CRYOPRESERVATION OF EUROPEAN CHESTNUT GERMPLASM

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.

Efficient Protocol for Improving the Development of Cryopreserved Embryonic Axes of Chestnut (Castanea sativa Mill.) by Encapsulation-Vitrification

An optimized cryopreservation protocol for embryonic axes (EAs) of chestnut (Castanea sativa Mill.) has been developed based on the encapsulation–vitrification procedure. EAs of mature seeds were aseptically dissected and encapsulated in alginate beads with or without 0.3% (w/v) activated charcoal (AC). Embedded EAs were dehydrated with Plant Vitrification Solution 2 for different treatment times up to 120 min, followed by direct immersion in liquid nitrogen. Cryopreserved embryonic axes encapsulated with AC showed higher survival (70%) compared to those encapsulated without AC (50%). Sixty-four percent of embryonic axes, from synthetic seeds with AC, subsequently developed as whole plants. Plantlet regrowth was faster in AC-encapsulated EAs and showed enhanced postcryopreservation shoot and root regrowth over 2 cm after five weeks from rewarming. Results indicate that encapsulation–vitrification with activated charcoal added to the beads is an effective method for the long-term preservation of Castaneasativa embryonic axes.

Cryopreservation of embryos: an overview

Cryopreservation (liquid nitrogen, -196°C) is the only safe and cost-effective option for long-term -conservation of genetic resources of non-orthodox seed species. Cryopreservation protocols have been developed for various materials including seeds, dormant buds, cell suspensions, calli, apices, zygotic, and somatic embryos of numerous plant species. Zygotic embryos or embryonic axes of almost 100 different species and somatic embryos of almost 40 different species from both temperate and tropical climates, comprising crops, fruit, and forest trees as well as wild species, whose seeds displayed orthodox, intermediate, and recalcitrant storage characteristics, have been successfully cryopreserved. With zygotic embryos and embryonic axes, the desiccation technique has been used with the majority of the species tested, leading to highly variable survival and recovery after freezing, especially during earlier experiments. More recently, new cryopreservation techniques viz. encapsulation-dehydration and vitrification have been employed, leading to generally improved results. With somatic embryos, different cryopreservation methods have been used viz. desiccation, pre-growth-desiccation, encapsulation-dehydration, vitrification, encapsulation-vitrification, and droplet-vitrification. There are also a few examples of the utilisation of slow controlled freezing, which correspond to the earlier experiments performed with somatic embryos. The development and application of cryopreservation is significantly more advanced for somatic embryos, in comparison with zygotic embryos, mainly because of the different origin and characteristics of the species treated. In most cases, zygotic embryos originate from tropical, wild species, for which knowledge and techniques relevant to the development of cryopreservation protocols are limited, or even non-existent. By contrast, somatic embryos are generally produced from cultivated species, which have already been studied extensively and for which propagation techniques are already operational. A number of technical possibilities to explore exist in order to improve the development of cryopreservation protocols for zygotic embryos and embryonic axes. For both categories of materials, the utilisation of analytical techniques has proved to be extremely useful to assist in the development of cryopreservation protocols.