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The genus Quercus, which belongs to the family Fagaceae, is native to the northern hemisphere and includes deciduous and evergreen species. The trees of the different species are very important from both economic and ecological perspectives. Application of new technological approaches (which span the fields of plant developmental biology, genetic transformation, conservation of elite germplasm and discovery of genes associated with complex multigenic traits) to these long-rotation hardwoods may be of interest for accelerating tree improvement programs. This review provides a summary of the advances made in the application of biotechnological tools to specific oak species. Significant progress has been made in the area of clonal propagation via organogenesis and somatic embryogenesis (SE). Standardized procedures have been developed for micropropagating the most important European (Q. robur, Q. petarea, Q. suber) and American (Q. alba, Q. bicolor, Q. rubra) oaks by axillary shoot growth. Although regenerated plantlets are grown in experimental trials, large-scale propagation of oak species has not been carried out. The induction of SE in oaks from juvenile explants is generally not problematic, although the use of explants other than zygotic embryos is much less efficient. During the last decade, enormous advances have been made in inducing SE from selected adult trees, mainly specimens of pedunculate oak (Q. robur) and cork oak (Q. suber). Advances in the understanding of the maturation and germination steps are required for better use of embryogenic process in clonal forestry. Quercus species are late-maturing and late-flowering, exhibit irregular seed set, and produce seeds that are recalcitrant to storage by conventional procedures. Vitrification-based cryopreservation techniques were used successfully in somatic embryos of pedunculate oak and cork oak, and an applied genbank of cork oak selected genotypes is now under development. The feasibility of genetic transformation of pedunculate oak and cork oak somatic embryos by means of co-culture techniques with several strains of Agrobacterium tumefaciens has also been demonstrated. To date, most research on the genomics of Quercus species has concerned population genetics. Approaches using functional genomics to examine the molecular and cellular mechanisms that control organogenesis and or somatic embryogenesis are still scarce, and efforts on the isolation and characterization of genes related to other specific traits should be intensified in the near future, as this would help improve the practical application of clonal forestry in recalcitrant species such as oaks.

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Somatic embryogenesis was induced in expanding leaf explants excised from epicormic shoots forced from branch segments taken at four different times of year from a mature oak (Quercus robur L.). Branch segments 2-4 cm in diameter produced most shoots when collected in March. Somatic embryos were induced on explants derived from branches of all collection dates, although collection in November seemed to afford the best results. Germination and conversion ability of embryos of embryogenic lines derived from six oak trees depended heavily on genotype, conversion rates ranging from 0 to 70%. RAPD analyses found no evidence of genetic variation either within or between the embryogenic lines established from three of these trees, or between these lines and the trees of origin, or between somatic embryo derived plantlets and the trees of origin. The embryogenic system used in this study appears to be suitable for true-to-type clonal propagation of mature oak genotypes.

Somatic embryogenesis from in vitro leaf and shoot apex explants excised from axillary shoot cultures established from two mature Quercus ilex trees has been developed. Somatic embryos (SE) were obtained from both explant types and genotypes evaluated, although embryogenic frequencies were influenced by the genotype, auxin concentration, and explant type. The explants were cultured on Murashige and Skoog salts and vitamins, supplemented with 500 mg L -1 casein hydrolysate (CH) and different concentrations of indole-3-acetic acid or α-naphthalene acetic acid (NAA) in combination with 2.22 µM 6-benzylaminopurine (BA). In both genotypes, shoot apex explants were more responsive than leaf explants. The best results were obtained with apex explants of clone Q3 (11%) cultured on medium with 21.48 µM NAA plus 2.22 µM BA. This combination was also effective for initiating SE from leaf explants, although the induction rates were lower (1-3%). Embryogenic lines were maintained by repetitive embryogenesis following culture of nodular embryogenic structures on Schenk and Hildebrand medium without plant growth regulators. Low embryo multiplication rates were obtained when torpedo or early cotyledonary SE were used as initial explant for embryo proliferation, or when glutamine or CH (500 mg L -1 ) was added to proliferation medium. For germination, cotyledonary-stage SE were isolated and stored at 4ºC for 2 months. After cold storage, SE were cultured on germination medium consisting of Gresshoff and Doy medium, supplemented with 0.44 μM BA and 20 μM silver thiosulphate. Under these conditions, plantlets were regenerated from 21-66.7% of the SE generated for both genotypes.