Mutagenesis of in vitro explants of Coffea spp. to induce fungal resistance.

Abstract Coffee is one of the most valuable commodity tree crops worldwide. However, it suffers from several devastating diseases and pests, for example coffee leaf rust and coffee berry borer, whose impact is being amplified by changing climatic conditions. Development of new adapted varieties remains a laborious effort by conventional breeding due to the long juvenile period in tree crops. Plant cell/tissue culture represents the ultimate method to produce large amounts of true-to-type healthy plants and of explants for mutation breeding. In fact, mutation induction combined with in vitro cell/tissue culture techniques has proved to be effective for developing improved cultivars of perennial crops. Prior to mutation breeding, cell and tissue radiosensitivity tests to various mutagens need to be performed, so that optimal treatments can be applied for large population development. Thus, different in vitro explants (plantlet, leaf, callus, embryogenic callus, globular and torpedo stage embryos) of Coffea arabica and Coffea canephora were exposed to different gamma-ray doses (0, 10, 15, 20, 40, 60 and 80 Gy). After 9-21 weeks incubation, a radiosensitivity test was conducted on the different explants and LD50 doses corresponding to 50% of viability or survival of callus, embryogenic callus, globular and torpedo stage embryos and 50% growth reduction (GR50) of shoot were also determined. Callus explants showed a relatively high radio-resistance (LD30-LD50 50-100 Gy) in comparison with entire plantlets or embryos (LD30-GR50 8-46 Gy). Globular embryo development into plantlets and also leaf area of irradiated plantlets were more severely affected by irradiation than other explants. It was possible to confirm the relative radio-resistance of unicellular explants compared with multicellular explants. Estimation of optimal mutation induction dosage range for various in vitro explants is important for tree crops, especially for coffee improvement.