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  • Alex Cloherty

Mean, green, plant-infecting machines

Updated: Jun 8, 2020

This week's Microbial Mondays post is in response to two questions: 1) "Are there viruses for plant cells?" from D’Arcy, and 2) "Can you please finally write about a different virus?" from Robert (Sassy. But Rob is my boyfriend, he's allowed to be).

Plant viruses are even passed from plant to plant in a way that is remarkably similar to how many human viruses are transmitted. Just like with humans, plant virus transmission can be split up into "vertical transmission", which means viral transmission from parents to progeny, and "horizontal transmission", which means passing viruses to new humans or plants in basically any other way.

In humans, "vertical transmission" usually happens from mother to child. This includes virus transmission to fetuses, such as we now know can happen with Zika virus, with detrimental effects on the growth of the unborn children's brains, and transmission at the messy moments of birth or during breastfeeding, which can happen with HIV if the mother is not taking state-of-the-art antiviral medication. The difference with plants, is that vertical transmission means that the virus spreads through infected seeds or pollen. But the effect remains the same: infected plant parents can also spread the infection to their plant kids.

For both plants and humans, bugs are an important horizontal transmitter of viruses. If we're talking about humans, mosquitoes are a major pesky virus transmitter, as I've written about before in the case of Dengue virus. With plants, it's different bugs but bugs nonetheless. For example, insects like aphids that gnaw on plants, just as mosquitoes gnaw on humans, can pick up viruses and bring them from one plant to another.

You might be thinking, well that must be it, right? Plants don't really meet up with each other, so they're natural social distancers, no? Wrong! I was also surprised to learn that plants can also transmit viruses horizontally by contact transmission. Viruses can jump from plant to plant when infected roots or leaves meet uninfected ones under or above the ground - especially when there is a bite or a scratch already on those roots or leaves, where the virus can get out of the infected plant and into the uninfected plant more easily. Humans and other animals can also contribute to a sort of contact transmission by pruning, carving, trampling, or grafting plants, and thereby leaving sort of open wounds that are also open doors for viruses to enter plant cells.

So it's not just humans who are infected by viruses. Ok, ok. But why should we care about plant viruses?

Well, they cause pandemics too.

Because plant and animal cells are very different, plant viruses don't infect humans. But, humans are heavily reliant on plants. We rely on plants like cotton, which has been devastated in multiple epidemics of cotton leaf curl virus, for clothing. We rely on plants like tomatoes, which suffer from "the spotted wilt disease" caused by tomato spotted wilt virus, for food. As a side note, tomato spotted wilt virus is a particularly nasty one that can also infect peppers, lettuce, peanuts, and even some flowers. And, we rely on plants for beauty. Not much is yet known about viruses of wild plants, but scientists are a little worried that climate change could mean that natural environments don't only suffer from having to deal with different climates, but also different disease-causing microbes.

And, to make it worse, all of this has huge social and economic impacts. Plant viruses cause billions of dollars of losses in terms of food and materials production every year. This is especially problematic in places where food security is already unstable, and in places where there is a lot of the exact same plant around. For instance, for a huge proportion of the world's population, a starchy potato-like vegetable called cassava is a major source of calories. This also means that in some countries, there is a lot of cassava very close together. And that is an ideal breeding ground for cassava mosaic virus. To make matters worse, farming cassava depends on cutting off stems to propagate, which gives a perfect opening for the virus to invade. The pandemic of cassava mosaic virus that has lasted decades in sub-Saharan Africa destroys more than 12 million tonnes of cassava, equivalent to over one billion US dollars, every single year, which, on a more human level, destroys individual cassava farms, forces farmers to find new crops, and leaves people without dinner.

Luckily, just like with the 2019 novel coronavirus, scientists are working on combating plant viruses. To be honest, though, viruses have so far proved tricky to combat. We haven't come up with any traditional chemical pesticides, like those that are used against bacteria, fungi, and insects that damage plants, that can be used to prevent viral diseases. In the past, the main strategy for preventing viral outbreaks in plants has been to try to stop insects from munching on them and leaving open wounds in the plants where viruses could get in.

Recently, though, we've started learning from the plants themselves. It turns out, that the old-school, wild ancestors of plants that humans have bred for hundreds to years to make them bigger, juicer, and tastier (add any intensifier, really), were not too bad at resisting viruses. Some of these old school plant great-great-grandparents actually have genes that helped them fight back against viruses. Basically, the wild plants evolved to have some antiviral tricks up their sleeves, but we humans have in some cases accidentally cultivated some of our favourite fruit and veg to the point that they have now forgotten these tricks. To come full circle, modern genetic engineering may help us to give these old tricks back to the new plants. Just like with human grandmas, sometimes plant grandmas know best.

Until next week, see if you can learn something from your Grandma, too ;)

~ Alex

These are the papers that I relied on most heavily for writing this blog post. A big shout-out and thank you to their authors!

The evolution of plant virus transmission pathways.

Global dimensions of plant virus diseases: Current status and future perspectives.

Engineering plant virus resistance: from RNA silencing to genome editing strategies.

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