Plant vitrification solution 2 lowers water content and alters freezing behavior in shoot tips during cryoprotection

Osmotic priming-induced cryotolerance uncovers rejuvenation of grapevine cell cultures: morphogenetic changes and gene expression pattern highlighting enhanced embryogenic potential

Cryopreservation is a reliable technique for the long-term storage and preservation of embryogenic cells, maintaining their viability without loss of their embryogenic capacity. However, the large-scale conservation of grapevine embryogenic lines in cryobanks remains limited. A significant challenge is understanding somatic cell rejuvenation. Here, we investigate the encapsulation/dehydration and encapsulation/vitrification for cryopreserving embryogenic material. Cell rejuvenation and enhanced embryogenic competence were observed after cryopreservation, as evidenced through structural cellular changes observed by histology and electron scanning microscopy. Results showed that cryopreserved samples of 110-Richter, Riesling, and Tempranillo using encapsulation/dehydration had better survival rates, averaging 81%, 62%, and 48%, respectively, while encapsulation/vitrification yielded lower survival rates, averaging 58%, 42%, and 32%, respectively. Cryopreservation also improved post-thaw recovery and regeneration efficiency assessed through regrowth of proembryogenic masses and somatic embryo conversion reaching 54-72% against 11-17% in control samples. Cryopreservation triggered changes in gene expression patterns and exhibited considerable increase at genotype-specific basis of 1.5- to 4.5-fold in SERK1, BBM, and WOX associated to embryogenic competence as well as in ChitIV and LEA involved in stress response. Membrane stability index, hydrogen peroxide, and proline contents were used as indicators of oxidative stress uncovering a key role of an osmotic trans-priming effect leading to cryotolerance. Our finding highlighted that cryopreservation enhances embryogenic capacity in senescent callus and probably acts as a screening process allowing safe maintenance of proembryogenic cells and promoting their recovery. This study provides a high throughput innovation to set up cryolines for cell rejuvenation of grapevine and other important plant species.

Direct evidence that cryoprotectant mixtures facilitate individual component permeation into living plant cells

The fundamental interactions between plant cells and cryoprotectants during vitrification are understudied in the field of plant cryopreservation. Within this area of research, real time cryoprotectant permeation into plant cells is even less documented. In this study, we monitor the real time permeation of individual cryoprotectants into rice callus cells when in mixtures with other cryoprotectants. Specifically, we use coherent anti-Stokes Raman scattering (CARS) microscopy to observe the permeation of individually deuterated DMSO, ethylene glycol, and glycerol in plant vitrification solution 2 (PVS2) by probing vibrational frequencies that correspond to C-D stretching modes of the cryoprotectant molecules. Additionally, we measure cell plasma membrane responses to PVS2 exposure using brightfield microscopy. We conclude that the permeation of PVS2 components into plant cells occurs faster than the first cell plasma membrane responses observed and therefore permeation and cell plasma membrane response do not appear to be directly correlated. In addition, we observe that cryoprotectant permeation into plant cells occurs more quickly and more uniformly when cryoprotectants are in PVS2 solution than when they are in single component aqueous solutions.

Analysis of Thermal Characteristics of Potato and Hop Pollen for Their Cryopreservation and Cross-Breeding

This study investigated the thermal properties of potato and hop pollen for cryopreservation and subsequent cross-breeding. Phase transitions and frozen water content in selected pollen samples were measured using a differential scanning calorimeter (DSC). Unlike hop pollen, potato pollen showed high variability in thermal properties and water content. Three specific types of pollen samples based on their thermal characteristics and water content were distinguished by DSC in potato: (1) 'glassy', with a water content lower than 0.21 g water per g dry matter; (2) 'transient', with a water content between 0.27 and 0.34 g of water per g of dry matter; (3) 'frozen', with a water content higher than 0.34 g of water per g of dry matter. Only the 'glassy' pollen samples with a low water content showed suitable properties for its long-term storage using cryopreservation in potato and hops. Cryopreservation of pollen did not significantly reduce its viability, and cryopreserved pollen was successfully used to produce both potato and hop hybrids. The results indicate that cryopreservation is a feasible technique for the preservation and utilization of pollen of these crops in the breeding process.

A droplet vitrification cryopreservation protocol for conservation of hops (Humulus lupulus) genetic resources

Hops (Humulus lupulus L.) is essentially used in the brewing industry as it contributes to flavor, and aroma of beer. However, the genetic diversity of hops is increasingly threatened by diseases, environmental changes, and urbanization. Cryopreservation has emerged as a pivotal strategy for safeguarding and maintaining the genetic diversity of hops. The present work presents a comprehensive study on the cryopreservation of hops, focusing on the development and optimization of a droplet vitrification based cryopreservation protocol. Shoot tips excised from one month old in vitro cultures were precultured on 0.3 M sucrose, dehydrated in a loading solution followed by treatment with PVS2 solution for different durations. Significant effect of PVS2 dehydration was observed on post-thaw survival and regeneration after cryoconservation with maximum 50% post-thaw regeneration observed in shoot tips dehydrated in PVS2 for 30 min. Genetic fidelity of the regenerated plants was confirmed using 30 ISSR markers. Reproducibility of the developed protocol was tested on seven other accessions and post thaw regeneration ranging from 43 to 70% was observed across the accessions. The present study reports a highly efficient protocol for conservation of hops germplasm. The results indicate that droplet vitrification can be used as a reliable and sustainable approach for hop genetic preservation, with high survival rates and minimal genetic alterations observed in cryopreserved samples. To the best of our knowledge, this is the first report on DV based cryopreservation of hops germplasm.

Enhancing cryopreservation survival in Petunia × Calibrachoa 'Light Yellow' callus: Insights into material characteristics and genetic integrity

Petunia × Calibrachoa 'Light Yellow' (× Petchoa 'Light Yellow') is a kind of perennial herbaceous flower obtained through intergeneric hybridization of Petunia and Calibrachoa with high ornamental value and wide application, facing challenges in seed acquisition. Expanding propagation through tissue culture is an economically efficient means. Hence, establishing an effective procedure for the storage of callus is essential for × Petchoa 'Light Yellow'. Cryopreservation is an effective method for the in vitro propagation and long-term preservation of × Petchoa 'Light Yellow' germplasms. For formulating the optimization of the vitrification procedure, first, an orthogonal experimental design was employed to pinpoint critical steps in the vitrification protocol (pre-culture, osmoprotection, dehydration, and dilution) for Petunia × Calibrachoa callus tissues and then five additional factors (pre-culture, osmoprotection I and II, dehydration, and dilution) were optimized to further reduce the sample water content and enhance cell viability levels. The vitrification procedure was described as follows: callus tissues were precultured in MS solid medium with 0.3 M sucrose for 5 d, incubated with osmoprotection solution I and II for 15 min at 25 °C, respectively, cryoprotected with PVS2 for 30 min at 0 °C, and rapidly immersed in liquid nitrogen. Cryopreserved callus tissues were then diluted in MS liquid medium with 1.2 M sucrose for 20 min at 25 °C and recovered on MS solid medium with 0.5 mg/L 6-BA and 0.1 mg/L NAA, and sucrose. The cell viability measured by TTC staining was approximately 16 %-18 % after 72 h-recovery. Following 45 days, the relative survival of callus reached up to 49.48 %. Furthermore, EST-SSR analysis showed no significant difference in the genetic stability of cryopreserved callus compared to the control. Based on the cryopreservation of × Petchoa 'Light Yellow' callus, we further evaluated the response of callus water contents to the osmotic stress in the optimized and original protocols (CK) for a higher cryopreservation survival. A comparative analysis of water content demonstrated that the procedure of gradual and gentle dehydration significantly improved water content and cell survival. Ultrastructural changes between cryopreserved and non-cryopreserved callus were examined and high vacuolation emerged as a key determinant, indicating its substantial impact on the low survival of cryopreserved cells, which should help us to understand the effectiveness of osmotic protectants in dehydration.

