Plant virus reported in new crop hosts
Scientists at the University of Minnesota in Saint Paul have identified the first Mastrevirus, a plant virus causing disease in crops worldwide, to infect both monocotyledonous and dicotyledonous plants. While this finding has interesting implications for gene editing via viral vectors, it may also impact pathogen management and food security.
Mastreviruses are a genus in the family Geminiviridae, a group of plant viruses responsible for the destruction of billions of dollars of crops every year. Most Mastreviruses infect monocotyledonous plants (monocots) such as rice, wheat, and maize. While monocots and dicots are both flowering plants, monocots have one cotyledon– the structure in the seed that stores nutrients– and dicots have two. Dicots include crops such as peppers, papayas, beans, and tomatoes.
The virus in question, Wheat dwarf India virus (WDIV), was first identified in wheat, a monocot, in India in 2012 [1]. However, WDIV was also reported in Solanum nigrum, or black nightshade, a dicotyledonous berry in the Solanaceae family, which includes other cultivated crops such as tomatoes and peppers. The berries and leaves of S. nigrum are used in some foods and traditional medicines.
When testing for sequence similarity between the wheat-infecting and S. nigrum-infecting WDIV strains, researchers identified only a 1% difference in the nucleotide sequences, which was randomly distributed throughout the genome. This confirms that the same virus is infecting the two different plant species as the Geminiviridae species demarcation threshold (the value at which two Gemini viruses are considered of the same species) is <94% similarity [2].
Wheat dwarf India virus is additionally unique because it is one of only two known Mastreviruses to associate with a satellite. Satellites are small, circular, single-stranded DNA molecules that rely on their association with a virus for replication and encapsidation. This means the virus and its satellite are most often found causing infection together. The satellite with which WDIV associates is Ageratum yellow leaf curl betasatellite (AYLCB).
WDIV’s ability to infect both monocotyledonous and dicotyledonous plants means it has the potential to become a very successful viral vector for gene editing. Often separate vectors have to be developed for each new host-virus interaction.
But in this study, a single WDIV vector was designed for use in barley, wheat, oat, corn (all monocots), soybean, and tobacco (dicots). The vector was created using the wheat-infecting strain of WDIV and included a copy of the phytoene desaturase (PDS) gene. When the PDS gene is silenced in host plants, the resulting photobleached phenotype is a visually identifiable sign that the gene editing has been successful.
The viral vector was introduced into wheat and tobacco via Agrobacterium tumefaciens-mediated transformation for a mechanism called virus-induced gene silencing (VIGS). VIGS utilizes the wheat or tobacco plant’s immune system, which innately senses the viral vector and generates double-stranded DNA that is then further cleaved into short-interfering RNA molecules (siRNAs). The siRNAs are produced to degrade the viral genes. However, since the virus has been modified to include the PDS gene, a gene also present in the host plant, the plant ends up degrading–or silencing–its own gene.
In transformed plants, the photobleached phenotype was observed on wheat leaves, spikes, and seeds, and tobacco leaves between three and eight weeks post-inoculation, confirming the success of the viral vector in both a monocot and dicot host plant.
The barley, wheat, oat, corn, soybean, and tobacco inoculated with WDIV showed varying amounts of systemic infection. A systemic infection occurs when the virus enters the plant’s vascular tissue (the same tissue that transports water and sugars throughout the plant) and spreads beyond the initial point of infection. In each of the species, over 50% of the inoculated plants became systemically infected. Each of these plants also tested positive for the presence of AYLCB. WDIV levels were higher in plants inoculated with both the virus and its satellite compared to plants inoculated only with the virus. This tells us that the betasatellite is important for WDIV’s virulence.
An expanding host range of Geminiviruses could cause even more loss than the billions of pounds of food already being destroyed each year. As climate change alters insect vector distribution and behavior, so too will viruses’ relationship with their vectors change [3]. Scientists, agronomists, and policy makers alike must take into account environmental changes and vector lifestyle in addition to virus biology in order to successfully manage plant diseases.
So much remains unknown about how plant viruses interact with their host plants, insect vectors, and even other viruses. The findings of this study raise concerns about mutated viruses with increased fitness, altered pathogenicity, and capabilities of overcoming host resistance mechanisms. In the face of climate change and an ever-increasing global population, managing plant viruses is paramount to crop viability and food security.
But these findings also have a silver lining: the possibility of a single vector system for the study of genetics in multiple plant species. The ability to study plant genes and how they affect phenotype gives scientists a fighting chance against crop loss and food insecurity.
About the Author: Shaina Eagle
Shaina, a class of 2024 Global Disease Biology major, is passionate about combating disease using the One Health Model, a multifaceted approach that addresses the full spectrum of human, animal, plant, and environmental health risks. She loves editing papers from all disciplines–whether it’s original research, science news, or her friends’ medical school personal statements. In addition to her time at The Aggie Transcript, Shaina worked in the Drakakaki Lab, studying hull dehiscence in pistachios.
References
- Sabelli PA. 2012. Seed development: a comparative overview on biology of morphology, physiology, and biochemistry between monocot and dicot plants. In: G Agrawal, R Rakwal, editors. Seed Development: OMICS technologies toward improvement of seed quality and crop yield. Dordrecht: Springer. 3-25.
- International Committee on Taxonomy of Viruses. Genus: Mastrevirus. Accessed December 5, 2023. Available from: https://ictv.global/report/chapter/geminiviridae/geminiviridae/mastrevirus
- Vilanova ES, Ramos A, de Oliveira MCS, Esteves MB, Gonçalves MC, and Lopes JRS. 2022. First Report of a Mastrevirus (Geminiviridae) Transmitted by the Corn Leafhopper. Plant Dis. 106(5):1330-1333.