Direct Agroinoculation of Maize Seedlings by Injection with Recombinant Foxtail Mosaic Virus and Sugarcane Mosaic Virus Infectious Clones
February 27th, 2021
An Agrobacterium-based injection (agroinjection) protocol is presented for the inoculation of foxtail mosaic virus and sugarcane mosaic virusclones into maize seedlings. Inoculation in this manner leads to viral infection, virus-induced gene silencing of marker genes, and viral overexpression of GFP.
This protocol allows the inoculation of viral vectors directly into maize plants that can be readily applied in most lab settings without the need for any expensive specialized equipment. Direct inoculation eliminates the use of an alternative host as an inoculum source, saving both time and resources. This method could be applied to additional viral vectors, maize lines, and potentially other monocot species.
Begin by planting 1 to 2 maize seeds in the peat-based growing medium in small inserts placed inside trays. Place the trays in a growth chamber under 16 hour days at 25 degrees Celsius and 8 hour nights at 22 to 25 degrees Celsius, or in a greenhouse at 22 to 25 degrees Celsius with 16 hour days and 8 hour nights. Regularly water plants and fertilize once a week with 15-5-15 liquid fertilizer at 330 parts per million concentration.
Prepare agrobacterium for injection by inoculating the agrobacterium strain containing the desired viral construct in LB media with antibiotics. Then, incubate the flask or tubes at 28 degrees Celsius while shaking at 225 RPM for 24 hours. On the next day, pellet the bacteria by centrification at 4, 000 times G for 10 minutes at room temperature and discard the supernatant.
Then, wash the pellet with one milliliter of deionized water. Re-suspend the pellet in one milliliter of 10 millimolar magnesium sulfate solution, and measure the optical density at 600 nanometers. Then adjust it to 1.0.
Use 4 to 7 day old maize seedlings for injecting agrobacterium. Assemble the syringe and needle. Gently inject the bacterial suspension 2-3 millimeters above the coleoptilular node until it fills up the coleoptile, or is visible in the whorl, pending on the growth stage of the plants.
Inject all seedlings and change the syringe and needles for injecting each construct. Confirm phenotypic infection by observing lesions from silencing the control genes, lesion mimic 22 or phytoene desaturase on the leaves. Use a fluorescence imaging device for quick screening of plants and fluorescence microscopy to detect the presence of GFP expression.
Perform gene expression analysis for molecular detection of infection. Extract total RNA from leaves of infected plants 14 to 21 days after infection, and synthesized first strand cDNA as described in the text manuscript. Perform reverse transcriptase PCR to confirm infection and determine the gene's integrity or gene fragment of interest using specific primers designed for different viral constructs.
Visualize the PCR product on a 1%agarose gel containing a nucleic acid stain to determine the presence or absence of virus and gene or gene fragment. This protocol was used to insert recombinant viruses engineered for gene into maize seedlings. Approximately 12 days after injection, silencing phenotypes were observed on leaves as necrosis and a photobleaching.
The presence of construct in leaves after infection was detected by observing green fluorescent protein expression under a fluorescence using a different GFP filter. Green fluorescent protein expression in foxtail mosaic virus infected leaves was visualized as small punctate areas of fluorescence distributed across the leaves and as large patches in sugarcane mosaic virus infected leaves. Using the gene expression analysis, systemic foxtail mosaic virus infection was confirmed and gene silencing or suppression of phytoene desaturase and lesion mimic 22 was observed.
The construct also used influenced infection efficiency. In the case of foxtail mosaic virus infection, foxtail mosaic virus empty vector, and foxtail mosaic virus lesion mimic 22 typically had the highest infection efficiencies at 53%and 54%respectively. Foxtail mosaic virus phytoene desaturase at a slightly lower efficiency at 39%and foxtail mosaic virus green fluorescent protein, at the lowest efficiency at 16%Meanwhile, the infection efficiency of sugarcane mosaic virus green fluorescent protein was 8%Timing and symptoms will be based on the viral vector and maize line used.
Lack of a visual phenotype may not mean a lack of silencing or expression. This method can also be applied to gene editing technologies by improving delivery methods for guide RNAs.
Overview
This article presents a detailed protocol for Agrobacterium-mediated delivery of recombinant viral vectors into maize seedlings. The method enables direct inoculation of viral constructs for gene silencing and gene expression studies, eliminating the need for alternative host plants and specialized equipment. The approach is optimized for foxtail mosaic virus (FoMV) and sugarcane mosaic virus (SCMV) vectors and is adaptable to various maize genotypes and potentially other monocot species.