Conservation of Green and White Ash Germplasm Using the Cryopreservation of Embryogenic Cultures

Green ash (Fraxinus pennsylvanica) and white ash (F. americana) populations are currently experiencing major declines across their native ranges in North America due to infestation by the exotic insect pest emerald ash borer (Agrilus planipennis). The development of a reliable method for the long-term storage of green and white ash germplasm in the form of embryogenic cultures using cryopreservation would be a considerable aid to ash conservation efforts. We compared recovery percentages of cryopreserved green and white ash embryogenic cultures using vitrification versus slow cooling methods. Three Plant Vitrification Solution 2 (PVS2) exposure durations (40, 60, and 80 min) for vitrification and three DMSO concentrations (5%, 10%, and 15%) for slow cooling were tested for their effects on the percentage of cultures that regrew following cryostorage. Vitrification resulted in a higher overall culture recovery percentage (91%) compared to cultures that were cryostored using the slow cooling approach (39%), and a more rapid initiation of regrowth (5 days versus 2-3 weeks) resulted. Recovery from cryostorage by cultures using the slow cooling approach varied significantly (p < 0.05) between experiments and with genotype (p < 0.05). The recovery of vitrified tissue from cryostorage did not vary with genotype, species, or PVS2 exposure duration (p > 0.05). The vitrification cryopreservation protocol provides a reliable and versatile alternative to the traditional slow cooling method, strengthening our ability to preserve valuable ash germplasm for conservation and restoration.

Cryopreservation of Duckweed Genetic Diversity as Model for Long-Term Preservation of Aquatic Flowering Plants

Vegetatively propagating aquatic angiosperms, the Lemnaceae family (duckweeds) represents valuable genetic resources for circular bioeconomics and other sustainable applications. Due to extremely fast growth and laborious cultivation of in vitro collections, duckweeds are an urgent subject for cryopreservation. We developed a robust and fast DMSO-free protocol for duckweed cryopreservation by vitrification. A single-use device was designed for sampling of duckweed fronds from donor culture, further spin-drying, and subsequent transferring to cryo-tubes with plant vitrification solution 3 (PVS3). Following cultivation in darkness and applying elevated temperatures during early regrowth stage, a specific pulsed illumination instead of a diurnal regime enabled successful regrowth after the cryopreservation of 21 accessions of Spirodela, Landoltia, Lemna, and Wolffia genera, including interspecific hybrids, auto- and allopolyploids. Genome size measurements revealed no quantitative genomic changes potentially caused by cryopreservation. The expression of CBF/DREB1 genes, considered as key factors in the development of freezing tolerance, was studied prior to cooling but was not linked with duckweed regrowth after rewarming. Despite preserving chlorophyll fluorescence after rewarming, the rewarmed fronds demonstrated nearly zero photosynthetic activity, which did not recover. The novel protocol provides the basis for future routine application of cryostorage to duckweed germplasm collections, saving labor for in vitro cultivation and maintaining characterized reference and mutant samples.

Tracking Permeation of Dimethyl Sulfoxide (DMSO) in Mentha × piperita Shoot Tips Using Coherent Raman Microscopy

Cryopreservation has emerged as a low-maintenance, cost-effective solution for the long-term preservation of vegetatively propagated crops. Shoot tip cryopreservation often makes use of vitrification methods that employ highly concentrated mixtures of cryoprotecting agents; however, little is understood as to how these cryoprotecting agents protect cells and tissues from freezing. In this study, we use coherent anti-Stokes Raman scattering microscopy to directly visualize where dimethyl sulfoxide (DMSO) localizes within Mentha × piperita shoot tips. We find that DMSO fully penetrates the shoot tip tissue within 10 min of exposure. Variations in signal intensities across images suggest that DMSO may interact with cellular components, leading to its accumulation in specific regions.

Comparative Cryopreservation of Indian Wild Orange (Citrus Indica Tanaka) Embryonic Axes

BACKGROUND: Indian Wild Orange (Citrus indica Tanaka) is an endangered and endemic species from northeast India for which effective ex situ conservation strategies, including embryo cryopreservation, are urgently needed. MATERIALS AND METHODS: Desiccation tolerance and cryopreservation ability for embryonic axes of Citrus indica was determined using three techniques (air desiccation-freezing, PVS2 vitrification-freezing and encapsulation-dehydration-freezing). Success was assessed as survival and recovery in vitro. RESULTS: Successful cryopreservation of embryonic axes was achieved using all three methods, with the highest survival achieved when using air desiccationfreezing (90%) followed by encapsulation-dehydration (85%) and PVS2 vitrification cryopreservation (80%). Regeneration levels were lower than survival levels for all three proceedures. Post-cryo regeneration success was: encapsulation-dehydration (64%) > air desiccation-freezing (55%) > PVS2 vitrification (52%). CONCLUSION: Although there was relatively high post-cryopreservation recovery growth obtained using all the three techniques, the air desiccation-freezing technique is preferred, as it is a simple, practical and reproducible technique for the long-term cryobanking of this important wild species.

Seed Storage Physiology of Lophomyrtus and Neomyrtus, Two Threatened Myrtaceae Genera Endemic to New Zealand

There is no published information on the seed germination or seed storage physiology of Lophomyrtus bullata, Lophomyrtus obcordata, and Neomyrtus pedunculata. This lack of information is hampering conservation efforts of these critically endangered species. This study investigated the seed morphology, seed germination requirements, and long-term seed storage methods for all three species. The impact of desiccation, desiccation and freezing, as well as desiccation plus storage at 5 °C, -18 °C, and -196 °C on seed viability (germination) and seedling vigour was assessed. Fatty acid profiles were compared between L. obcordata and L. bullata. Variability in storage behaviour between the three species was investigated through differential scanning calorimetry (DSC) by comparing thermal properties of lipids. L. obcordata seed were desiccation-tolerant and viability was retained when desiccated seed was stored for 24 months at 5 °C. L. bullata seed was both desiccation- and freezing-sensitive, while N. pedunculata was desiccation-sensitive. DSC analysis revealed that lipid crystallisation in L. bullata occurred between -18 °C and -49 °C and between -23 °C and -52 °C in L. obcordata and N. pedunculata. It is postulated that the metastable lipid phase, which coincides with the conventional seed banking temperature (i.e., storing seeds at -20 ± 4 °C and 15 ± 3% RH), could cause the seeds to age more rapidly through lipid peroxidation. Seeds of L. bullata, L. obcordata and N. pedunculata are best stored outside of their lipid metastable temperature ranges.

