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

Potential role of ice-binding protein in mitochondria-lipid and ATP mechanisms during freezing of plant callus

Plant calli, a perpetually undifferentiated cell culture, have defects in maintaining their genetic fidelity during prolonged tissue culture. Cryopreservation using ice-binding proteins (IBP) is a potential solution. Despite a few studies on cryopreservation using IBPs in plant calli, detailed insights into the intracellular metabolism during freezing, thawing, and re-induction remain sparse. This study investigated and employed IBP from polar yeast Leucosporidium sp. (LeIBP) in the cryopreservation process across diverse taxa, including gymnosperms, monocots, dicots, and woody plants. Molecular-level analyses encompassing reactive oxygen species levels, mitochondrial function, and ATP and lipophilic compounds content were conducted. The results across nine plant species revealed the effects of LeIBP on callus competency post-thawing, along with enhanced survival rates, reactive oxygen species reduction, and restored metabolic activities to the level of those of fresh calli. Moreover, species-specific survival optimization with LeIBP treatments and morphological assessments revealed intriguing extracellular matrix structural changes post-cryopreservation, suggesting a morphological strategy for maintaining the original cellular states and paracrine signaling. This study pioneered the comprehensive application of LeIBP in plant callus cryopreservation, alleviating cellular stress and enhancing competence. Therefore, our findings provide new insights into the identification of optimal LeIBP concentrations, confirmation of genetic conformity post-thawing, and the intracellular metabolic mechanisms of cryopreservation advancements in plant research, thereby addressing the challenges associated with long-term preservation and reducing labor-intensive cultivation processes. This study urges a shift towards molecular-level assessments in cryopreservation protocols for plant calli, advocating a deeper understanding of callus re-induction mechanisms and genetic fidelity post-thawing.

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.

Dynamics of Organic Acids during the Droplet-Vitrification Cryopreservation Procedure Can Be a Signature of Oxidative Stress in Pogostemon yatabeanus

Cryopreservation in liquid nitrogen (LN, -196 °C) is a unique option for the long-term conservation of threatened plant species with non-orthodox or limitedly available seeds. In previous studies, a systematic approach was used to develop a droplet-vitrification (DV) cryopreservation protocol for Postemon yatabeanus shoot tips that includes preculture with 10% sucrose, osmoprotection with C4-35%, cryoprotection with A3-80% vitrification solution, and a three-step regrowth starting with the ammonium-free medium. The tricarboxylic acid (TCA) cycle is a crucial component of plant cell metabolism as it is involved in redox state regulation and energy provision. We hypothesized that organic acids (OAs) associated with the TCA and its side reactions indirectly indicate metabolism intensity and oxidative stress development in shoot tips under the cryopreservation procedure. In this study, the contents of 14 OAs were analyzed using gas chromatography-tandem mass spectrometry (GC-MS/MS) in P. yatabeanus shoot tips in a series of treatments including individual steps of the DV procedure, additional stress imposed by non-optimum protocol conditions (no preculture, no osmoprotection, various vitrification solution composition, using vials instead of aluminum foils, etc.) and regrowth on different media with or without ammonium or growth regulators. The possible relation of OA content with the total cryoprotectant (CPA) concentration and shoot tips regeneration percentage was also explored. Regeneration of cryopreserved shoot tips reduced in descending order as follows: standard protocol condition (91%) > non-optimum vitrification solution (ca. 68%) > non-optimum preculture (60-62%) > regrowth medium (40-64%) > no osmoprotection, cryopreservation in vials (28-30%). Five OAs (glycolic, malic, citric, malonic, and lactic) were the most abundant in the fresh (control) shoot tips. The dynamic pattern of OAs during the DV procedure highly correlated (r = 0.951) with the total CPA concentration employed: it gradually increased through the preculture, osmoprotection, and cryoprotection, peaked at cooling/rewarming (6.38-fold above control level), and returned to the fresh control level after 5 days of regrowth (0.89-fold). The contents of four OAs (2-hydroxybutyric, 3-hydroxypropionic, lactic, and glycolic) showed the most significant (10-209-fold) increase at the cooling/rewarming step. Lactic and glycolic acids were the major OAs at cooling/rewarming, accounting for 81% of the total OAs content. The OAs were categorized into three groups based on their dynamics during the cryopreservation protocol, and these groups were differently affected by protocol step modifications. However, there was no straightforward relationship between the dynamics of OAs and shoot tip regeneration. The results suggest that active modulation of OAs metabolism may help shoot tips to cope with osmotic stress and the chemical cytotoxicity\ of CPAs. Further intensive studies are needed to investigate the effect of cryopreservation on cell primarily metabolism and identify oxidative stress-related biomarkers in plant materials.

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.

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.

