Agrobacterium-mediated Transformation of Tropical Maize Using Seedling Leaf Whorl Explants.

Conventional maize transformation has largely relied on immature embryos as explants, and is thus often hampered by the limited access to high-quality immature embryos year-round. Here, we present a detailed protocol using seedling leaf whorls as alternative explants for tropical maize inbred transformation. This approach involves the use of a cassette that drives the expression of the morphogenic transcription factors (MTFs) Baby boom (Bbm) and Wuschel2 (Wus2), which have been shown to greatly enhance transformation efficiency. We outline here the steps required for the preparation of seedling leaf whorl explants and subsequent Agrobacterium infection, and describe the tissue culture regimen that results in transgenic plant regeneration. Because constitutive expression of Bbm and Wus2 prevents normal plant regeneration and the production of fertile plants, the cassette containing these genes must be excised. As such, we include the steps for the Cre/loxP-mediated excision of the MTF gene cassette. The protocol outlines a year-round, more affordable, and efficient approach for carrying out maize transformation for crop improvement.