How do we transport plant species with desiccation-sensitive germplasm in space?

Many useful plant species with potential for plant-based bioregenerative life support systems produce extremophile seeds with tolerance to multiple stressors, including desiccation, which allows for their transport through space in a dried state. However, other valuable species produce desiccation-sensitive seeds or are propagated clonally, and life sciences research in space has not yet addressed the challenge of alternative transport methods in microgravity for such material. Although liquid nitrogen storage is used on Earth for desiccation-sensitive germplasm, it poses atmospheric leakage problems to crewed spacecraft and therefore liquid nitrogen-free cryogenic freezing could be an alternative. Another promising approach is slow growth tissue culture, with subculture intervals extended to months or years through the precise control of the culture environment. Whilst the design of innovative systems for the transport of species with desiccation-sensitive germplasm will be demanding, the prospect still remains for their successful growth beyond Earth.

Seed Longevity-The Evolution of Knowledge and a Conceptual Framework

The lifespan or longevity of a seed is the time period over which it can remain viable. Seed longevity is a complex trait and varies greatly between species and even seed lots of the same species. Our scientific understanding of seed longevity has advanced from anecdotal 'Thumb Rules,' to empirically based models, biophysical explanations for why those models sometimes work or fail, and to the profound realisation that seeds are the model of the underexplored realm of biology when water is so limited that the cytoplasm solidifies. The environmental variables of moisture and temperature are essential factors that define survival or death, as well as the timescale to measure lifespan. There is an increasing understanding of how these factors induce cytoplasmic solidification and affect glassy properties. Cytoplasmic solidification slows down, but does not stop, the chemical reactions involved in ageing. Continued degradation of proteins, lipids and nucleic acids damage cell constituents and reduce the seed's metabolic capacity, eventually impairing the ability to germinate. This review captures the evolution of knowledge on seed longevity over the past five decades in relation to seed ageing mechanisms, technology development, including tools to predict seed storage behaviour and non-invasive techniques for seed longevity assessment. It is concluded that seed storage biology is a complex science covering seed physiology, biophysics, biochemistry and multi-omic technologies, and simultaneous knowledge advancement in these areas is necessary to improve seed storage efficacy for crops and wild species biodiversity conservation.

Impacts of Rapid Desiccation on Oxidative Status, Ultrastructure and Physiological Functions of Syzygium maire (Myrtaceae) Zygotic Embryos in Preparation for Cryopreservation

Syzygium maire is a highly threatened Myrtaceae tree species endemic to New Zealand. Due to its recalcitrant seed storage behaviour, cryopreservation is the only viable long-term ex situ conservation option for this species. This study investigated viability, oxidative stress, thermal properties, and ultrastructure of zygotic embryo axes (EAs) desiccated to various moisture contents (MC). Fresh EAs had a MC of c. 1.9 g/g with 100% viability but rapid desiccation to MC < 0.3 g/g significantly reduced viability and decreased the activities of the enzymatic antioxidants superoxide dismutase, catalase and glutathione peroxidase, with a sevenfold increase in the production of protein carbonyls and lipid peroxides. Differential Scanning Calorimetry analysis showed no thermal events in EAs desiccated to a MC of <0.2 g/g, indicating that all freezable water had been removed, but this was lethal to both EAs and enzymatic antioxidants. The ultrastructure of desiccated EAs showed signs of plasmolysis, while fully hydrated EAs exposed to cryogenic temperature had ultrastructural disintegration and membrane damage. The decline in enzymatic antioxidant activities and the increase in lipid peroxidation suggest that S. maire EA viability loss is due to oxidative stress rather than structural impacts.

In Vitro Technologies for American Chestnut (Castanea dentata (Marshall) Borkh) Conservation

American chestnut (Castanea dentata), a native species of eastern North America, is an economically important deciduous hardwood tree that has been designated as endangered in Canada. The population of American chestnut trees has dwindled significantly across Southern Ontario due to chestnut blight and many of the surviving trees continue to show blight disease symptoms. American chestnut requires efficient strategies for propagation and preservation for species recovery. The objective of this study was to develop a long-term plant conservation program using micropropagation and cryopreservation protocols. An in vitro technology using a liquid-based temporary immersion system (TIS) was developed for micropropagation of American chestnut. The highest rate of shoot multiplication was observed in cultures grown in the DKW (Driver and Kuniyuki 1984) basal medium supplemented with 2.2 µM 6-benzylaminopurine and 1.0 µM gibberellic acid. More than 95% of proliferated microshoots, about 40-50 mm in size, developed roots after 30 days of culture within bioreactor vessels containing DKW basal medium supplemented with 15 µM 3-Indolebutyric acid. Rooted plantlets transplanted to the greenhouse had a survival efficiency of 82% after one month of growth. The cryopreservation protocol for germplasm preservation was developed through droplet vitrification of shoots. Optimal regeneration of shoot tips occurred from explants precultured on stepwise concentrations of sucrose and subsequent dehydration in PVS3 for 30 min. Cryopreserved shoot tips were regenerated to whole plants using pre-optimized conditions of micropropagation. This study confirms the potential of TIS for micropropagation in ex situ conservation and reintroduction of endangered American chestnuts and possibly other woody plant species.

Vitrification Solutions for Plant Cryopreservation: Modification and Properties

Many plants cannot vitrify themselves because they lack glassy state-inducing substances and/or have high water content. Therefore, cryoprotectants are used to induce vitrification. A cryoprotectant must have at least the following primary abilities: high glass-forming property, dehydration strength on a colligative basis to dehydrate plant cells to induce the vitrification state, and must not be toxic for plants. This review introduces the compounds used for vitrification solutions (VSs), their properties indicating a modification of different plant vitrification solutions, their modifications in the compounds, and/or their concentration. An experimental comparison is listed based on the survival or regeneration rate of one particular species after using more than three different VSs or their modifications. A brief overview of various cryopreservation methods using the Plant Vitrification Solution (PVS) is also included. This review can help in alert researchers to newly introduced PVSs for plant vitrification cryoprotocols, their properties, and the choice of their modifications in the compounds and/or their concentration.