Effect of Plant Preservative MixtureTM on Endophytic Bacteria Eradication from In Vitro-Grown Apple Shoots

Endophytic contaminants are a common problem for the in vitro propagation of woody plants and have significant economic repercussions for the conservation of plant genetic resources and commercial micropropagation. In this study, first, the microbial contamination that appeared around the base of in vitro-grown apple shoots was identified as Bacillus megaterium. Then, plant preservative mixture (PPMTM) was used as a bactericidal agent in plant tissue culture. Its efficacy for eradicating endophytic B. megaterium in in vitro cultures of apple was tested. In vitro-contaminated shoots were grown in tissue culture medium supplemented with 0.2% v/v PPMTM for 12 weeks and then transferred to medium without any PPMTM and cultured for 24 weeks. This study showed that PPMTM is an effective agent for controlling the growth of B. megaterium. Our results highlight the species-specific response of apple shoots to PPMTM. PPMTM was effective in controlling endogenous microbial contaminations from apple varieties 'Golden Delicious', 'Landsberger Renette', 'Suislepper', and 'Aport krovavo-krasnyi'; meanwhile, in 'KG 7' and 'Gold Rush', all the plants grown in the absence of PPMTM were still bacterially contaminated, even though they were pre-treated for 12 weeks in PPMTM-supplemented medium. These results therefore suggest the essentiality of further testing of extended incubation of PPMTM in these cultivars that had outbreaks of bacterial contamination.

Unearthing the alleviatory mechanisms of hydrogen sulfide in aluminum toxicity in rice

Hydrogen sulfide (H₂S) improves aluminum (Al) resistance in rice, however, the underlying mechanism remains unclear. In the present study, treatment with 30-μM Al significantly inhibited rice root growth and increased the total Al content, apoplastic and cytoplasm Al concentration in the rice roots. However, pretreatment with NaHS (H₂S donor) reversed these negative effects. Pretreatment with NaHS significantly increased energy production under Al toxicity conditions, such as by increasing the content of ATP and nonstructural carbohydrates. In addition, NaHS stimulated the AsA-GSH cycle to decrease the peroxidation damage induced by Al toxicity. Pretreatment with NaHS significantly inhibited ethylene emissions in the rice and then inhibited pectin synthesis and increased the pectin methylation degree to reduce cell wall Al deposition. The phytohormones indole-3-acetic and brassinolide were also involved in the alleviation of Al toxicity by H₂S. The transcriptome results further confirmed that H₂S alleviates Al toxicity by increasing the pathways relating to material and energy metabolism, redox reactions, cell wall components, and signal transduction. These findings improve our understanding of how H₂S affects rice responses to Al toxicity, which will facilitate further studies on crop safety.

Microbial occurrence in liquid nitrogen storage tanks: a challenge for cryobanking?

Abstract

Modern biobanks maintain valuable living materials for medical diagnostics, reproduction medicine, and conservation purposes. To guarantee high quality during long-term storage and to avoid metabolic activities, cryostorage is often conducted in the N₂ vapour phase or in liquid nitrogen (LN) at temperatures below − 150 °C. One potential risk of cryostorage is microbial cross contamination in the LN storage tanks. The current review summarises data on the occurrence of microorganisms that may compromise the safety and quality of biological materials during long-term storage. We assess the potential for the microbial contamination of LN in storage tanks holding different biological materials based on the detection by culture-based and molecular approaches. The samples themselves, the LN, the human microbiome, and the surrounding environment are possible routes of contamination and can cause cross contaminations via the LN phase. In general, the results showed that LN is typically not the source of major contaminations and only a few studies provided evidence for a risk of microbial cross contamination. So far, culture-based and culture-independent techniques detected only low amounts of microbial cells, indicating that cross contamination may occur at a very low frequency. To further minimise the potential risk of microbial cross contaminations, we recommend reducing the formation of ice crystals in cryotanks that can entrap environmental microorganisms and using sealed or second sample packing. A short survey demonstrated the awareness for microbial contaminations of storage containers among different culture collections. Although most participants consider the risk of cross contaminations in LN storage tanks as low, they prevent potential contaminations by using sealed devices and − 150 °C freezers. It is concluded that the overall risk for cross contaminations in biobanks is relatively low when following standard operating procedures (SOPs). We evaluated the potential sources in detail and summarised our results in a risk assessment spreadsheet which can be used for the quality management of biobanks.

Key points

• Identification of potential contaminants and their sources in LN storage tanks.

• Recommendations to reduce this risk of LN storage tank contamination.

• Development of a risk assessment spreadsheet to support quality management.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11531-4.

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.

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.

Challenges and Prospects for the Conservation of Crop Genetic Resources in Field Genebanks, in In Vitro Collections and/or in Liquid Nitrogen

The conservation of crop genetic resources, including their wild relatives, is of utmost importance for the future of mankind. Most crops produce orthodox seeds and can, therefore, be stored in seed genebanks. However, this is not an option for crops and species that produce recalcitrant (non-storable) seeds such as cacao, coffee and avocado, for crops that do not produce seeds at all; therefore, they are inevitably vegetatively propagated such as bananas, or crops that are predominantly clonally propagated as their seeds are not true to type, such as potato, cassava and many fruit trees. Field, in vitro and cryopreserved collections provide an alternative in such cases. In this paper, an overview is given on how to manage and setup a field, in vitro and cryopreserved collections, as well as advantages and associated problems taking into account the practical, financial and safety issues in the long-term. In addition, the need for identification of unique accessions and elimination of duplicates is discussed. The different conservation methods are illustrated with practical examples and experiences from national and international genebanks. Finally, the importance of establishing safe and long-term conservation methods and associated backup possibilities is highlighted in the frame of the global COVID-19 pandemic.