Cryopreservation is a technique to preserve cells, tissues, and organs in liquid nitrogen (− 196 °C) and is widely used in the long-term preservation of animal, plant, and microbial germplasm resources. Ultra-low temperature preservation of plants is usually combined with in vitro culture technology, which can achieve long-term safe preservation of pollen, injured tissues, somatic embryos, zygotic embryos, seeds, meristems and dormant buds. The ultra-low temperature preservation technology of important crops and ornamental plants is relatively mature, and the main wild species and varieties of potatoes and bananas worldwide have been stored at ultra-low temperature.
Germplasm banks are the most effective and economical means of ex situ conservation of wild plants. Most of the seeds of wild plants can be stored at low temperature (− 20 °C) for a long time after dehydration, and their lifespan has been greatly extended. Seeds of some plants do not tolerate dehydration and cannot be stored at low temperatures and are usually called recalcitrant seeds. According to the review article "Seed banking not an option for many threatened plants" published by the UK Millennium Seed Bank in Nature Plants in 2018, 8% of plants worldwide produce recalcitrant seeds, but this proportion is as high as 36% and 33% in critically endangered plants and tree species, respectively. Goal 7 of the Global Strategy for Plant Conservation requires relocation protection for 75% of endangered species, and the realization of the goal requires investment and support in plant ultra-low temperature preservation technology.
Water becomes larger after crystallization. The formation of large crystals of cell water at ultra-low temperatures can destroy the structure of the cell membrane and lead to cell death. The first generation of plant ultra-low temperature preservation technology induces extracellular crystals by controlled cooling rate and slowly dehydrates the cells, improves intracellular fluid concentration and viscosity, and enters the vitrification state at ultra-low temperature, avoiding intracellular crystals and ensuring cell survival. The first generation of ultra-low temperature preservation techniques is only suitable for cold hardy plants. The new vitrification ultra-low temperature technology treats the cells with a high concentration of plant vitrification solution. While osmotic dehydration, the entry of cryoprotectants into the cell interior changes the thermodynamic properties of the cell fluid, and intracellular and extracellular vitrification can be achieved during rapid cooling to maintain the integrity and regeneration ability of the cell structure. The new generation of plant ultra-low temperature preservation technology realizes the preservation of tropical plants and greatly expands the scope of plant ultra-low temperature preservation.
The cell membrane is the core site of injury and osmotic response to low temperature. In the vitrification ultra-low temperature preservation process, cells are subjected to drastic and complex osmotic and temperature changes, and the survival and death of cells depend on whether the cell membrane can effectively respond to the above stresses. For a long time, the response mechanism of cell membrane to ultra-low temperature process has not been analyzed, which also restricts the further development of ultra-low temperature technology. Recently, Dr. Lin Liang from the Seed Biology Research Group of Kunming Institute of Botany, Chinese Academy of Sciences, Associate Researcher Chen Hongying, Researcher Hugh W. Pritchard and Researcher Li Weiqi collaborated to use the cryopreservation system of embryonic cells of Magnolia officinalis as a model system. Using lipidomics analysis method, the differences of 12 different ultra-low temperature treatments were compared, and the response mode of cell membrane to the vitrification ultra-low temperature preservation process was revealed. The results showed that the head group exchange of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PA) and phosphatidylglycerol (PG) occurred and the cell membrane was remodeled by osmotic protection treatment and rehydration treatment. After treatment with PVS solution, the total lipid level is significantly increased, storing a large number of lipid molecules for the freezing process that the cells will undergo. The results of this study suggest that membrane lipid remodeling and prevention of lipid degradation are key to the success of PVS methods. The mechanism of plant ultra-low temperature based on PVS is analyzed from the perspective of membrane lipid molecules, which provides a new idea for improving vitrification ultra-low temperature preservation technology and expanding the versatility and application range of plant ultra-low temperature preservation. The above findings, entitled "Lipid Remodeling Confers Osmotic Stress Tolerance to Embryogenic Cells during Cryopreservation", were published online in Molecular Sciences.
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