5 ways viruses can help plants
Like all other types of microbes, viruses have the potential to be neutral or even beneficial to their host. As part of a recent RTÉ Brainstorm Article, Marta Niedzicka identified 5 ways viruses can help plants.
When we hear the word 'virus', we typically think ‘disease’. However, viruses do not necessarily cause diseases. Viruses, like all other types of microbes, have the potential to cause disease, but they may also be neutral, or even beneficial, to their host.
Although viruses have been responsible for various crop diseases, resulting in significant losses of crops and vegetables, not all viruses behave like villains. In fact, there are a number of viruses present in plants that do not cause visible disease symptoms – and we still have a great deal to learn about the role they play. While the complete picture of the function of these viruses remains unknown, some aspects have been illuminated.
1). Plant protectors
Occasionally, viral fragments get stuck in the plant genome – and in rare circumstances, these fragments retain functionality, producing proteins within plant cells. The impact of this integration, whether good or bad, depends on the specific virus and the active protein. For example, the tomato genome carries beneficial virus fragments that contribute to the plant’s immune system.
The plant immune system, which differs significantly from that of humans, utilises small RNA sequences to deactivate or silence the infectious viral sequences, thereby stopping them from replicating. The beneficial virus fragments play a similar role, generating small silencing RNA sequences that limit infection.
This phenomenon is not the only example of a virus providing protection to its host plant. While some viruses causing acute infection change the plant fragrance to attract insects to feed on and damage the plant, other viruses exhibit more positive behaviour. Pepper cryptic virus 1 renders infected Jalapeño pepper less appealing to aphids compared to uninfected peppers. As aphids perceive these infected peppers less suitable for feeding or reproduction, the virus effectively functions as a protective agent against damage from feeding aphids.
2). Tolerance to drought and cold
Plants respond poorly to drought and cold weather, which are outside of their typical environmental preferences, resulting in dehydration and oxidative stress. That may sound familiar – it is a phenomenon in which a cell is damaged by free radicals. Prolonged oxidative stress in our organisms is one of the risk factors in certain diseases. At the same time, oxidative stress is part of plant ageing process.
Surprisingly, common plant viruses that affect plants like beet, zucchini, cucumber or tomato actually improve the plants drought resistance. While plants free from infection tend to wilt or collapse entirely, those infected with these viruses maintain their condition for much longer periods, thus enhancing their survival chances during unfavourable weather.
The virus helps the plant retain water and reduce transpiration. Additionally, it promotes the build-up of antioxidants – which can stop production of free radicals, limiting the oxidative stress damage to the plant - and other protective compounds in the leaves, which helps combat oxidative stress.
3). Resource managers
Like other plants in the legume family, white clover is known for its ability to capture nitrogen from the air and transform it into a form that plants can use. Helpful bacteria living symbiotically in nodules on the clover’s roots make this feat possible.
The plant expends energy to create and sustain these nodules, and an overabundance of them can hinder plant growth. The white clover cryptic virus 1 plays an important role in this process. Its protein suppresses nodule formation when the soil already is abundant in nitrogen. This virus is passed down to future generations of clover through seeds.
4). Beauticians
Flowers with unusual decorative features or colourful patterns on their petals always capture our attention. We keep propagating these plants to preserve their visual appeal. Some decorative features are encoded in their genes, while others are caused by viral infections. If you ever admired camellia with white patches on their pink petals, you may have admired the work of Camellia yellow mottle virus. Likewise, the Pelargonium flower break virus is responsible for the striking white streaks on the red petals of ivy-leaf geranium commonly found in hanging baskets.
The viruses can also beautify leaves. Frequently admired flowering maples with mosaic patterns on their leaves carry Abutilon mosaic virus. Japanese honeysuckle cultivars may have distinctive yellow veins causing beautiful chequered patterns – and the pattern is caused by either Honeysuckle yellow vein mosaic virus or Honeysuckle yellow vein virus.
5). Biocontrols for plant diseases
Chemical agents like fungicides are commonly used for protecting our crops from various pathogens. However, scientists are also looking for alternative approaches, including biological control strategies that employ natural enemies or pathogens to our crop pests. Here, viruses can also be utilised.
One current application of viruses is in managing chestnut blight, a devastating fungal disease. The virus infects and weakens the fungi, stopping the spread of cankers. While this process often occurs naturally, in regions lacking the virus, it has been introduced as a treatment. This method is particularly effective on young trees and, as infection is less severe, trees start to actually recover.
What might be even stranger, viruses can be used to shield plants from other viruses through a method similar to live vaccination sometimes used for us. Plants are intentionally infected with a virus causing only mild symptoms, which prepares their immune system to combat more aggressive strains of the same virus. Some of these mild strains have been used effectively to protect oranges from severe strains of the citrus tristeza virus.
All these examples represent just the beginning of our understanding of the full potential of viruses and their impact on plant biology.
Dr Marta Niedzicka is a postdoctoral researcher in Teagasc’s Crop Science Department, studying plant viruses across different crops and wild plant communities on a Marie Skłodowska-Curie Postdoctoral Fellowship (grant agreement no. 101106728).