Vitrification Ability of Combined and Single Cryoprotective Agents

Cryoprotective agents (CPA) are an important part of many current vitrification methods. The vitrification ability of CPAs influences the probability of the glass transition and water crystallization occurrence. Thermal characteristics and the vitrification ability of two combined CPAs (PVS2 and PVS3), common plant vitrification solutions, and four single CPAs (ethylene glycol, DMSO, glycerol, and sucrose), the components of the mentioned PVSs, were evaluated utilizing a differential scanning calorimetry (DSC) during standard cooling/warming rates of 10 °C min-1. The effect of solute concentration on their vitrification ability was shown in the CPAs tested. Four typical concentration regions at which the glassy state and/or crystallization occurred were defined. We suggest the solute concentration of 0.7 g g-1 as the universal vitrification concentration, characterized by an actual Tg of CPA solution and limited water crystallization. Knowledge of the thermal properties of CPAs allows the design of new combined CPAs with the required vitrification ability respecting the cryopreservation method used and the characteristics of the cryopreserved sample.

Cryopreservation of Agronomic Plant Germplasm Using Vitrification-Based Methods: An Overview of Selected Case Studies

Numerous environmental and endogenous factors affect the level of genetic diversity in natural populations. Genetic variability is the cornerstone of evolution and adaptation of species. However, currently, more and more plant species and local varieties (landraces) are on the brink of extinction due to anthropopression and climate change. Their preservation is imperative for the sake of future breeding programs. Gene banks have been created worldwide to conserve different plant species of cultural and economic importance. Many of them apply cryopreservation, a conservation method in which ultra-low temperatures (−135 °C to −196 °C) are used for long-term storage of tissue samples, with little risk of variation occurrence. Cells can be successfully cryopreserved in liquid nitrogen (LN) when the adverse effect of ice crystal formation and growth is mitigated by the removal of water and the formation of the so-called biological glass (vitrification). This state can be achieved in several ways. The involvement of key cold-regulated genes and proteins in the acquisition of cold tolerance in plant tissues may additionally improve the survival of LN-stored explants. The present review explains the importance of cryostorage in agronomy and presents an overview of the recent works accomplished with this strategy. The most widely used cryopreservation techniques, classic and modern cryoprotective agents, and some protocols applied in crops are considered to understand which parameters provide the establishment of high quality and broadly applicable cryopreservation. Attention is also focused on the issues of genetic integrity and functional genomics in plant cryobiology.

Morphoanatomical aspects of the starting material for the improvement of pineapple cryopreservation by the droplet-vitrification technique

Cryopreservation of pineapple shoot tips has been established from various protocols, including droplet vitrification. Thus, this work aimed to evaluate the morphoanatomical conditions of the starting material over different times (30, 45 and 60 days) of culture before freezing and its correlation with the survival percentage of the cryopreserved shoot tips. Four accessions, Ananas comosus var. comosus (BGA-009); var. bracteatus (BGA-119); var. parguazensis (BGA-376), var. erectifolius (BGA-750) from the Pineapple Active Germplasm Bank (BGA Pineapple) and two hybrids from the Embrapa Genetic Breeding Program, FIB-ROX1 (var. bracteatus X var. erectifolius) and FIB-ROX2 ( var. erectifolius X var. bracteatus), recently introduced in the field from in vitro storage, were used. Histological sections before freezing and the percentages of survival after freezing were obtained taking into account the different times of cultivation of the donor plants. The results showed a significative interaction between genotypes (accessions and hybrids) and the culture period. The accessions BGA-009 and BGA-119 showed the highest survival rates, with 95% and 90% respectively for the 30-day culture time. Different results were obtained for each genotype, showing the need for improvements in the standardization of starting material, which would allow better repeatability of the protocol.

Lipid Remodeling Confers Osmotic Stress Tolerance to Embryogenic Cells during Cryopreservation

Plant species conservation through cryopreservation using plant vitrification solutions (PVS) is based in empiricism and the mechanisms that confer cell integrity are not well understood. Using ESI-MS/MS analysis and quantification, we generated 12 comparative lipidomics datasets for membranes of embryogenic cells (ECs) of Magnolia officinalis during cryogenic treatments. Each step of the complex PVS-based cryoprotocol had a profoundly different impact on membrane lipid composition. Loading treatment (osmoprotection) remodeled the cell membrane by lipid turnover, between increased phosphatidic acid (PA) and phosphatidylglycerol (PG) and decreased phosphatidylcholine (PC) and phosphatidylethanolamine (PE). The PA increase likely serves as an intermediate for adjustments in lipid metabolism to desiccation stress. Following PVS treatment, lipid levels increased, including PC and PE, and this effectively counteracted the potential for massive loss of lipid species when cryopreservation was implemented in the absence of cryoprotection. The present detailed cryobiotechnology findings suggest that the remodeling of membrane lipids and attenuation of lipid degradation are critical for the successful use of PVS. As lipid metabolism and composition varies with species, these new insights provide a framework for technology development for the preservation of other species at increasing risk of extinction.

Two Advanced Cryogenic Procedures for Improving Stevia rebaudiana (Bertoni) Cryopreservation

Cryopreservation is a useful tool for the long-term storage of plant genetic resources, and different cryogenic procedures have recently been developed. The present study focused on the use of the Droplet-vitrification (DV) and V cryo-plate protocol for the cryopreservation of Stevia rebaudiana in vitro-derived apical shoot tips and axillary shoot tips. A preliminary test showed that 90 and 120 min PVS2 (Plant Vitrification Solution 2) treatment significantly reduced the regrowth of the explants before immersion in liquid nitrogen (LN). For both procedures tested, the best osmoprotective condition for obtaining a higher regrowth of cryopreserved explants occurred when explants were PVS2 treated for 60 min. After direct immersion in LN, thawing and plating, the highest regrowth recorded was 80% with DV and 93% with V cryo-plate. Moreover, shoot tips proved to be a more suitable material for Stevia cryopreservation. A satisfactory vegetative regrowth was observed in the subcultures following cryopreservation by DV and V cryo-plate cryogenic procedures.

Cryopreservation of Plant Shoot Tips of Potato, Mint, Garlic, and Shallot Using Plant Vitrification Solution 3

Cryopreservation of shoot tips facilitates long-term storage of plant genetic resources which can otherwise only be propagated vegetatively. The vitrification approach using the cryoprotectant plant vitrification solution 3 (PVS3, 50% sucrose and 50% glycerol) is easy to handle, has shown to produce high regrowth percentages in a number of potato, mint, garlic, and shallot accessions, and is, thus, highly suitable for routine cryopreservation of plant genetic resources. In the current chapter, the vitrification procedure is described for potato, mint, garlic, and shallot and includes details about modifications for the different plant species. Special emphasis is given on the preparation of the different culture media, solutions, the culture conditions prior and post-cryopreservation, and the preparation of the shoot tips from different sources. Furthermore, protocols to introduce plants into in vitro culture and methods to estimate cryopreservation success are provided.

The need for novel cryoprotectants and cryopreservation protocols: Insights into the importance of biophysical investigation and cell permeability

BACKGROUND

Cryopreservation is a key method of preservation of biological material for both medical treatments and conservation of endangered species. In order to avoid cellular damage, cryopreservation relies on the addition of a suitable cryoprotective agent (CPA). However, the toxicity of CPAs is a serious concern and often requires rapid removal on thawing which is time consuming and expensive.

