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

Endophytic bacterial community dynamics in sweet cherry in vitro shoot culture and their role in shoot adaptation after cryopreservation

BACKGROUND

In vitro cultivation and cryopreservation techniques are essential tools for genetic diversity conservation and pathogen-free plant propagation of horticultural crops. The optimisation of cryopreservation protocols typically focuses on minimising the negative effects of pretreatment with cryoprotectors (CPs), cryogenic freezing (CF) treatment, and recovery procedures on explants. However, the impact of in vitro and CF techniques on plant-associated microbiota remains poorly understood, and their potential to improve plant adaptation after cryopreservation is underexplored. The aim of the present study was to investigate in vitro shoot culture and cryopreservation-induced changes in the endophytic bacterial diversity of two sweet cherry cultivars and to assess the potential of an inoculum of bacterial isolates to improve the growth of shoot culture after CF.

RESULTS

Cultivars 'Sunburst' and 'Mindaugė' showed different responses to cold hardening preconditioning as well as different survival and regrowth rates after cryopreservation. Metataxonomic analysis revealed variation in the abundance and taxonomic composition of bacteria assigned to 35 families in samples of field-grown tree leaves, dormant buds, and in vitro shoot culture before and after CF treatment. Bacillaceae and Enterobacteriaceae bacteria were predominant in the leaf samples of both cultivars. For 'Sunburst', Pseudomonadaceae and Sphingomonadaceae bacteria were dominant in dormant buds and in vitro shoots, respectively, while Burkholderiaceae was largely predominant in the shoots following CF treatment. Conversely, 'Mindaugė' tissues exhibited more consistent colonisation by Bacillaceae and Enterobacteriaceae across the experimental groups, except for in vitro shoots where Mycobacteriaceae prevailed. A pure bacterial isolate inoculum was applied to the 'Mindaugė' shoot culture to counter the CF treatment-induced suppression of shoot growth (~ 40%). Cocultivation with Brevibacterium sp. S1-2, Bacillus cereus S1-3, or B. toyonensis Nt18 increased the shoot leaf area from 48 to 75%.

CONCLUSIONS

This study revealed that endophytic bacterial diversity is significantly reduced under in vitro conditions, often leading to a genotype-specific increase in the abundance and dominance of bacteria attributed to a single bacterial family. Moreover, shoot cocultivation with endophytic bacterial isolates has potential for improving the recovery of in vitro shoots after cryopreservation.

Improvement and Innovation of Cryopreservation and In Vitro Methods in Plant Resource Protection

Plant genetic resources (PGRs) are perhaps the most precious gift of nature to humanity: they provide food, shelter, medicines, and many goods of high economic value, not to mention their key importance for healthy ecosystems and their aesthetic value [...].

Comparative Studies for Cryopreservation of Agave Shoot Tips by Droplet-Vitrification

The objective of this work was to assess the suitability of the Droplet-vitrification protocol previously developed with Agave peacockii shoot tips for the cryopreservation of six Agave species. Shoot tips were precultured for 1 day on a medium with 0.3 M sucrose in the dark, loaded in a solution with 1.6 M glycerol and 0.4 M sucrose for 20 min, and dehydrated by exposure to Plant Vitrification Solution 2 (PVS2) at 0 °C for 20 min. Complementary studies using histological analysis, Differential scanning calorimetry (DSC), and evaluation of morphological characteristics in cryo-derived plants were performed. Survival rates ranged from 84% to 100% and from 76% to 97% before and after cryopreservation regardless of the Agave species belonging to two taxonomic subgenera. Thermal analysis of shoot tips subjected to the successive steps of the Droplet-vitrification protocol identified ice crystal formation after loading treatment and glass transition after osmotic dehydration with PVS2. The average glass transition temperature (Tg) was -55.44 °C based on the results of four Agave species. The histological studies showed the anatomical differences that could be found in the meristematic structures depending on the loss of apical dominance. This is the most advanced research on cryopreservation of Agave shoot tips.

