Oxidative status and stress associated with cryopreservation of germplasm of recalcitrant-seeded species.

Abstract

Genetic diversity of cultivated species and their wild relatives, as well as of wild species encompasses plant genetic resources or germplasm, the ex situ preservation of which embodies a critical aspect of biological conservation. While seed storage affords an efficient ex situ conservation method, recalcitrant seeds are intolerant of desiccation and cannot be stored conventionally in seed banks. Seeds of the three indigenous tree species investigated in this study, viz. Trichilia emetica, T. dregeana and Protorhus longifolia are recalcitrant, with the species considered to be endangered. Cryopreservation, which involves storage at ultra-low temperatures of selected tissue(s) from which plants are subsequently able to be generated, is currently the only method available for long-term ex situ conservation of recalcitrant-seeded species and affords significant potential for the future. Many protocols that have been applied for the cryopreservation of the germplasm of recalcitrant zygotic embryonic axes excised from seeds of tropical/sub-tropical species have resulted in survival post-cryo which has been recorded only as root development or callus formation, with shoot formation seldom occurring. Successful cryostorage of genetic resources cannot be achieved until post-cryopreservation recovery facilitates normal seedling development, i.e. the formation of both a fully functional root and a shoot. Cryopreservation requires the utilisation of the smallest explant possible (greatest surface area to volume ratio), the most suitable for recalcitrant seeds in general being the zygotic embryonic axis. Based on preliminary studies it was demonstrated that shoot production by axes is inhibited in association with a burst of reactive oxygen species (ROS), produced in response to wounding upon excision of the axis from the cotyledons, when these are attached close to the shoot apical meristem. It was postulated that a combination of the oxidative burst at the site of excision coupled with inadequate antioxidant machinery within the recalcitrant axis tissue, precludes shoot production. It was further considered highly probable that each subsequent stressful manipulation throughout the cryopreservation process would be accompanied by a surge of uncontrolled oxidative activity within the tissue, in response to the stress. Therefore, the primary aim of the study was to investigate the underlying causes of failure of shoot production after procedures associated with cryopreservation and to focus on ways to ameliorate the consequences of unbalanced oxidative metabolism. Additionally, studies were carried out to optimise each step of the cryopreservation procedure, viz. cryoprotection, dehydration, rehydration and cooling, and subsequent recovery, in conjunction with assessment of oxidative responses, ultimately to achieve successful cryopreservation of the embryonic axes of these species. The experimental work conducted to achieve this aim assessed changes in various biomarkers of injury, those selected for this study being three ROS, viz. superoxide, the hydroxyl radical and hydrogen peroxide, after axes were exposed to various pre-treatments, cryopreservation and recovery. Concomitantly, the elicited responses of endogenous antioxidant systems accompanying these steps were assessed. Changes in the levels of ROS and antioxidant activity were determined using various biochemical assays, and these parametres, together with assessment of shoot development, were investigated after each step of the cryopreservation process. The effect of stress on oxidative metabolism was tested after exposure to pre-treatments with and without the provision of various antioxidants, viz. DMSO, ascorbic acid and cathodic water, so as to determine the efficacy of selected ROS scavengers and, in general, to develop the best protocol for cryopreservation of embryonic axes of the three species. Significant results, in terms of shoot development and regulated ROS generation, were obtained after three major processes of the cryopreservation procedure. The production of roots and shoots by excised axes of T. emetica, T. dregeana and P. longifolia after excision (75%, 80% and 75%, respectively), and by 40% of excised axes of T. dregeana after each of the two further stages, cryoprotection and desiccation, were major achievements towards cryopreservation of the recalcitrant germplasm. The modulation of ROS by ascorbic acid and cathodic protection significantly improved survival of axes of both Trichilia species. In its entirety, the present study made significant advancements towards cryopreservation of recalcitrant germplasm and also towards understanding oxidative events associated with cryogenic processing and exposure to cryogenic conditions. This study concludes that unregulated metabolism is one of the underlying causes of failure of recalcitrant germplasm represented by zygotic axes, to survive cryopreservation. The application of antioxidants and cathodic protection during cryopreservation facilitated survival that has been previously unattainable. The outcomes of this study provide an informative platform for further optimising cryopreservation procedures for the germplasm of the species investigated, and extending the work to other recalcitrant-seeded species, especially those of tropical/sub-tropical provenances.