SCOPE OF REVIEW

The principles of Cryopreservation are reviewed and recent advances in cryopreservation methods and new CPAs are described. The importance of understanding key biophysical properties to assess the cryoprotective potential of new non-toxic compounds is discussed.

MAJOR CONCLUSIONS

Knowing the biophysical properties of a particular cell type is crucial for developing new cryopreservation protocols. Similarly, understanding how potential CPAs interact with cells is key for optimising protocols. For example, cells with a large osmotically inactive volume may require slower addition of CPAs. Similarly, a cell with low permeability may require a longer incubation time with the CPA to allow adequate penetration. Measuring these properties allows efficient optimisation of cryopreservation protocols.

GENERAL SIGNIFICANCE

Understanding the interplay between cells and biophysical properties is important not just for developing new, and better optimised, cryopreservation protocols, but also for broader research into topics such as dehydration and desiccation tolerance, chilling and heat stress, as well as membrane structure and function.

Cryobiotechnology of Plants: A Hot Topic Not Only for Gene Banks

Agriculture has always been an important part of human evolution. Traditionally, farming is changing and developing with regard to challenges it faces. The major challenges of modern agriculture are food and nutrition safety for the growing world population. Promoting species and genetic diversity in agriculture appears to be an important approach to dealing with those challenges. Gene banks all around the world play a crucial role in preserving plant genetic resources for future crop improvements. The plant germplasm can be preserved in different ways, depending on the species or form of stored plant tissue. This review focuses on a special preservation method—cryopreservation. Cryopreservation is an effective technique for storing living systems at ultra-low temperatures, usually in liquid nitrogen or its vapor phase. This conservation method is crucial for plants that do not produce seeds or that produce non germinating seeds, as well as for plants that propagate vegetatively. Moreover, based on the cryopreservation method, a novel plant biotechnology tool for pathogen eradication called cryotherapy has been developed. The use of liquid nitrogen eliminates plant pathogens such as viruses, phytoplasmas, and bacteria. Our article reviews recent advances in cryo-biotechnologies such as cryopreservation and cryotherapy, with special focus on studies concerning fruit plants.

Single-wall carbon nanotubes improve cell survival rate and reduce oxidative injury in cryopreservation of Agapanthus praecox embryogenic callus

BACKGROUND

Cryopreservation is the best way for long-term in vitro preservation of plant germplasm resources. The preliminary studies found that reactive oxygen species (ROS) induced oxidative stress and ice-induced membrane damage are the fundamental causes of cell death in cryopreserved samples. How to improve plant cryopreservation survival rate is an important scientific issue in the cryobiology field.

RESULTS

This study found that the survival rate was significantly improved by adding single-wall carbon nanotubes (SWCNTs) to plant vitrification solution (PVS) in cryopreservation of Agapanthus praecox embryogenic callus (EC), and analyzed the oxidative response of cells during the control and SWCNTs-added cryopreservation protocol. The SWCNTs entered EC at the step of dehydration and mainly located around the cell wall and in the vesicles, and most of SWCNTs moved out of EC during the dilution step. Combination with physiological index and gene quantitative expression results, SWCNTs affect the ROS signal transduction and antioxidant system response during plant cryopreservation. The EC treated by SWCNTs had higher antioxidant levels, like POD, CAT, and GSH than the control group EC. The EC mainly depended on the AsA-GSH and GPX cycle to scavenge H₂O₂ in the control cryopreservation, but depended on CAT in the SWCNTs-added cryopreservation which lead to low levels of H₂O₂ and MDA. The elevated antioxidant level in dehydration by adding SWCNTs enhanced cells resistance to injury during cryopreservation. The ROS signals of EC were balanced and stable in the SWCNTs-added cryopreservation.

CONCLUSIONS

The SWCNTs regulated oxidative stress responses of EC during the process and controlled oxidative damages by the maintenance of ROS homeostasis to achieve a high survival rate after cryopreservation. This study is the first to systematically describe the role of carbon nanomaterial in the regulation of plant oxidative stress response, and provided a novel insight into the application of nanomaterials in the field of cryobiology.

Cryopreservation of Orchids - A Review

BACKGROUND

The orchids are one of the beautiful creations of nature which stand apart from any other assemblage of flowering plants. They are highly evolutionary and ecologically significant group of plants that have effectively occupied almost every habitat on the earth. Indiscriminate collections and extermination of their natural habitats have threatened many species of orchids with extinction, resulting in a severe reduction of their genetic resources in nature according to recent patents. It is necessary to adopt sound scientific protocols for the preservation of orchid species.

METHOD

This cost-effective technique provides large storage time for the conservation of germplasm. Presently, efforts have been made to explore various cryopreservation techniques utilized so far and factors affecting the longevity of the propagules (in vivo and in vitro) while cryopreserving them. The sample to be cryopreserved is freeze-preserved in two ways, a) stepwise at two different subzero temperatures and b) in the rapid method, the samples are placed directly in the liquid nitrogen.

RESULTS

The orchid seeds and pollen are the most suitable propagules for cryopreservation of orchids due to their minute size and less space requirement.

CONCLUSION

Among the tissues (such as seeds, pollen, protocorms etc.) seeds are the most reliable. The present article reviews the cryopreservation techniques and factors effecting the cryopreservation, for in vitro conservation of orchid gene pool.

Changes of soluble sugars and ATP content during DMSO droplet freezing and PVS3 droplet vitrification of potato shoot tips

The potato's great genetic diversity needs to be maintained for future agricultural applications and can be preserved at ultra-low temperatures. To decipher detailed physiological processes, the aim of the study was to analyze the regrowth in 28 gene bank accessions and to reveal metabolite changes in a subset of four accessions that showed pronounced differences after shoot tip cryopreservation using DMSO droplet freezing and PVS3 droplet vitrification. Regrowth varied in all 28 genotypes ranging from 5% ('Kagiri') to 100% ('Karakter') and was higher after PVS3 droplet vitrification (71 ± 19%) than after cryopreservation using DMSO (54 ± 17%). Sucrose, glucose, and fructose were analyzed and showed significant increases after pre-culture in combination with PVS3 or DMSO and liquid nitrogen treatment and were reduced during regeneration. In contrast, adenosine triphosphate (ATP) reached its minimum concentration after cryoprotection and liquid nitrogen treatment and recovered most quickly after PVS3 droplet vitrification. A shortening of the explant pre-culture period reduced dramatically the regrowth after PVS3 vitrification. However, correlations between the shoot tip regrowth and sugar concentration were absent and significant at a low extent with ATP (r = 0.4, P < 0.01). Interestingly, several sub-cultivations of the donor plants from the previous stock affected negatively the regrowth. In conclusion, the cryopreservation protocol, genotypes, pre-culture period and number of sub-cultures affect the regrowth ability of explants, which was best estimated by the ATP concentration after low-temperature treatment. Due to the superior performance of PVS3, the routine potato cryopreservation at the Gatersleben gene bank was changed to PVS3 droplet vitrification.