Effect of Vitis vinifera zygotic embryo cryopreservation and post-cryopreservation on the gene expression of DNA demethylases

Grapevine (Vitis vinifera L.) crops are continuously exposed to biotic and abiotic stresses, which can cause genetic and epigenetic alterations. To determine the possible effects of grapevine cryopreservation on the regulation of DNA demethylase genes, this work studied the expression of DNA demethylase genes in cryopreserved and post-cryopreserved grapevine tissues. V. vinifera DNA demethylases were characterized by in silico analysis, and gene expression quantification was conducted by RT‒qPCR. Three DNA demethylase sequences were found: VIT_13s0074g00450 (VvDMT), VIT_08s0007g03920 (VvROS1), and VIT_06s0061g01270 (VvDML3). Phylogenetic analysis revealed that the sequences from V. vinifera and A. thaliana had a common ancestry. In the promoters of responsive elements to transcription factors such as AP-2, Myb, bZIP, TBP, and GATA, the conserved domains RRM DME and Perm CXXC were detected. These responsive elements play roles in the response to abiotic stress and the regulation of cell growth. These data helped us characterize the V. vinifera DNA demethylase genes. Gene expression analysis indicated that plant vitrification solution 2 (PVS2) treatment does not alter the expression of DNA demethylase genes. The expression levels of VvDMT and VvROS1 increased in response to cryopreservation by vitrification. Furthermore, in post-cryopreservation, VvROS1 was highly induced, and VvDML3 was repressed in all the treatment groups. Gene expression differences between different treatments and tissues may play roles in controlling methylation patterns during gene regulation in tissues stressed by cryopreservation procedures and in the post-cryopreservation period during plant growth and development.

Liquid Overlay-Induced Donor Plant Vigor and Initial Ammonium-Free Regrowth Medium Are Critical to the Cryopreservation of Scrophularia kakudensis

Cryopreservation, storing biological material in liquid nitrogen (LN, -196 °C), offers a valuable option for the long-term conservation of non-orthodox seeds and vegetatively propagated species in the sector of agrobiodiversity and wild flora. Although the large-scale cryobanking of germplasm collections has been increasing worldwide, the wide application of cryopreservation protocols in wild flora is hampered by difficulties in vitro propagation and a lack of universal cryopreservation protocols, among others. This study established a systematic approach to developing an in vitro culture and droplet-vitrification cryopreservation procedure for shoot tips of Scrophularia kakudensis. The standard procedure includes a two-step preculture with 10% sucrose for 31 h and with 17.5% sucrose for 16 h, osmoprotection with loading solution C4-35% (17.5% glycerol + 17.5% sucrose, w/v) for 30 min, cryoprotection with A3-80% (33.3% glycerol + 13.3% dimethyl sulfoxide + 13.3% ethylene glycol + 20.1% sucrose, w/v) at 0 °C for 60 min, and cooling and rewarming using aluminum foil strips. After unloading, a three-step regrowth procedure starting with an ammonium-free medium with growth regulators was essential for developing normal plantlets from cryopreserved shoot tips. Liquid overlay on the gelled medium two weeks after inoculation resulted in vigorous growth during subcultures. Moreover, liquid overlay increased LN regeneration by up to 80%, i.e., 23% higher than no liquid overlay.

Elimination of sugarcane mosaic virus, sugarcane yellow leaf virus, and co-infections in sugarcane (Saccharum spp. hybrids) shoot tips via osmo- and cryo-therapy

Cryopreservation for sugarcane (Saccharum spp. hybrids) germplasm conservation is well established. Virus elimination using droplet-vitrification (D-V) and cryo- or osmo-therapy has only been recently reported for sugarcane mosaic virus (SCMV). In this study, exposing large (3 mm) in vitro shoot tips of cultivars N12, N19, N58, and NCo376 infected with sugarcane yellow leaf virus (SCYLV) and NCo376 co-infected with SCMV and SCYLV were tested for virus elimination using both of the above-mentioned techniques. Cryo-therapy involved the exposure of infected in vitro shoot tips to the D-V protocol followed by recording recovery and virus-free shoot tips 16 wk after treatment. Osmo-therapy, consisting of the same treatment as cryo-therapy without immersion in liquid nitrogen (LN), was included for comparative purposes. Cryo-therapy resulted in 100% of the recovered shoots being SCYLV-free in cultivars N19, N58, and NCo376 and 83% in N12 when compared with untreated material. Osmo-therapy showed 58% (N12), 91% (N19 and N58), and 100% (NCo376) of shoots being clear of SCYLV when compared with untreated in vitro control plants (0 to 8%). Both techniques reduced the regrowth levels of treated shoot tips (22 to 57% recovery) when compared with untreated controls (92 to 97%). A novel finding of the study was that NCo376 co-infected with SCMV and SCYLV showed 100% virus-free recovered shoots after cryo-therapy and 92 to 100% of healthy shoots after osmo-therapy, compared with controls, which had 17 to 42% virus-free shoots. Plants from all cultivars that were re-tested 4 mo after hardening maintained their virus-free status. The described techniques for virus eradication offer a promising solution for the provision of clean vegetative planting propagules and safer germplasm exchange.

