Parameters influencing the regeneration capacity of calluses derived from mature indica and japonica rice seeds after microprojectile bombardment

The microprojectile bombardment procedure has allowed the stable transforma tion of indica and japonica rice varieties, although at different frequenci es of transformation depending mainly on their regeneration capacity and on the specific parameters of the transformation protocol. A study of the pro cess of regeneration to whole plants from primary calli derived from mature indica and japonica rice seeds, via embryogenesis, has shown that somatic embryos are produced by division and differentiation of the external cell l ayers of callus tissues. Adjusting the bombardment conditions to optimize g ene delivery to those regenerable cells, we have evaluated the influence of parameters such as the target distance, particle penetration and the effec t of osmotic treatment on the regeneration capacity of bombarded cells.