Using vaccines might be common practice to protect livestock from damaging diseases, but research is showing that vaccination programs could be effective for crops, too.
New options are springing up that apply immunisation principles to managing some of arable farming’s biggest viral, bacterial and fungal threats — many of which are non-native species, meaning that crops’ natural defences can be limited.
Plant vaccination, while a useful term, is far from a direct parallel to animal vaccination. There are no needles here; instead, seeds or seedlings are infected with non-harmful pathogens or other compounds, which provoke immune responses in plants when they encounter more destructive threats in the field.
Viral vaccines have value
As one of the hotbeds of plant vaccine research, scientists in Japan have been working on weakened versions of viruses or bacteria, known as attenuated vaccines, against viruses affecting some of the country’s most important crops.
There are no antiviral compounds available for when plants are infected with viruses, so as an alternative, novel vaccines have been proposed, where weakened versions of viruses are used to infect host plants and offer cross-protection against different viral strains.
In a review of progress to date, Japanese experts have highlighted the potential of this approach, while also pointing out a few flaws. They note that plant vaccination has a long history, tracing it back to 1929, where tobacco plants were deliberately infected with tobacco mosaic virus light green isolate to protect against the yellowing symptoms caused by the yellow mosaic isolate.
“Plant vaccines serve as effective tools for controlling viral diseases in crops that lack native or introduced resistance genes. Despite being a relatively traditional approach, plant vaccines also represent an environmentally sustainable technique,” the researchers wrote in the journal Viruses.
Following extensive research, attenuated viruses for plant inoculation against cucumber and watermelon mosaic viruses were commercialised in 2018, with seedlings inoculated with the pathogens, as well as zucchini yellow mosaic virus available to farmers from 2022.
Among the limitations of such approaches, the researchers note, are that sometimes plants aren’t completely infected with the protective strains, leaving them vulnerable. Meanwhile cross protection can break down, and mutations can occur that make a protective agent damaging. There’s also the fact that farmers can simply be reticent to deliberately infect their crops, no matter how well-tested the solution might be.
Still, research continues to further convince the doubters. The scientists said that as well as targeting more damaging viruses in a greater range of plant species, developments in viral plant vaccines need to focus on creating plant vaccines, which combine a number of isolates, for greater protection.
Bolstering against major bacterial pathogen
There has also been major progress in designing a vaccine against bacterial wilt disease, which is caused by Ralstonia solanacearum and hits a wide range of economically important crops such as potatoes, tomatoes and bananas.
Infecting potato tubers with avirulent strains of R. solanacearum seems to be effective in reducing disease and increasing growth. Researchers have also found ways to tackle the possibility that the bacterium could revert to virulence, using boiling and producing cell-free liquids for use in the vaccination process.
A team in China has further advanced possibilities of applying vaccination more widely against the pathogen by optimising production techniques for a typical R. solanacearum strain tested in research to date.
Another group of Chinese scientists have explored more novel approaches to induce an immune response against R. solanacearum, this time in tobacco. They found that using nanoparticles to treat the crop induced increased the activities in its defence enzymes and genes, suppressing the development of bacterial wilt.
Taking on fungus using virus
Collaborative work between researchers in China and US showed that viruses can also initiate immune responses against a major fungal pathogen, albeit via a different route.
In the work, the scientists explored using a mycovirus —virus which infects fungi — against white mould, which is considered one of the most devastating crop diseases worldwide.
Rather than priming the plant against the fungal infection, the fungus itself, Sclerotinia sclerotiorum, is infected with a virus known as S. sclerotiorum hypovirulence-associated DNA virus 1 (SsHADV-1), which effectively turns it from plant crop destroyer to defender.
The scientists showed that not only do strains of the virus isolated in China work equally well to this effect in the US, but that it is possible to then inoculate beans successfully with the virus, which increase the expression of genes associated with immunity and initiate rapid response to S. sclerotiorum infection.
Beyond solutions for plant diseases, it’s worth mentioning that plants are also seen as having great potential as ‘green factories’ for the creation of edible vaccines against our own diseases and those of our livestock; further illustrating just how versatile plant tissues are, and their myriad possible applications in a sustainable future.
