Cryopreservation of Malus and Pyrus Wild Species in the 'Fruit Genebank' in Dresden-Pillnitz, Germany

High-Efficiency In Vitro Root Induction in Pear Microshoots (Pyrus spp.)

Extensive research has been conducted on the in vitro mass propagation of pear (Pyrus spp.) trees through vegetative propagation, demonstrating high efficiency in shoot multiplication across various pear species. However, the low in vitro rooting rates remain a significant barrier to the practical application and commercialization of mass propagation. This study aims to determine the favorable conditions for inducing root formation in the in vitro microshoots of Pyrus genotypes. The base of the microshoots was exposed to a high concentration (2 mg L-1) of auxins (a combination of IBA and NAA) for initial root induction at the moment when callus formation begins. The microshoots were then transferred to an R1 medium (1/2 MS with 30 g L-1 sucrose without PGRs) to promote root development. This method successfully induced rooting in three European pear varieties, one Asian pear variety, and a European-Asian hybrid, resulting in rooting rates of 66.7%, 87.2%, and 100% for the European pear (P. communis), 60% for the Asian pear (P. pyrifolia), and 83.3% for the hybrid pear (P. pyrifolia × P. communis) with an average of 25 days. In contrast, the control group (MS medium) exhibited rooting rates of 0-13.3% after 60 days of culture. These findings will enhance in vitro root induction for various pear varieties and support the mass propagation and acclimatization of pear. The in vitro root induction method developed in this study has the potential for global commercial application in pear cultivation.

Current status of the cryopreservation of embryogenic material of woody species

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

Conservation of the Bird Cherry (Padus Mill.) Germplasm by Cold Storage and Cryopreservation of Winter Cuttings

Conservation at cryogenic temperatures, usually in liquid nitrogen (LN) or in its vapor, is the only reliable method for the long-term ex situ conservation of fruit and berry crops with vegetative reproduction. In this study, five bird cherry (Padus Mill.) varieties of different genetic origin from the bird cherry genebank at the N.I. Vavilov All-Russian Institute of Plant Genetic Resources (VIR, Russia) were tested for their response to cryopreservation in LN vapor (-183--185 °C). The response included viability under laboratory and field conditions, morphological assessment of the developed plants and biochemical analysis of fruits produced during three consecutive years by plants developed from cryopreserved cuttings. All parameters were compared to those recorded after cold storage of cuttings (-5 °C), a routine mid-term conservation method currently used at the VIR genebank. The initial viability of winter cuttings varied from 86.7% to 93.3%. Six-month cold storage and cryopreservation reduced viability to 53.3-86.7% and 43.3-60.0%, respectively, which was above the 40% viability threshold in all varieties tested. Cuttings after cold storage showed better viability when recovered in the laboratory (80% mean viability) than in the field (58% mean viability); viability of cryopreserved cuttings was not affected by recovery conditions. The results of a two-way analysis of covariance suggested that storage and recovery conditions had the most significant effect on viability (p < 0.0001), while the effects of genotype (p = 0.062) and factor interactions (p = 0.921) were minor. Cryopreservation had little or no influence on morphological parameters of the plants recovered in the field, including plant height, number of shoots, internodes and roots, and root length. Similarly, no effect of cryopreservation was recorded on dry matter content, total sugar content and ascorbic acid concentration in fruits produced by plants developed from the cryopreserved cuttings. These results suggest that cryopreservation in LN vapor is a reliable method for conservation of the bird cherry genetic collection and is worth testing with a broader variety of genotypes.

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.