Assessment of biolistic and Agrobacterium-mediated genetic transformation methods in Cenchrus ciliaris

Plant regeneration through somatic embryogenesis in cell suspensions of Cenchrus ciliaris L

BACKGROUND

This study aims to establish cell suspension culture, its maintenance and induction of somatic embryogenesis, and in vitro plant regeneration in Cenchrus ciliaris L. Suspension cultures are relatively homogenous cell lines facilitating uniform access to nutrition. These are ideal sources of competent cells for genetic transformation.

RESULTS

Callus was initiated by culturing immature inflorescences of Cenchrus ciliaris cv. IGFRI-3108 on Murashige and Skoog (MS) medium containing 3 mg l-1 2,4-dichlorophenoxyacetic acid (2,4-D) and 0.5 mg l-1 6-benzylaminopurine (BAP) with 30 g l-1 sucrose. Cell suspension cultures were established in liquid MS medium using an inoculum size of 10 g l-1. These were maintained to achieve embryogenic cell/regeneration competent cultures. Growth curve analysis and a subculture interval of 20 days were determined to harvest cells at the end of the exponential phase. The cell doubling time was found to be 11 days. Somatic embryogenesis was accomplished in MS medium containing 1 mg l-1 2,4-D, 1 mg l-1 BAP along with growth adjuvants, 300 mg l-1 casein hydrolysate, 400 mg l-1 glutamine and 300 mg l-1 proline. The highest number (16 ± 3.78/per inoculum) of shoots regenerated on this medium. The elongation and rooting of shoots were recorded on basal MS and ½ MS media, respectively. Rooted plants were successfully transferred to pots containing a Soilrite and cocopeat mixture in a 3:1 proportion for 3-4 weeks and later successfully acclimatized in the greenhouse with a 60% survival rate. The genetic fidelity of 12 regenerated plants was analysed using RAPD primers that were genetically identical to the mother plant.

CONCLUSION

Cell suspension culture-based in vitro plant regeneration of C. ciliaris involved the establishment, maintenance and progression of somatic embryogenesis during shoot and root development. The inherent limitation of callus-mediated in vitro plant regeneration reducing the regeneration potential due to the aging of the calli has been overcome.

Genetic Transformation of Apomictic Grasses: Progress and Constraints

The available methods for plant transformation and expansion beyond its limits remain especially critical for crop improvement. For grass species, this is even more critical, mainly due to drawbacks in in vitro regeneration. Despite the existence of many protocols in grasses to achieve genetic transformation through Agrobacterium or biolistic gene delivery, their efficiencies are genotype-dependent and still very low due to the recalcitrance of these species to in vitro regeneration. Many plant transformation facilities for cereals and other important crops may be found around the world in universities and enterprises, but this is not the case for apomictic species, many of which are C4 grasses. Moreover, apomixis (asexual reproduction by seeds) represents an additional constraint for breeding. However, the transformation of an apomictic clone is an attractive strategy, as the transgene is immediately fixed in a highly adapted genetic background, capable of large-scale clonal propagation. With the exception of some species like Brachiaria brizantha which is planted in approximately 100 M ha in Brazil, apomixis is almost non-present in economically important crops. However, as it is sometimes present in their wild relatives, the main goal is to transfer this trait to crops to fix heterosis. Until now this has been a difficult task, mainly because many aspects of apomixis are unknown. Over the last few years, many candidate genes have been identified and attempts have been made to characterize them functionally in Arabidopsis and rice. However, functional analysis in true apomictic species lags far behind, mainly due to the complexity of its genomes, of the trait itself, and the lack of efficient genetic transformation protocols. In this study, we review the current status of the in vitro culture and genetic transformation methods focusing on apomictic grasses, and the prospects for the application of new tools assayed in other related species, with two aims: to pave the way for discovering the molecular pathways involved in apomixis and to develop new capacities for breeding purposes because many of these grasses are important forage or biofuel resources.