Key Study Components
Area of Science
- Plant molecular biology
- Plant virology
- Genetic engineering
Background
- Agrobacterium-based methods are widely used for introducing DNA into plant tissues.
- Viral vectors facilitate gene silencing and expression in plants.
- Traditional inoculation often requires an alternative host as a virus source.
- Direct delivery methods can save time and resources in plant genetic studies.
Purpose of Study
- To develop and optimize a protocol for direct Agrobacterium-mediated delivery of viral vectors into maize seedlings.
- To demonstrate the method's effectiveness for gene silencing and expression using FoMV and SCMV constructs.
- To provide a broadly accessible technique for plant molecular research.
Methods Used
- Growth of maize seedlings in controlled conditions (growth chamber or greenhouse).
- Preparation of Agrobacterium tumefaciens GV3101 carrying recombinant viral constructs in pCAMBIA1380.
- Injection of bacterial suspension near the coleoptilar node of 4–7 day old maize seedlings.
- Phenotypic observation (lesion mimic 22, phytoene desaturase silencing, GFP expression) and molecular confirmation via RT-PCR and agarose gel electrophoresis.
Main Results
- Successful systemic infection and spread of recombinant viruses in maize seedlings.
- Observable phenotypes: necrosis and photobleaching for gene silencing, GFP fluorescence for gene expression.
- Infection efficiencies varied by construct: FoMV empty vector and les22 (53–54%), FoMV pds (39%), FoMV GFP (16%), SCMV GFP (8%).
- RT-PCR confirmed the presence and integrity of viral constructs in infected plants.
Conclusions
- The protocol enables efficient, direct delivery of viral vectors into maize without specialized equipment.
- It is effective for both gene silencing and expression studies in maize.
- The method is adaptable to other viral vectors, maize lines, and potentially other monocots, and may facilitate gene editing applications.
Frequently Asked Questions
Executive Industry Relevance
Direct agroinoculation of maize seedlings with recombinant viral vectors streamlines gene function interrogation and trait validation in monocot crops. This approach eliminates the need for alternative hosts and specialized equipment, accelerating early discovery and enabling rapid hypothesis testing in plant biotechnology pipelines. The method supports scalable, reproducible workflows for gene silencing and expression studies, enhancing predictive confidence in target validation and trait engineering.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Enables rapid functional genomics studies by direct delivery of gene silencing or expression constructs.
- Facilitates mechanistic de-risking through observable phenotypic changes linked to specific gene targets.
- Supports predictive confidence in trait selection and target prioritization for crop improvement.
Screening & Assay Development
- Prepares validated maize systems for downstream screening of gene function or trait expression.
- Standardizes infection and expression protocols, improving reproducibility across genotypes and constructs.
- Generates quantitative outputs via RT-PCR and fluorescence imaging for reliable construct evaluation.
Translational & Preclinical Research
- Aligns gene function studies with disease-relevant or agronomically important traits in monocot crops.
- Enables continuity from early discovery to preclinical trait validation in plant biotechnology pipelines.
- Provides mechanistic insights that inform risk-adjusted advancement of engineered traits.
Pipeline & Workflow Integration
This direct agroinoculation protocol integrates into the discovery-to-validation continuum for plant trait engineering and functional genomics.
- Discovery Biology: Supports hypothesis testing and pathway clarification by enabling direct gene silencing or expression in maize.
- Screening: Delivers reproducible, quantitative infection and expression data for construct comparison.
- Analytics: Employs RT-PCR and fluorescence readouts to confirm construct presence and functional outcomes.
- Translational Research: Bridges early gene function studies with preclinical trait validation in relevant crop systems.
- Enterprise Reuse: Offers a broadly accessible, adaptable platform for diverse viral vectors and monocot genotypes.
Operational & Enterprise Impact
- Scientific Value: Increases predictive confidence and reduces mechanistic ambiguity in gene function studies.
- Operational Value: Standardizes and scales gene delivery without specialized equipment or alternative hosts.
- Strategic Value: Accelerates go/no-go decisions and improves capital efficiency in trait development pipelines.
- Portfolio Impact: Enables risk-adjusted prioritization and advancement of engineered traits or targets.
Implementation Considerations
- Requires expertise in plant molecular biology and Agrobacterium-mediated transformation.
- Needs basic laboratory infrastructure for plant growth, bacterial culture, and molecular analysis.
- Demands cross-team standardization of injection and analysis protocols for reproducibility.
- Adaptable to various maize genotypes and potentially other monocot species with protocol optimization.
- Infection efficiency and phenotype visibility may vary by construct and genotype, requiring molecular confirmation.
