 'Signal', 'Utro', 'Ûna') and 'Аlyj Parus' breeding clone had regeneration rate above 40%. The regeneration rate in the studied subset was genotype independent according to the ANOVA results (p=0.711). Viability and regeneration rate were significantly correlated (r=0.86). As a result of the experiments, explants of 14 modern cultivars and two breeding clones with the known post-thaw regeneration rate were successfully cryopreserved in the VIR cryobank. Four cultivars ('Grand', 'Gusar', 'Signal', 'Utro') were monitored for their regeneration capacity after the long-term (seven months) preservation in the VIR cryobank. On an average, these four cultivars demonstrated a post-thaw regeneration capacity of 41.8%. It can be concluded that the use of the modified method of droplet vitrification is relevant for increasing the VIR potato cryo-collection.
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