This work was carried out to optimize the conditions for highly effective embryogenic callus induction from mature seeds, plantlet regeneration and genetic transformation of Citrus sinensis L. by Agrobacterium tumefaciens strain EHA105 (pCAMBIA 1305.1). Embryogenic calli could be successfully induced from mature seeds employing the MT medium supplemented with 500 mg/l malt extract. The percentage of embryogenic callus induction was 85. With the same medium, the high proliferation rate of embryogenic callus was achieved. The liquid MT medium containing 500 mg/l malt extract in combination with 50 mg/l lactose could be used as the embryoid development medium. Somatic embryos, however, could be regenerated with normal shoots and roots in the MS medium, with the regeneration percentage of 60. The delivery of an antisense ACC oxidase gene into the species C. sinensis mediated by Agrobacterium tumefaciens strain EHA 105 was successful by co-cultivating explants with the strain EHA 105 for 10 min, following that by eliminating the bacterium with 200 mg/l cefotaxime, and subsequently selecting transformed embryoid with 20 mg/l hygromycin. Verified histochemically by GUS assay, putative transformants showed the percentage of gus gene expression of 100. Molecular analysis using PCR confirmed the integration of the antisense ACC oxidase gene into plant genome.
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