Olive (Olea europaea L.) Genetic Transformation: Current Status and Future Prospects

Abstract

Olive (Olea europaea L.) is the most characteristic and important oil crop of the Mediterranean region. Traditional olive cultivation is based on few tens cultivars of ancient origin. To improve this crop, novel selections with higher tolerance to biotic and abiotic stress, adaptable to high-density planting systems and resilient to climate change are needed; however, breeding programs are hindered by the long juvenile period of this species and few improved genotypes have been released so far. Genetic transformation could be of great value, in the near future, to develop new varieties or rootstocks in a shorter time; in addition, it has currently become an essential tool for functional genomic studies. The recalcitrance of olive tissues to their in vitro manipulation has been the main bottleneck in the development of genetic transformation procedures in this species; however, some important traits such as fungal resistance, flowering or lipid composition have successfully been manipulated through the genetic transformation of somatic embryos of juvenile or adult origin, providing a proof of the potential role that this technology could have in olive improvement. However, the optimization of these protocols for explants of adult origin is a prerequisite to obtain useful materials for the olive industry. In this review, initially, factors affecting plant regeneration via somatic embryogenesis are discussed. Subsequently, the different transformation approaches explored in olive are reviewed. Finally, transgenic experiments with genes of interest undertaken to manipulate selected traits are discussed.

Keywords: Agrobacterium rhizogenes; Agrobacterium tumefaciens; biolistic; olive; somatic embryogenesis; transgenic plant.

Figure 1

Images of the steps of olive somatic embryos (SE) maturation and plant regeneration protocol [28].

Figure 2

Workflow chart of A. tumefaciens-mediated transformation of olive SE using the protocol described by [56].

Figure 3

Red fluorescence in olive embryogenic callus transformed with the DsRed marker gene. (A,B) Control non-transformed callus. (C,D) DsRed transformed embryogenic callus. Pictures were taken under white light (A,C) and fluorescent light using a DsRed excitation filter (B,D).

Figure 4

Early flowering olive plants expressing the FT gene MtFTa1 from Medicago truncatula. (A) Control (left) and MtFTa1-transformed (right) olive plants. (B,C) Details of MtFTa1-transformed plants.