Expanding the toolbox for cryopreservation of marine and freshwater diatoms

Diatoms constitute the most diverse group of microalgae and have long been recognised for their large biotechnological potential. In the wake of growing research interest in new model species and development of commercial applications, there is a pressing need for long-term preservation of diatom strains. While cryopreservation using dimethylsulfoxide (DMSO) as a cryoprotective agent is the preferred method for long-term strain preservation, many diatom species cannot be successfully cryopreserved using DMSO. Therefore, in this study, we studied cryopreservation success in six different diatom species, representing the major morphological and ecological diatom groups, using a range of DMSO concentrations and Plant Vitrification Solution 2 (PVS2) as an alternative cryoprotectant to DMSO. In addition, we tested whether suppressing bacterial growth by antibiotics accelerates the post-thaw recovery process. Our results show that the effects of cryoprotectant choice, its concentration and the addition of antibiotics are highly species specific. In addition, we showed that PVS2 and antibiotics are useful agents to optimize cryopreservation of algae that cannot survive the traditional cryopreservation protocol using DMSO. We conclude that a species-specific approach will remain necessary to develop protocols for diatom cryopreservation and to increase their representation in public culture collections.

Changes in transcript expression patterns as a result of cryoprotectant treatment and liquid nitrogen exposure in Arabidopsis shoot tips

Transcripts related to abiotic stress, oxidation, and wounding were differentially expressed in Arabidopsis shoot tips in response to cryoprotectant and liquid nitrogen treatment. Cryopreservation methods have been implemented in genebanks as a strategy to back-up plant genetic resource collections that are vegetatively propagated. Cryopreservation is frequently performed using vitrification methods, whereby shoot tips are treated with cryoprotectant solutions, such as Plant Vitrification Solution 2 (PVS2) or Plant Vitrification Solution 3 (PVS3); these solutions remove and/or replace freezable water within the meristem cells. We used the model system Arabidopsis thaliana to identify suites of transcripts that are up- or downregulated in response to PVS2 and PVS3 treatment and liquid nitrogen (LN) exposure. Our results suggest that there are many changes in transcript expression in shoot tips as a result of cryoprotection and that these changes exceed the number detected as a result of LN exposure. In total, 180 transcripts showed significant changes in expression level unique to treatment with either the cryoprotectant or cryopreservation followed by recovery. Of these 180 transcripts, 67 were related to stress, defense, wounding, lipid, carbohydrate, abscisic acid, oxidation, temperature (cold/heat), or osmoregulation. The responses of five transcripts were confirmed using qPCR methods. The transcripts responding to PVS2 + LN suggest an oxidative response to this treatment, whereas the PVS3 + LN treatment invoked a more general metabolic response. This work shows that the choice of cryoprotectant can have a major influence on the patterns of transcript expression, presumably due to the level and extent of stress experienced by the shoot tip. As a result, there may be divergent responses of study systems to PVS2 and PVS3 treatments.

Conservation Strategies in the Genus Hypericum via Cryogenic Treatment

In the genus Hypericum, cryoconservation offers a strategy for maintenance of remarkable biodiversity, emerging from large inter- and intra-specific variability in morphological and phytochemical characteristics. Long-term cryostorage thus represents a proper tool for preservation of genetic resources of endangered and threatened Hypericum species or new somaclonal variants with unique properties. Many representatives of the genus are known as producers of pharmacologically important polyketides, namely naphthodianthrones and phloroglucinols. As a part of numerous in vitro collections, the nearly cosmopolitan Hypericum perforatum - Saint John's wort - has become a suitable model system for application of biotechnological approaches providing an attractive alternative to the traditional methods for secondary metabolite production. The necessary requirements for efficient cryopreservation include a high survival rate along with an unchanged biochemical profile of plants regenerated from cryopreserved cells. Understanding of the processes which are critical for recovery of H. perforatum cells after the cryogenic treatment enables establishment of cryopreservation protocols applicable to a broad number of Hypericum species. Among them, several endemic taxa attract a particular attention due to their unique characteristics or yet unrevealed spectrum of bioactive compounds. In this review, recent advances in the conventional two-step and vitrification-based cryopreservation techniques are presented in relation to the recovery rate and biosynthetic capacity of Hypericum spp. The pre-cryogenic treatments which were identified to be crucial for successful post-cryogenic recovery are discussed. Being a part of genetic predisposition, the freezing tolerance as a necessary precondition for successful post-cryogenic recovery is pointed out. Additionally, a beneficial influence of cold stress on modulating naphthodianthrone biosynthesis is outlined.

Effects of droplet-vitrification cryopreservation based on physiological and antioxidant enzyme activities of Brassidium shooting star orchid

Protocorm-like bodies (PLBs) of Brassidium Shooting Star orchid were successfully cryopreserved using droplet-vitrification method. Vitrification based cryopreservation protocol is comprised of preculture, osmoprotection, cryoprotection, cooling, rewarming, and growth recovery and each and every step contributes to the achievement of successful cryopreservation. In order to reveal the lethal and nonlethal damage produced by cryopreservation, histological observation, scanning electron microscopy (SEM), and biochemical analysis were carried out in both cryopreserved and noncryopreserved PLBs of Brassidium Shooting Star orchid comparing with the control PLBs stock culture. Histological and scanning electron microscopy analyses displayed structural changes in cryopreserved PLBs due to the impact of cryoinjury during exposure to liquid nitrogen. Total soluble protein significantly increased throughout the dehydration process and the highest value was achieved when PLBs were stored in liquid nitrogen. Ascorbate peroxidase (APX) and catalase (CAT) showed the highest enzyme activities in both dehydration and cryostorage treatments indicating that stress level of PLBs was high during these stages.

Molecular mechanism of the synergistic effects of vitrification solutions on the stability of phospholipid bilayers

The vitrification solutions used in the cryopreservation of biological samples aim to minimize the deleterious formation of ice by dehydrating cells and promoting the formation of the glassy state of water. They contain a mixture of different cryoprotective agents (CPAs) in water, typically polyhydroxylated alcohols and/or dimethyl sulfoxide (DMSO), which can damage cell membranes. Molecular dynamics simulations have been used to investigate the behavior of pure DPPC, pure DOPC, and mixed DOPC-β-sitosterol bilayers solvated in a vitrification solution containing glycerol, ethylene glycol, and DMSO at concentrations that approximate the widely used plant vitrification solution 2. As in the case of solutions containing a single CPA, the vitrification solution causes the bilayer to thin and become disordered, and pores form in the case of some bilayers. Importantly, the degree of thinning is, however, substantially reduced compared to solutions of DMSO containing the same total CPA concentration. The reduction in the damage done to the bilayers is a result of the ability of the polyhydroxylated species (especially glycerol) to form hydrogen bonds to the lipid and sterol molecules of the bilayer. A decrease in the amount of DMSO in the vitrification solution with a corresponding increase in the amount of glycerol or ethylene glycol diminishes further its damaging effect due to increased hydrogen bonding of the polyol species to the bilayer headgroups. These findings rationalize, to our knowledge for the first time, the synergistic effects of combining different CPAs, and form the basis for the optimization of vitrification solutions.

