Cryobiotechnology for the long-term preservation of oak (Quercus sp.) genetic resources.

Oaks (genus Quercus) are dominant and iconic trees in most European, American, Asian and North African forests, from cool temperate to tropical environments. There are at least 600 species of Quercus globally with high ecological and economical importance. However, 45% of the species evaluated by IUCN are considered threatened, being affected by habitat destruction or diseases and pests, such as acute oak decline or chronic oak dieback in the UK. As a result, conservation of oaks is becoming increasingly important for many countries. Oak seeds are desiccation sensitive (i.e., recalcitrant), and conventional seed bank strategies are not suitable for their long-term preservation. Tissue culture is a useful tool for Quercus species conservation, supporting micropropagation and in vitro conservation. However, cryopreservation is accepted as the most favourable and cost-effective option for oak long-term conservation ex situ. Numerous tissues can be used as germplasm to preserve the genetic diversity of oaks, including winter dormant buds, shoot tips, plumules, embryogenic lines, somatic embryos and embryonic axes. Pollen cryopreservation could also be used as a complementary technology to support conservation and breeding programmes. Interestingly, whilst the seeds of oak are recalcitrant it appears that the pollen has much greater desiccation tolerance. Seed embryonic axes (with shoot and root meristems) are the preferred explants for oak preservation as high genetic diversity can be captured and axes can be grown into full plants with relatively simple micropropagation procedures. However, successful cryopreservation depends on controlling many factors, including physical damage to the embryo during excision, oxidative stress associated to excision and cryopreservation procedures, differential response of shoot and root meristems (within and amongst species) to the stresses imposed by partial desiccation and cooling, and the need to improve in vitro growth and acclimation procedures. Here we review the main cryobiotechnological options for the successful banking of species in the genus Quercus and the needs for future research are highlighted.