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Abstract 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. 

Summary The ability of plant somatic cells to dedifferentiate, form somatic embryos and regenerate whole plantsin vitrohas been harnessed for both clonal propagation and as a key component of plant genetic engineering systems. Embryogenic culture response is significantly limited, however, by plant genotype in most species. This impedes advancements in both plant transformation‐based functional genomics research and crop improvement efforts. We utilized natural variation among maize inbred lines to genetically map somatic embryo generation potential in tissue culture and identify candidate genes underlying totipotency. Using a series of maize lines derived from crosses involving the culturable parent A188 and the non‐responsive parent B73, we identified a region on chromosome 3 associated with embryogenic culture response and focused on three candidate genes within the region based on genetic position and expression pattern. Two candidate genes showed no effect when ectopically expressed in B73, but the geneWox2awas found to induce somatic embryogenesis and embryogenic callus proliferation. Transgenic B73 cells with strong constitutive expression of the B73 and A188 coding sequences ofWox2awere found to produce somatic embryos at similar frequencies, demonstrating that sufficient expression of either allele could rescue the embryogenic culture phenotype. Transgenic B73 plants were regenerated from the somatic embryos without chemical selection and no pleiotropic effects were observed in theWox2aoverexpression lines in the regenerated T0 plants or in the two independent events which produced T1 progeny. In addition to linking natural variation in tissue culture response toWox2a, our data support the utility ofWox2ain enabling transformation of recalcitrant genotypes. 

Summary Agrobacterium tumefaciens, the causal agent of plant crown gall disease, has been widely used to genetically transform many plant species. The inter‐kingdom gene transfer capability madeAgrobacteriuman essential tool and model system to study the mechanism of exporting and integrating a segment of bacterial DNA into the plant genome. However, many biological processes such asAgrobacterium‐host recognition and interaction are still elusive. To accelerate the understanding of this important plant pathogen and further improve its capacity in plant genetic engineering, we adopted a CRISPR RNA‐guided integrase system forAgrobacteriumgenome engineering. In this work, we demonstrate thatINsertion ofTransposableElements byGuideRNA–AssistedTargEting (INTEGRATE) can efficiently generate DNA insertions to enable targeted gene knockouts. In addition, in conjunction with Cre‐loxPrecombination system, we achieved precise deletions of large DNA fragments. This work provides new genetic engineering strategies forAgrobacteriumspecies and their gene functional analyses. 

Agrobacterium-mediated transformation is an essential tool for functional genomics studies and crop improvements. Recently developed ternary vector systems, which consist of a T-DNA vector and a compatible virulence (vir) gene helper plasmid (ternary helper), demonstrated that including an additionalvirgene helper plasmid into disarmedAgrobacteriumstrains significantly improves T-DNA delivery efficiency, enhancing plant transformation. Here, we report the development of a new ternary helper and thymidine auxotrophicAgrobacteriumstrains to boostAgrobacterium-mediated plant transformation efficiency. AuxotrophicAgrobacteriumstrains are useful in reducingAgrobacteriumovergrowth after the co-cultivation period because they can be easily removed from the explants due to their dependence on essential nutrient supplementation. We generated thymidine auxotrophic strains from publicAgrobacteriumstrains EHA101, EHA105, EHA105D, and LBA4404. These strains exhibited thymidine-dependent growth in the bacterial medium, and transientGUSexpression assay using Arabidopsis seedlings showed that they retain similar T-DNA transfer capability as their original strains. Auxotrophic strains EHA105Thy- and LBA4404T1 were tested for maize B104 immature embryo transformation using our rapid transformation method, and both strains demonstrated comparable transformation frequencies to the control strain LBA4404Thy-. In addition, our new ternary helper pKL2299A, which carries thevirAgene from pTiBo542 in addition to othervirgene operons (virG,virB,virC,virD,virE, andvirJ), demonstrated consistently improved maize B104 immature embryo transformation frequencies compared to the original version of pKL2299 (33.3% vs 25.6%, respectively). Therefore, our improvedAgrobacteriumsystem, including auxotrophic disarmedAgrobacteriumstrains and a new ternary helper plasmid, can be useful for enhancing plant transformation and genome editing applications. 

Efficient genetic transformation is a prerequisite for rapid gene functional analyses and crop trait improvements. We recently demonstrated that new T-DNA binary vectors with NptII/G418 selection and a compatible helper plasmid can efficiently transform maize inbred B104 using our rapid Agrobacterium-mediated transformation method. In this work, we implemented the non-integrating Wuschel2 (Wus2) T-DNA vector method for Agrobacterium-mediated B104 transformation and tested its potential for recalcitrant inbred B73 transformation and gene editing. The non-integrating Wus2 (NIW) T-DNA vector-assisted transformation method uses two Agrobacterium strains: one carrying a gene-of-interest (GOI) construct and the other providing an NIW construct. To monitor Wus2 co-integration into the maize genome, we combined the maize Wus2 expression cassette driven by a strong constitutive promoter with a new visible marker RUBY, which produces the purple pigment betalain. As a GOI construct, we used a previously tested CRISPR-Cas9 construct pKL2359 for Glossy2 gene mutagenesis. When both GOI and NIW constructs were delivered by LBA4404Thy- strain, B104 transformation frequency was significantly enhanced by about two-fold (10% vs. 18.8%). Importantly, we were able to transform a recalcitrant inbred B73 using the NIW-assisted transformation method and obtained three transgene-free edited plants by omitting the selection agent G418. These results suggest that NIW-assisted transformation can improve maize B104 transformation frequency and provide a novel option for CRISPR technology for transgene-free genome editing. 

Core Ideas Manual pollinations in breeding and genetics research requires pollen available when recipient silks are viable. The method collects and stores maize pollen for at least 5 days and facilitates efficient pollination. Pollen is mixed with polyetheretherketone and uses field‐collected pollen and simple storage conditions. The method can increase the number of pollinations per tassel and generates a reasonable number of viable seeds. 

For maize genome-editing and bioengineering, genetic transformation of inbred genotypes is most desired due to the uniformity of genetic background in their progenies. However, most maize inbred lines are recalcitrant to tissue culture and transformation. A public, transformable maize inbred B104 has been widely used for genome editing in recent years. This is primarily due to its high degree of genetic similarity shared with B73, an inbred of the reference genome and parent of many breeding populations. Conventional B104 maize transformation protocol requires 16–22 weeks to produce rooted transgenic plants with an average of 4% transformation frequency (number of T0 plants per 100 infected embryos). In this Method paper, we describe an advanced B104 transformation protocol that requires only 7–10 weeks to generate transgenic plants with an average of 6.4% transformation frequency. Over 66% of transgenic plants carried CRISPR/Cas9-induced indel mutations on the target gene, demonstrating that this protocol can be used for genome editing applications. Following the detailed and stepwise procedure described here, this quick and simplified method using the Agrobacterium ternary vector system consisting of a T-DNA binary vector and a compatible helper plasmid can be readily transferable to interested researchers.