Efficient soybean regeneration and Agrobacterium-mediated transformation using a whole cotyledonary node as an explant

CRISPR/Cas genome editing in soybean: challenges and new insights to overcome existing bottlenecks

BACKGROUND

Soybean is a worldwide-cultivated crop due to its applications in the food, feed, and biodiesel industries. Genome editing in soybean began with ZFN and TALEN technologies; however, CRISPR/Cas has emerged and shortly became the preferable approach for soybean genome manipulation since it is more precise, easy to handle, and cost-effective. Recent reports have focused on the conventional Cas9 nuclease, Cas9 nickase (nCas9) derived base editors, and Cas12a (formally Cpf1) as the most commonly used genome editors in soybean. Nonetheless, several challenges in the complex plant genetic engineering pipeline need to be overcome to effectively edit the genome of an elite soybean cultivar. These challenges include (1) optimizing CRISPR cassette design (i.e., gRNA and Cas promoters, gRNA design and testing, number of gRNAs, and binary vector), (2) improving transformation frequency, (3) increasing the editing efficiency ratio of targeted plant cells, and (4) improving soybean crop production.

AIM OF REVIEW

This review provides an overview of soybean genome editing using CRISPR/Cas technology, discusses current challenges, and highlights theoretical (insights) and practical suggestions to overcome the existing bottlenecks.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The CRISPR/Cas system was discovered as part of the bacterial innate immune system. It has been used as a biotechnological tool for genome editing and efficiently applied in soybean to unveil gene function, improve agronomic traits such as yield and nutritional grain quality, and enhance biotic and abiotic stress tolerance. To date, the efficiency of gRNAs has been validated using protoplasts and hairy root assays, while stable plant transformation relies on Agrobacterium-mediated and particle bombardment methods. Nevertheless, most steps of the CRISPR/Cas workflow require optimizations to achieve a more effective genome editing in soybean plants.

Soybean genetics, genomics, and breeding for improving nutritional value and reducing antinutritional traits in food and feed

Soybean [Glycine max (L.) Merr.] is a globally important crop due to its valuable seed composition, versatile feed, food, and industrial end-uses, and consistent genetic gain. Successful genetic gain in soybean has led to widespread adaptation and increased value for producers, processors, and consumers. Specific focus on the nutritional quality of soybean seed composition for food and feed has further elucidated genetic knowledge and bolstered breeding progress. Seed components are historical and current targets for soybean breeders seeking to improve nutritional quality of soybean. This article reviews genetic and genomic foundations for improvement of nutritionally important traits, such as protein and amino acids, oil and fatty acids, carbohydrates, and specific food-grade considerations; discusses the application of advanced breeding technology such as CRISPR/Cas9 in creating seed composition variations; and provides future directions and breeding recommendations regarding soybean seed composition traits.

In vitro regeneration and Agrobacterium-mediated genetic transformation of Dragon's Head plant (Lallemantia iberica)

Dragon’s head plant (Lallemantia iberica), is a flowering species belongs to the mint family (Lamiaceae). The species contains valuable essential oils, mucilage and oil which are used in pharmaceutical and food industries. Tissue culture is a feasible strategy to attain large‐scale production of plantlets with a huge potential to produce plants with superior quality. The objective of this study was to develop a simple and efficient method for regeneration and transformation of L. iberica. To reach this goal, the regeneration ability of various explants including leaf, cotyledonary node, hypocotyl and cotyledon segments was investigated in MS medium supplemented with diverse concentrations of NAA (Naphthalene acetic acid) and BAP (6-Benzyl Amino Purine). According to the results, cotyledonary nodes showed the best regeneration response. The maximum rate of regeneration (and number of induced shoots was achieved in 1 mg l⁻¹ BAP in combination with 0.05 mg l⁻¹ NAA from the cotyledonary nodes. Additionally, through the optimized regeneration technique Agrobacterium-mediated transformation of L. iberica was successfully accomplished. Gene transfer was assessed on leaf samples from regenerated plantlets under a fluorescent microscope to detect the GFP signals. Moreover, transgene integration and its expression were confirmed by PCR and RT-PCR analysis, respectively. The establishment of these efficient regeneration and genetic transformation methods paved the way for further application such as plant improvement, functional analysis and gene editing.

