Recent Advances inAgrobacterium-Mediated Maize Genetic Transformation Using Immature Embryos and Seedling Leaf Whorl Explants

The introduction of maize genetic transformation in the 1990s brought forth a powerful tool for crop improvement and a deeper understanding of plant genetics. Despite decades of genetics research, however, and the promise of CRISPR-mediated gene editing, maize transformation currently faces several challenges, such as genotype dependence and limitations in explant availability. Indeed, although the most commonly used method, immature embryo transformation, has been improved through optimization of tissue culture media composition and selection methods, the approach is only applicable to a limited number of public genotypes, including B104 and Hi II. Recently, genotype-flexible methods have been developed using coexpression cassettes of morphogenic transcription factors (MTFs) Baby boom ( Bbm ) and Wushel2 ( Wus2 ), which have enabled the successful transformation of many previously recalcitrant maize lines. This MTF-based transformation method has also allowed for the use of alternate explants, such as seedling leaf whorl, whose production is cost-effective and requires only minimum controlled growth space. In this review, we summarize recent advances in Agrobacterium -mediated maize transformation methods that use immature embryos or seedling leaf whorls as starting material.