The Cryobiotechnology of Oaks: An Integration of Approaches for the Long-Term Ex Situ Conservation of Quercus Species

Abstract

Conventional dry seed storage is unlikely for about one third of all tree species (and nearly half of evergreen rain forest trees) as they probably produce desiccation sensitive (recalcitrant) seeds. Consequently, international ex situ conservation targets for threatened trees will be difficult to achieve without innovation, especially in cryobiotechnology. We assessed progress in the development of various cryobiotechnology approaches for the preservation of oaks (Quercus), which are keystone species of functioning landscapes, important to the bioeconomy and under increasing threats from the spread of pests and diseases under a changing climate. Various tissues of oaks can be used for banking, from pollen grains to embryo axes. Pollen from five oak species have been shown to be highly desiccation tolerant, making dry pollen storage at low temperatures (including in liquid nitrogen) a valuable technology to support conservation and breeding programs. Somatic embryo (SE) technology and/or shoot tip in vitro technology is available for 39 species, and SE cryopreservation is routinely performed on three commercial species and shoot tips cryopreservation successful in two more species. Seed embryonic axes are the preferred explants for oak ex situ conservation, with tissue survival and regeneration of plants after cryopreservation recorded for 14 and seven species respectively; although differential responses between the shoot and root meristems in the axes are known. Dormant bud preservation seems promising, but is under-researched. Overall, these results indicate the possibility of establishing an integrated platform for the ex situ conservation of oak species based on cryobiotechnology. Challenges of explant choice, optimization of methodologies and large-scale application do remain. However, multiple approaches for the cryopreservation of oak genetic resources are available and implementation programmes should not be delayed, particularly in the centres of species diversity.

Plant Diversity Conservation Challenges and Prospects—The Perspective of Botanic Gardens and the Millennium Seed Bank

“…These discrepancies, in both the lack of a universal method for plant cryopreservation and in the levels of initial percentage of plant regeneration between cryopreserved plant collections and conventional seed banking, have often been a barrier for the establishment of cryopreserved collections of wild species within seed banks. However, this has not been the case for crop plants [12,82] and we think similar standards of success should be applied to wild and threatened 'exceptional species' [83]. In this regard, cryopreserved plant accessions of wild species can be viewed like the elements of metacollections described in a previous section of this paper.…”

“…Secondly, conventional seed banks and botanic gardens need to invest in infrastructure and specialised training to increase the taxonomic and geographic variation of cryopreserved collections of wild plant species (the scale of the investment will depend on the scale of the cryobank desired). Thirdly, cryobiotechnological research needs to be increased if more species and tissues are to be successfully cryopreserved [12,[83][84][85][89][90][91]. Fourthly, we must strengthen networking between wild species genebanks and crop genebanks to facilitate the preservation of wild species collections at the regional level in their crop genebank cryobank facilities, particularly in tropical areas where funding, a stable supply of liquid nitrogen and training for the development of wild species cryopreserved collections may be challenging [84].…”

Can Forest Trees Cope with Climate Change?—Effects of DNA Methylation on Gene Expression and Adaptation to Environmental Change

“…The importance of using appropriate, matching, diverse, and improved germplasm in forestry systems and the need for appropriate seed and seedling production systems are now being appreciated more. Ex-situ conservation ensures the survival of genetic resources [27][28][29][30]; in the case of forest genetic resources, this involves the storage of seeds, ensuring that they remain in suitable, species-specific conditions [31][32][33][34][35][36][37][38][39][40][41]. Skillful, effective seed storage provides a stable source of genetic diversity of forest trees needed for forest conservation.…”

Micropropagation, Characterization, and Conservation of Phytophthora cinnamomi-Tolerant Holm Oak Mature Trees

“…Axillary budding enables conservation of selected genotypes but is expensive and has the additional risk of contamination and somaclonal variation [29]. To prevent these risks, the period between each subculture event could be increased (thus reducing costs) by applying slow growth methods [30,31] or by completely inhibiting growth (cryobiotechnology) [29,32]. To date, very few studies have investigated preservation procedures in holm oak, such as cold storage to preserve established axillary shoots from juvenile plants [30], while in [33], the authors reported the cryopreservation of holm oak somatic embryos in a procedure that did not enable long-term preservation.…”