Progress in Soybean Genetic Transformation Over the Last Decade

Soybean is one of the important food, feed, and biofuel crops in the world. Soybean genome modification by genetic transformation has been carried out for trait improvement for more than 4 decades. However, compared to other major crops such as rice, soybean is still recalcitrant to genetic transformation, and transgenic soybean production has been hampered by limitations such as low transformation efficiency and genotype specificity, and prolonged and tedious protocols. The primary goal in soybean transformation over the last decade is to achieve high efficiency and genotype flexibility. Soybean transformation has been improved by modifying tissue culture conditions such as selection of explant types, adjustment of culture medium components and choice of selection reagents, as well as better understanding the transformation mechanisms of specific approaches such as Agrobacterium infection. Transgenesis-based breeding of soybean varieties with new traits is now possible by development of improved protocols. In this review, we summarize the developments in soybean genetic transformation to date, especially focusing on the progress made using Agrobacterium-mediated methods and biolistic methods over the past decade. We also discuss current challenges and future directions.

GmRAV1 regulates regeneration of roots and adventitious buds by the cytokinin signaling pathway in Arabidopsis and soybean

The division and differentiation of cells are the basis of growth and development. Cytokinin plays an active role in cell growth division and differentiation. The Related to ABI3/VP1 (RAV) family comprises transcription factors in plants and all contain both AP2- and B3-like domains. In this study, GmRAV1 (Glycine max), which belongs to the AP2/ERF transcription factor family, was isolated and functionally characterized. Subcellular localization showed that GmRAV1 was localized to the nucleus and quantitative real-time polymerase chain reaction (qRT-PCR) analysis indicated that GmRAV1 was induced by cytokinin. Furthermore, compared with wild-type plants, plants overexpressing GmRAV1 showed dwarfism and late maturity. In contrast, the mutant of TEMPRANILLO (tem1) and GmRAV-i plants had an opposite phenotype. More interestingly, a root and shoot regeneration experiment indicated that GmRAV1 is one of the most important positive regulators of the cytokinin signaling pathway, which is involved in promoting root and shoot regeneration. In addition, RNA-seq and qRT-PCR results indicated that GmRAV1 is related to the key factors involved in the cytokinin signaling pathway, namely, cytokinin oxidase (GmCKX6 and GmCKX7), purine permease (GmPUP1), cell cyclin-related genes (GmCycA2;4, GmCycD3 and GmCYC1), cyclin-dependent kinase (GmCDKB2), cell division cycle (GmCDC20), E2F transcription factors (GmE2FE) and authentic response regulator (GmARR9). In conclusion, GmRAV1, one of the most important positive regulators involved in promoting root and shoot regeneration, was induced by cytokinin and is related to the key factors of the cytokinin signaling pathways.

Vacuum and Co-cultivation Agroinfiltration of (Germinated) Seeds Results in Tobacco Rattle Virus (TRV) Mediated Whole-Plant Virus-Induced Gene Silencing (VIGS) in Wheat and Maize

Tobacco rattle virus (TRV)-mediated virus-induced gene silencing (VIGS) has been frequently used in dicots. Here we show that it can also be used in monocots, by presenting a system involving use of a novel infiltration solution (containing acetosyringone, cysteine, and Tween 20) that enables whole-plant level VIGS of (germinated) seeds in wheat and maize. Using the established system, phytoene desaturase (PDS) genes were successfully silenced, resulting in typical photo-bleaching symptoms in the leaves of treated wheat and maize. In addition, three wheat homoeoalleles of MLO, a key gene repressing defense responses to powdery mildew in wheat, were simultaneously silenced in susceptible wheat with this system, resulting in it becoming resistant to powdery mildew. The system has the advantages generally associated with TRV-mediated VIGS systems (e.g., high-efficiency, mild virus infection symptoms, and effectiveness in different organs). However, it also has the following further advantages: (germinated) seed-stage agroinfiltration; greater rapidity and convenience; whole-plant level gene silencing; adequately stable transformation; and suitability for studying functions of genes involved in seed germination and early plant development stages.

Overexpression of a Chitinase Gene from Trichoderma asperellum Increases Disease Resistance in Transgenic Soybean

In the present study, a chi gene from Trichoderma asperellum, designated Tachi, was cloned and functionally characterized in soybean. Firstly, the effects of sodium thiosulfate on soybean Agrobacterium-mediated genetic transformation with embryonic tip regeneration system were investigated. The transformation frequency was improved by adding sodium thiosulfate in co-culture medium for three soybean genotypes. Transgenic soybean plants with constitutive expression of Tachi showed increased resistance to Sclerotinia sclerotiorum compared to WT plants. Meanwhile, overexpression of Tachi in soybean exhibited increased reactive oxygen species (ROS) level as well as peroxidase (POD) and catalase (SOD) activities, decreased malondialdehyde (MDA) content, along with diminished electrolytic leakage rate after S. sclerotiorum inoculation. These results suggest that Tachi can improve disease resistance in plants by enhancing ROS accumulation and activities of ROS scavenging enzymes and then diminishing cell death. Therefore, Tachi represents a candidate gene with potential application for increasing disease resistance in plants.