Biophysical characteristics of successful oilseed embryo cryoprotection and cryopreservation using vacuum infiltration vitrification: an innovation in plant cell preservation

Heterogeneity in morphology, physiology and cellular chemistry of plant tissues can compromise successful cryoprotection and cryopreservation. Cryoprotection is a function of exposure time × temperature × permeability for the chosen protectant and diffusion pathway length, as determined by specimen geometry, to provide sufficient dehydration whilst avoiding excessive chemical toxicity. We have developed an innovative method of vacuum infiltration vitrification (VIV) at 381 mm (15 in) Hg (50 kPa) that ensures the rapid (5 min), uniform permeation of Plant Vitrification Solution 2 (PVS2) cryoprotectant into plant embryos and their successful cryopreservation, as judged by regrowth in vitro. This method was validated on zygotic embryos/embryonic axes of three species (Carica papaya, Passiflora edulis and Laurus nobilis) up to 1.6 mg dry mass and 5.6 mm in length, with varying physiology (desiccation tolerances) and 80 °C variation in lipid thermal profiles, i.e., visco-elasticity properties, as determined by differential scanning calorimetry. Comparisons between the melting features of cryoprotected embryos and embryo regrowth indicated an optimal internal PVS2 concentration of about 60% of full strength. The physiological vigour of surviving embryos was directly related to the proportion of survivors. Compared with conventional vitrification, VIV-cryopreservation offered a ∼ 10-fold reduction in PVS2 exposure times, higher embryo viability and regrowth and greater effectiveness at two pre-treatment temperatures (0 °C and 25 °C). VIV-cryopreservation may form the basis of a generic, high throughput technology for the ex situ conservation of plant genetic resources, aiding food security and protection of species from diverse habitats and at risk of extinction.

Intracellular ice and cell survival in cryo-exposed embryonic axes of recalcitrant seeds of Acer saccharinum: an ultrastructural study of factors affecting cell and ice structures

BACKGROUND AND AIMS

Cryopreservation is the only long-term conservation strategy available for germplasm of recalcitrant-seeded species. Efforts to cryopreserve this form of germplasm are hampered by potentially lethal intracellular freezing events; thus, it is important to understand the relationships among cryo-exposure techniques, water content, structure and survival.

METHODS

Undried embryonic axes of Acer saccharinum and those rapidly dried to two different water contents were cooled at three rates and re-warmed at two rates. Ultrastructural observations were carried out on radicle and shoot tips prepared by freeze-fracture and freeze-substitution to assess immediate (i.e. pre-thaw) responses to cooling treatments. Survival of axes was assessed in vitro.

KEY RESULTS

Intracellular ice formation was not necessarily lethal. Embryo cells survived when crystal diameter was between 0·2 and 0·4 µm and fewer than 20 crystals were distributed per μm(2) in the cytoplasm. Ice was not uniformly distributed within the cells. In fully hydrated axes cooled at an intermediate rate, the interiors of many organelles were apparently ice-free; this may have prevented the disruption of vital intracellular machinery. Intracytoplasmic ice formation did not apparently impact the integrity of the plasmalemma. The maximum number of ice crystals was far greater in shoot apices, which were more sensitive than radicles to cryo-exposure.

CONCLUSIONS

The findings challenge the accepted paradigm that intracellular ice formation is always lethal, as the results show that cells can survive intracellular ice if crystals are small and localized in the cytoplasm. Further understanding of the interactions among water content, cooling rate, cell structure and ice structure is required to optimize cryopreservation treatments without undue reliance on empirical approaches.

Peroxidation due to cryoprotectant treatment is a vital factor for cell survival in Arabidopsis cryopreservation

Cryopreservation can be a safe and cost-effective tool for the long-term storage of plant germplasm. In Arabidopsis, the ability to recover from cryogenic treatment was lost as growth progressed. Growth could be restored in 48-h seedlings, whereas 72-h seedlings died after cryogenic treatment. Why seedling age and survival are negatively correlated is an interesting issue. A comparative transcriptomics was performed to screen differentially expressed genes between 48- and 72-h seedlings after exposure to cryoprotectant. Among differentially expressed genes, oxidative stress response genes played important roles in cryoprotectant treatment, and peroxidation was a key factor related to cell survival. Seedlings underwent more peroxidation at 72-h than at 48-h. A comprehensive analysis indicated that peroxidation injured membrane systems leading to photophosphorylation and oxidative phosphorylation damage. Furthermore, the apoptosis-like events were found in cryogenic treatment of Arabidopsis seedlings. 48- and 72-h seedlings underwent different degrees of membrane lipid peroxidation during cryoprotectant treatment, and reducing the injury of oxidative stress was an important factor to successful cryopreservation. This study provided a novel insight of genetic regulatory mechanisms in cryopreservation, and established an excellent model to test and evaluate the effect of exogenous antioxidants and conventional cryoprotectants in plant cryopreservation.

Glassy state and cryopreservation of mint shoot tips

Vitrification refers to the physical process by which a liquid supercools to very low temperatures and finally solidifies into a metastable glass, without undergoing crystallization at a practical cooling rate. Thus, vitrification is an effective freeze-avoidance mechanism and living tissue cryopreservation is, in most cases, relying on it. As a glass is exceedingly viscous and stops all chemical reactions that require molecular diffusion, its formation leads to metabolic inactivity and stability over time. To investigate glassy state in cryopreserved plant material, mint shoot tips were submitted to the different stages of a frequently used cryopreservation protocol (droplet-vitrification) and evaluated for water content reduction and sucrose content, as determined by ion chromatography, frozen water fraction and glass transitions occurrence by differential scanning calorimetry, and investigated by low-temperature scanning electron microscopy, as a way to ascertain if their cellular content was vitrified. Results show how tissues at intermediate treatment steps develop ice crystals during liquid nitrogen cooling, while specimens whose treatment was completed become vitrified, with no evidence of ice formation. The agreement between calorimetric and microscopic observations was perfect. Besides finding a higher sucrose concentration in tissues at the more advanced protocol steps, this level was also higher in plants precultured at 25/-1°C than in plants cultivated at 25°C.

Cryopreservation of shoot tips and meristems: an overview of contemporary methodologies

Cryopreservation is the storage of viable bioresources at ultra-low temperatures in liquid nitrogen (LN). This chapter provides an overview of those protocols most commonly used to cryopreserve in vitro derived shoot tips and meristems; they are described generically, as sequential technical steps, including preparative and cryogenic treatments and the morphogenetic assessment of recovery. The importance of translating research-generated methods into formal Standard Operating Procedures (SOPs) is considered.

