RECOVERY OF TRANSGENIC EVENTS FROM TWO HIGHLY RECALCITRANT MAIZE (Zea mays L.) GENOTYPES USING AGROBACTERIUM-MEDIATED STANDARD-BINARY-VECTOR TRANSFORMATION

Plant genetic engineering plays a unique role in improving the maize (Zea mays L.) gene pool for enhancing agronomical traits. Here we report the recov- ery of transgenic events from two maize genotypes high- ly recalcitrant to Agrobacterium-mediated transformation using standard binary vectors. Four maize genotypes were initially evaluated for the transformation: two U.S. public inbred genotypes, H99 and Oh43, and two elite Korean maize varieties, "Jangdaok" and "Chalok-2". Em- bryogenic and transformation responses were evaluated in H99 and Oh43 under two different culture regimes and in Jangdaok and Chalok-2 under three different cul- ture regimes. These different culture regimes included MS-based, N6-based and alternation of MS and N6. Even under an optimal regime condition, no transgenic was re- covered in H99, Oh43, or Jangdaok genotypes; whereas the Korean sticky maize "Chalok-2" was successfully transformed, albeit at an extremely low (0.16%) frequen- cy. Furthermore, we evaluated the impact of several transformation conditions on stable transformation using H99 as a test genotype. These conditions included the further reduction of salt concentrations in inoculation and co-cultivation media and varying co-cultivation tem- peratures under two herbicide selection schemes. Under optimal conditions, a reproducible transgenic recovery at frequency of 1.8% was achieved for H99. Transgene inte- gration and inheritance were confirmed using an herbi- cide (Liberty®) leaf painting assay and Southern analyses in Chalok-2 and H99 genotypes. Our results suggest that transformation of highly recalcitrant maize genotypes is achievable, although it is subject to the limited genetic potential of their regeneration or transformation capaci- ties.