Maize Transformation: From Plant Material to the Release of Genetically Modified and Edited Varieties

Abstract

Over the past decades, advances in plant biotechnology have allowed the development of genetically modified maize varieties that have significantly impacted agricultural management and improved the grain yield worldwide. To date, genetically modified varieties represent 30% of the world’s maize cultivated area and incorporate traits such as herbicide, insect and disease resistance, abiotic stress tolerance, high yield, and improved nutritional quality. Maize transformation, which is a prerequisite for genetically modified maize development, is no longer a major bottleneck. Protocols using morphogenic regulators have evolved significantly towards increasing transformation frequency and genotype independence. Emerging technologies using either stable or transient expression and tissue culture-independent methods, such as direct genome editing using RNA-guided endonuclease system as an in vivo desired-target mutator, simultaneous double haploid production and editing/haploid-inducer-mediated genome editing, and pollen transformation, are expected to lead significant progress in maize biotechnology. This review summarises the significant advances in maize transformation protocols, technologies, and applications and discusses the current status, including a pipeline for trait development and regulatory issues related to current and future genetically modified and genetically edited maize varieties.

Leaf transformation for efficient random integration and targeted genome modification in maize and sorghum

“…In addition, a consistent year-round supply of immature embryos as a transformation explant requires greenhouse infrastructure that makes it resource-and cost-prohibitive for most academic institutions. Consequently, transformation service facilities have become a necessity 8 . Improved transformation processes with high frequencies of regeneration therefore become a prerequisite for these complex genome editing applications 9 .…”

Genome editing in maize: Toward improving complex traits in a global crop

“…This complexity challenges dissecting the molecular mechanisms of gene actions and accurately measuring phenotypes, making it difficult to use genomic engineering tools to develop superior cultivars for those traits. Transgenic maize cultivars aiming at increased insect and herbicide tolerance have been on the market for decades, in contrast to only a few examples developed for complex traits ( Yassitepe et al ., 2021 ). The difficulty of applying a transgenic approach to manipulate complex traits stable in several environments has limited the development of biotech cultivars that could be widely used ( Simmons et al, 2021 ).…”

“…Most genome editing studies in maize have been performed using genetic transformation protocols based on Agrobacterium tumefaciens or biolistic delivery methods in a couple of temperate genotypes suitable for Agrobacterium infection and regeneration ( Kausch et al, 2021a ; Yassitepe et al ., 2021 ). However, the lack of a transformation protocol applicable to a wider range of genotypes and the typical low transformation efficiency hamper the broad application of genome editing in maize.…”

“…One of the main bottlenecks for applying biotechnological tools in maize breeding is the recalcitrance of maize lines to Agrobacterium -mediated transformation. Although there are some maize lines amenable to this standard transformation method, such as Hi-II and B104, most of these lines are not suitable for proper field trials of GE events and/or for commercial application ( Kausch et al, 2021a ; b ; Yassitepe et al ., 2021 ). In addition, even among these transformable maize lines, transformation efficiency with standard protocols is usually low.…”

OMICS in Fodder Crops: Applications, Challenges, and Prospects

“…Globally, it is one of the most important cereal crops, and it is adaptable to grow in different environmental conditions, generating billions of dollars in revenue annually [ 139 , 140 , 141 ]. Maize has evolved into one of the most essential model crops for understanding the genetics and genomes of plants [ 142 , 143 , 144 ]. The complexity of the diverse maize genome has kept researchers interested in this crop studying cytogenetics and genomics.…”