Coffee is an economically important crop grown in ≈80 countries across the world (Los Santos-Brones and Hernandez-Sotomayor, 2006). The genus Coffee comprises ≈100 species of which only two species, Arabica coffee and Robusta coffee, are commercially cultivated (Los Santos-Brones and Hernandez-Sotomayor, 2006). Disease control is one of the major constraints to sustainable and economic coffee production (Gichuru et al., 2012). Breeding work in Kenya has resulted in the release of a new improved cultivar—Ruiru 11. Moreover, the hybrid combines resistance to CBD and leaf rust with high yield and good cup quality attributes (Van der Vossen and Walyaro, 1980). The development and dissemination of Ruiru 11 has made a significant contribution not only to the coffee farmers, but also to the Kenyan community as a whole for the following reasons:
- Its availability and use has the potential to reduce the cost of coffee production by up to 30% and thereby saves the Kenyan government considerable amounts of foreign currency usually spent on the importation of fungicides and spraying equipment.
- Its use will increase and stabilize the incomes of small-scale farmers for whom disease control using chemicals is frequently impossible to realize.
- Most importantly it offers potential environmentally friendly approach to the long-term control of coffee diseases and in production of organically grown coffee.
The propagation of Ruiru 11 is by F1 hybrid seed, production through hand artificial cross-pollination (Van der Vossen and Walyaro, 1981), cuttings, and tip grafting (Wamatu and Kingoro, 1991). The number of planting materials produced by the above methods is far below those needed to supply farmer’s demand. Therefore, there is need to accelerate the production of Ruiru11 planting materials by using alternative methods such as tissue culture.
A great deal of work has been carried out on coffee somatic embryogenesis using the indirect method. However, direct somatic embryogenesis (DSE) has not received much attention. This is although DSE is preferred, as it has the potential for maintaining genomic stability of regenerated plants. The former method that is via an intermediate callus phase increases the possibility of somaclonal variations (Tang and Guo, 2001).
Of the several factors that influence the process of somatic embryogenesis, the most important are the origin of the explant and composition of the culture medium, particularly with respect to the type and concentration of growth regulators. Somatic embryogenesis is known to be highly dependent on the explant used. The kind of explant chosen, its size, age, and the manner in which it is placed on the media can all determine whether morphogenesis can be induced (Rossin and Rey, 2011). Explants taken from juvenile plant tissues, particularly those from seedlings, are usually highly responsive. For example, while working on somatic embryogenesis of chickpea, Rao (1991) observed that more embryos were produced from leaf explants of 9-d-old seedlings than from older explants. In general, explants from newly originating organs are known to be more responsive than those taken from older ones (George, 1993). In vitro plant-derived leaves are regarded as rejuvenated plant materials (Nozeran, 1978) in which the endogenous hormonal balance is probably different from that of physiologically older leaves from glasshouse or field-grown plants. The embryogenic potential of this type of explant is considered to be substantially higher than the glasshouse and field-grown explants (Franclet, 1983). Micropropagated leaf explants from C. arabica and Anthurium scherzarium plants were found to be better explants than all the others tested (Bieysse et al., 1993; Hamida et al., 1997).
Most researchers working on coffee somatic embryogenesis have used leaves from the field or glasshouse plants as source of explant (Boxtel and Berthouly, 1996; De Almeida et al., 2014; Tahara et al., 1994). The production of embryos and plantlets from leaf explants offers a number of advantages including, uniform leaf material, and abundantly available and easily renewable source of explants.
Growth in vitro is highly dependent on the interaction between naturally occurring endogenous growth substances and the analogous synthetic growth regulators that may be added to the medium (George, 1993). The induction of DSE in C. arabica is strongly associated with the addition of cytokinins to the culture medium, in the absence of auxins (Almeida and Silvarolla, 2009; De Almeida et al., 2014; Kahia, 1999). The objective of the current study was to evaluate the ideal explant and the best type and concentration of cytokinins (phenylurea-type vs. adenine-type) for induction and regeneration of direct somatic embryos in the new cultivar.
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