Did you know that plant viruses can be more than pathogens that cause harm to crops? They might also be used as “allies” to increase food production safely and sustainably. Viruses are obligate intracellular parasites that infect all cellular species. Assuming that plant viruses are simple pathogenic entities vastly underestimates their incredible diversity, their non-pathogenic roles in natural ecosystems and their potential to be used as biotechnological tools. Engineered viruses are currently used to safely delivery vaccines to humans and realizing the immense potential for using engineered viruses as our allies, rather than just fighting them as enemies, is just beginning.
Plant pathogens and climate change are threatening our ability to feed the world, with the United Nations estimating that, by 2050, food production must increase by 60% to feed nearly 10 billion people. Development of safe and sustainable technologies that rapidly and transiently enable crops to respond to adverse conditions that threaten food production is critical. The crops that feed humans and livestock represent 80% of food consumed. World-wide, plant pathogens and pests cause annual losses in major crops of 8 to 41%. Traditional approaches to controlling pathogens and pests in crops are either slow (breeding for resistance) or bad for the environment (pesticides, fungicides, etc.). Both approaches can lose effectiveness due to rapid evolution of pathogens.
Delivery of molecules that increase crop resistance to biotic and abiotic factors using engineered viruses could be a safe, sustainable, and rapid way to reduce yield losses. To begin to test the utility of this approach, we need to develop the required molecular tools. Most plant viruses have RNA genomes, so we make cDNA plasmid clones of the virus, plus any virus proteins required for initiating an infection in the plant host.
In a recent Molecular Plant Pathology article, we reported the first infectious clone of maize mosaic virus (MMV), a rhabdovirus that infects maize and its insect vector, the corn planthopper. Like all rhabdoviruses, MMV has a negative sense RNA genome that must first be transcribed into the positive sense RNA in the host cell for production of the structural viral proteins required for virion assembly and host infection. For MMV, transcription of the genomic RNA requires three viral proteins: the nucleocapsid protein (N), phosphoprotein (P) and the polymerase (L, for large protein). After these four cDNAs were inserted into plasmids, the plasmids were co-delivered into cells of a tobacco plant (Nicotiana benthamiana).
We were excited to find MMV-infected cells within 2-weeks of inoculation! Extracts of the tobacco leaves could be used to inoculate infectious MMV into maize and insects, confirming its viability in its original hosts. To track virus infection (and demonstrate the potential of MMV to act as vehicle for foreign protein expression in plants and insects), the green fluorescent protein (GFP) was inserted in the MMV genome. Plants inoculated with the modified clone had the bright fluorescence characteristic of stable GFP expression (and fluorescence that remained stable over successive infectious cycles in plant and insect hosts).
Creation of this system for introducing modified MMV into maize and insects opens the door to a range of applications for expressing foreign proteins or RNAs in maize and insects. This has the potential for genome editing in maize to enhance crop productivity. Direct modification using this vector could decrease the time needed to modify important resistance traits by several years. In addition, the MMV infectious clone will allow us to probe fundamental aspects of MMV infection in both plants and insects to produce applied solutions for improving food security.
Blog by César A.D. Xavier, Anna E. Whitfield and Margaret G. Redinbaugh.
Surapathrudu Kanakala, César A. D. Xavier, Kathleen M. Martin, Hong Hanh Tran, Margaret G. Redinbaugh and Anna E. Whitfield published this study in Molecular Plant Pathology:
TITLE IMAGE: MMV infectious clone in maize and insects. Full-length genome and the viral nucleocapsid (N), phospho (P) and polymerase (L) proteins were cloned into binary plasmids (left panel) then transformed into Agrobacterium tumefaciens (right panel). As maize is not a host of Agrobacterium, media containing each of the three bacterial cultures was agroinfiltrated into leaves of tobacco (N. benthamiana) and GFP fluorescence of leaf cells was monitored as a reporter for virus replication (middle panel). A crude extract of N. benthamiana leaves was then mechanically inoculated into germinating maize kernels and leaf fluorescence was monitored (right panel, lower image). The ability of the modified MMV to replicate in and be transmitted by insects was assessed by injecting virions purified from infected maize leaves into adult maize planthoppers and monitoring fluorescence. GFP was efficiently expressed in both maize and planthoppers (right panel). All images used with permission of the author.