Dehydration improves cryopreservation of coconut (Cocos nucifera L.)

Cryopreservation of coconut can be used as a strategy to back up the establishment of living collections which are expensive to maintain and are under constant threat from biotic and abiotic factors. Unfortunately, cryopreservation protocols still need to be developed that are capable of producing a sizeable number of field-grown plants. Therefore, we report on the development of an improved cryopreservation protocol which can be used on a wide range of coconut cultivars. The cryopreservation of zygotic embryos and their recovery to soil-growing plants was achieved through the application of four optimised steps viz.: (i) rapid dehydration; (ii) rapid cooling; (iii) rapid warming and recovery in vitro and (iv) acclimatization and soil-supported growth. The thermal properties of water within the embryos were monitored using differential scanning calorimetry (DSC) in order to ensure that the freezable component was kept to a minimum. The feasibility of the protocol was assessed using the Malayan Yellow Dwarf (MYD) cultivar in Australia and then tested on a range of cultivars which were freshly harvested and studied in Indonesia. The most efficient protocol was one based on an 8-h rapid dehydration step followed by rapid cooling step. Best recovery percentages were obtained when a rapid warming step and an optimised in vitro culture step were used. Following this protocol, 20% (when cryopreserved 12 days after harvesting) and 40% (when cryopreserved at the time of harvest) of all MYD embryos cryopreserved could be returned to normal seedlings growing in soil. DSC showed that this protocol induced a drop in embryo fresh weight to 19% and significantly reduced the amount of water remaining that could produce ice crystals (0.1%). Of the 20 cultivars tested, 16 were found to produce between 10% and 40% normal seedlings while four cultivars generated between 0% and 10% normal seedlings after cryopreservation. This new protocol is applicable to a wide range of coconut cultivars and is useful for the routine cryopreservation of coconut genetic resources.

Desiccation tolerance in the vegetative tissues of the fern Mohria caffrorum is seasonally regulated

As there is limited information on the mechanisms of vegetative desiccation tolerance in pteridophytes, we undertook a comprehensive anatomical, ultrastructural, physiological and biochemical study on the fern Mohria caffrorum. Our data show that this species is desiccation-tolerant during the dry season, and desiccation-sensitive in the rainy season. This system allows the verification of protection mechanisms by comparison of tolerant and sensitive tissues of the same species at the same developmental age. Tolerant fronds acquire protection mechanisms during drying that are mostly similar to those reported for angiosperms. These include: (i) chlorophyll masking by abaxial scales and frond curling; (ii) increased antioxidant capacity that is maintained in dry tissues; (iii) mechanical stabilization of vacuoles in the dry state; (iv) de novo production of heat stable proteins (at least one identified as a putative chaperonin); (v) accumulation of protective carbohydrates (sucrose, raffinose family oligosaccharides and cyclitols). This study has implications for the biotechnological production of drought-tolerant crops, and allows speculation on the evolution of vegetative desiccation tolerance.

Calorimetric properties of water and triacylglycerols in fern spores relating to storage at cryogenic temperatures

Storing spores is a promising method to conserve genetic diversity of ferns ex situ. Inappropriate water contents or damaging effects of triacylglycerol (TAG) crystallization may cause initial damage and deterioration with time in spores placed at -15 degrees C or liquid nitrogen temperatures. We used differential scanning calorimetry (DSC) to monitor enthalpy and temperature of water and TAG phase transitions within spores of five fern species: Pteris vittata, Thelypteris palustris, Dryopteris filix-mas, Polystichum aculeatum, Polystichum setiferum. The analyses suggested that these fern spores contained between 26% and 39% TAG, and were comprised of mostly oleic (P. vittata) or linoleic acid (other species) depending on species. The water contents at which water melting events were first observable ranged from 0.06 (P. vittata) to 0.12 (P. setiferum)gH(2)Og(-1)dry weight, and were highly correlated with water affinity parameters. In spores containing more than 0.09 (P. vittata) to 0.25 (P. setiferum)gH(2)Og(-1)dry weight, some water partitioned into a near pure water fraction that melted at about 0 degrees C. These sharp peaks near 0 degrees C were associated with lethal freezing treatments. The enthalpy of water melting transitions was similar in fern spores, pollen and seeds; however, the unfrozen water content was much lower in fern spores compared to other forms of germplasm. Though there is a narrow range of water contents appropriate for low temperature storage of fern spores, water content can be precisely manipulated to avoid both desiccation and freezing damage.

Plasmolysis and recovery of different cell types in cryoprotected shoot tips of Mentha X piperita

Successful cryopreservation of plant shoot tips is dependent upon effective desiccation through osmotic or physical processes. Microscopy techniques were used to determine the extent of cellular damage and plasmolysis that occurs in peppermint (Mentha x piperita) shoot tips during the process of cryopreservation, using the cryoprotectant plant vitrification solution 2 (PVS2) (30% glycerol, 15% dimethyl sulfoxide, 15% ethylene glycol, 0.4 M sucrose) prior to liquid-nitrogen exposure. The meristem cells were the smallest and least plasmolyzed cell type of the shoot tips, while the large, older leaf and lower cortex cells were the most damaged. When treated with cryoprotectant solutions, meristem cells exhibited concave plasmolysis, suggesting that this cell type has a highly viscous protoplasm, and protoplasts have many cell wall attachment sites. Shoot tip cells were most severely plasmolyzed after PVS2 treatment, liquid-nitrogen exposure, and warming in 1.2 M sucrose. Successful recovery may be dependent upon surviving the plasmolytic conditions induced by warming and diluting treated shoot tips in 1.2 M sucrose solutions. In peppermint shoot tips, clumps of young meristem or young leaf cells survive the cryopreservation process and regenerate plants containing many shoots. Cryoprotective treatments that favor survival of small, meristematic cells and young leaf cells are most likely to produce high survival rates after liquid-nitrogen exposure.

Survival of mint shoot tips after exposure to cryoprotectant solution components

Many plant species can be cryopreserved by treating shoot tips with complex cryoprotectant solutions before rapidly cooling them to liquid nitrogen temperatures. Plant vitrification solution 2 (PVS2), a commonly selected cryoprotectant, can be lethal with extended exposure times. To determine potentially toxic combinations, we have exposed mint shoot tips to one-, two-, three-, and four-component solutions of PVS2 chemicals (30% glycerol, 15% ethylene glycol, 15% dimethyl sulfoxide, and 0.4 M sucrose) at 0 and 22 degrees C. Overall, solution exposures at 22 degrees C were more damaging than exposures at 0 degree C. Solutions with glycerol, particularly in combination with ethylene glycol and dimethyl sulfoxide, were also damaging. Cryoprotectant solutions PGluD (10% PEG8000, 10% glucose, and 10% dimethyl sulfoxide) and PVS3 (50% glycerol, 50% sucrose) were less damaging than PVS2 at 22 degrees C. When plant cryoprotectants are characterized on a toxicological and biophysical basis, less damaging cryoprotectant solutions could be developed.