Plant Cryopreservation: Principles, Applications, and Challenges of Banking Plant Diversity at Ultralow Temperatures

Progressive loss of plant diversity requires the protection of wild and agri-/horticultural species. For species whose seeds are extremely short-lived, or rarely or never produce seeds, or whose genetic makeup must be preserved, cryopreservation offers the only possibility for long-term conservation. At temperatures below freezing, most vegetative plant tissues suffer severe damage from ice crystal formation and require protection. In this review, we describe how increasing the concentration of cellular solutes by air drying or adding cryoprotectants, together with rapid cooling, results in a vitrified, highly viscous state in which cells can remain viable and be stored. On this basis, a range of dormant bud-freezing, slow-cooling, and (droplet-)vitrification protocols have been developed, but few are used to cryobank important agricultural/horticultural/timber and threatened species. To improve cryopreservation efficiency, the effects of cryoprotectants and molecular processes need to be understood and the costs for cryobanking reduced. However, overall, the long-term costs of cryopreservation are low, while the benefits are huge.

Current status of the cryopreservation of embryogenic material of woody species

Cryopreservation, or the storage at liquid nitrogen temperatures (-196°C), of embryogenic cells or somatic embryos allows their long-term conservation without loss of their embryogenic capacity. During the last decade, protocols for cryopreservation of embryogenic material of woody species have been increasing in number and importance. However, despite the large experimental evidence proved in thousands of embryogenic lines, the application for the large-scale conservation of embryogenic material in cryobanks is still limited. Cryopreservation facilitates the management of embryogenic lines, reducing costs and time spent on their maintenance, thus limiting the risk of the appearance of somaclonal variation or contamination. Somatic embryogenesis in combination with cryopreservation is especially useful to preserve the juvenility of lines while the corresponding clones are being field-tested. Hence, when tree performance has been evaluated, selected varieties can be propagated from the cryostock. The traditional method of slow cooling or techniques based on vitrification are mostly applied procedures. For example, slow cooling methods are widely applied to conserve embryogenic lines of conifers. Desiccation based procedures, although simpler, have been applied in a smaller number of species. Genetic stability of the cryopreserved material is supported by multiloci PCR-derived markers in most of the assayed species, whereas DNA methylation status assays showed that cryopreservation might induce some changes that were also observed after prolonged subculture of the embryogenic lines. This article reviews the cryopreservation of embryogenic cultures in conifers, fruit species, deciduous forest species and palms, including a description of the different cryopreservation procedures and the analysis of their genetic stability after storage in liquid nitrogen.

The Cryopreservation of Medicinal and Ornamental Geophytes: Application and Challenges

Nowadays, plant genetic resources are often at risk of loss and destruction. Geophytes are herbaceous or perennial species that are annually renewed by bulbs, rhizomes, tuberous roots, or tubers. They are often subject to overexploitation, which, combined with other biotic and abiotic stresses, can make these plants more vulnerable to a decline in their diffusion. As a result, multiple endeavors have been undertaken to establish better conservation strategies. Plant cryopreservation at ultra-low temperatures in liquid nitrogen (-196 °C) has proven to be an effective, long-term, low-cost, and suitable conservation method for many plant species. Over the last two decades, major advances in cryobiology studies have enabled successful explants of multiple genera and types, including pollen, shoot tips, dormant buds, and zygotic and somatic embryos. This review provides an update on recent advances and developments in cryopreservation and its application to medicinal and ornamental geophytes. In addition, the review includes a brief summary of factors limiting the success of bulbous germplasm conservation. The critical analysis underpinning this review will benefit biologists and cryobiologists in their further studies on the optimization of geophyte cryopreservation protocols and will support a more complete and wider application of knowledge in this area.