History and current status of embryogenic culture-based tissue culture, transformation and gene editing of maize (Zea mays L.)

The production of embryogenic callus and somatic embryos is integral to the genetic improvement of crops via genetic transformation and gene editing. Regenerable embryogenic cultures also form the backbone of many micro-propagation processes for crop species. In many species, including maize, the ability to produce embryogenic cultures is highly genotype dependent. While some modern transformation and genome editing methods reduce genotype dependence, these efforts ultimately fall short of producing truly genotype-independent tissue culture methods. Recalcitrant genotypes are still identified in these genotype-flexible processes, and their presence is magnified by the stark contrast with more amenable lines, which may respond more efficiently by orders of magnitude. This review aims to describe the history of research into somatic embryogenesis, embryogenic tissue cultures, and plant transformation, with particular attention paid to maize. Contemporary research into genotype-flexible morphogenic gene-based transformation and genome engineering is also covered in this review. The rapid evolution of plant biotechnology from nascent technologies in the latter half of the 20th century to well-established, work-horse production processes has, and will continue to, fundamentally changed agriculture and plant genetics research. Plain Language Summary The production of embryogenic callus and somatic embryos in tissue culture has been a critical component of crop genome engineering and editing systems. In many species, including maize, the ability to produce somatic embryo-forming (embryogenic) tissue cultures is highly genotype dependent, limiting the scope and efficiency of engineering and editing-based applications. Although some modern transformation and editing methods reduce genotype dependence, they are not yet completely genotype independent. This review describes the history of research into somatic embryogenesis, embryogenic cultures, and plant transformation, with a focus on how these technologies were developed and applied to maize. Current research into genotype-flexible, morphogenic gene-based transformation and gene editing is also discussed. The evolution of plant tissue culture, transformation, and gene editing systems has fundamentally changed agriculture and accelerated functional genomics research and genetic enhancement of maize and other crops.