Bifunctional selection–reporter systems for genetic transformation of citrus: mannose- and kanamycin-based systems

Agrobacterium-mediated transformation of Carrizo citrange [Citrus sinensis (L.) Osbeck × Poncirus trifoliata (L.) Raf.] with a binary vector containing a novel bifunctional reporter–selection fusion gene comprising an inframe fusion between the manA gene and egfp gene is detailed. This system combined the phosphomannose isomerase positive selection system with the ability to monitor gene expression in a non-destructive manner using EGFP. Transgenic plants stably expressing the EGFP protein were regenerated following Agrobacterium-mediated transformation using a vector containing this fusion gene. We also obtained comparable transformation efficiencies when Carrizo explants were transformed using another Agrobacterium strain containing a binary vector with a bifunctional egfp–nptII fusion gene. Regenerating shoots were selected on medium containing 15 g L?1 mannose supplemented with 5 g L?1 sucrose for the manA-based selection or on medium containing 100 mg L?1 kanamycin for the nptII-based selection. Our results indicated that the mannose-based antibiotic-free selection combined with visual identification of transgenic shoots using EGFP allows for early elimination of escape non-transgenic shoots and can provide a viable alternative to antibiotic-based selection systems in the genetic transformation of citrus and other crops.