By Massimiliano Borgia
I plant virus I am the great forgotten enemy. Yet even these they fight only by increasing the plant's resistance and avoiding contagions as much as possible.
Not much news but we are constantly on the lookout for viroses affecting agriculture. A he struggles relentlessly towards diseases which do not infect man, but which can make man lack food. This has always been the case, but today, in the connected world and in intensive agriculture, the "phyto" viruses can arrive in an instant and fold entire agricultural economies, entire territories and change the prices of food commodities with economic consequences on the production chains and on the food prices.
Therefore, plant viruses do not make man and animals sick but only plants. Every day, without knowing it, we eat and touch many viruses with the leaves of salad, biting the fruit, cutting the potatoes, cleaning vegetables, but always without any consequence for our health as long as fruit and vegetables are not already full of rot.
Yet many plant viruses are similar to those that make us sick and, above all, behave in the same way: they start the infection process of a plant by penetrating one or a few cells, they subvert its metabolism to force them to replicate viral RNA and to form new virions which, in turn, will infect nearby cells, making the whole plant sick.
So similar to our diseases that, in these days, there are those who is investigating certain mechanisms of the virus-plant cell relationship to apply the findings in the fight against Coronavirus.
Is exactly the discovery of viruses has come from the study of plant diseases.
To be precise, it was a tobacco disease that made people think that the smallest inhabitants of the natural world were not the known bacteria but something infinitely more microscopic. It was given the name "virus" which in Latin is synonymous with poison and their discovery is relatively recent. In 1879, Adolf Eduard Mayer, director of the experimental station of the then Agricultural School of Wageningen (NL), was called to study an unknown tobacco disease, which caused a patchwork of stains on infected leaves and which was not attributable to any known pathogen. In 1886 he called it "tobacco mosaic" (it is still known by this name) and proved its transmissibility through the inoculation with juice extracted from infected plants. Despite being able to transmit the disease, Mayer was unable to isolate the agent, but hypothesized the existence of bacteria of unknown nature, invisible under the microscope.
The existence of an infectious agent smaller than the bacteria was described in Germany by F. Loeffler and P. Frosch in the case of animal foot-and-mouth disease and, in 1901, by an American doctor, Walter Reed, who identified the yellow fever virus, the first known human virus.
But for another 30 years, science failed to identify viruses with certainty. TO Dimitri Ivanovsky descriptions of crystalline inclusion bodies in infected cells are due (1903), then identified as tobacco mosaic virus (TMV) aggregates, visible only with the first electron microscope images in 1939.
So we started from plants to get to Man.
In the agricultural world there are historical viruses and emerging viruses, just as in the case of the animal and human world.
Many do not yet know each other, but the world of viruses is also in eternal movement: during replication they can change generating new viruses with slightly different characteristics. New viruses can also arise from two "parent" virions by RNA recombination.
Plants show characteristic symptoms (as with human viral infections). They range from the spots on the leaves to the "mosaics", from the alteration of the color or shape of the plant which takes on non-natural aspects to the malformation of the flowers.
Viruses can be present in a plant asymptomatically and then manifest symptoms when they attack a plant belonging to a different species. Or, they can remain latent in asymptomatic plants and then "wake up" and infect the same host plant even very far in time. Usually, they have a very close relationship with the species they have adapted to, but they can also attack species from different families.
But plants know how to defend themselves.
"Unlike humans, a plant cannot produce antibodies - explains Luisa Rubino, a molecular virologist of the CNR Institute for Sustainable Plant Protection and a member of the Academy of Georgophiles - The defense by the plant is very different. For example, a hypersensitivity reaction may develop. The virus enters a cell and carries out the first cycle of replication, but the plant "feels" it and kills the cells surrounding the infected one and, in this way, also kills the virus. Sacrifice a small part of herself to save herself. Another mechanism was discovered just in the 90s and is called "gene silencing". About 80% of known viruses have a genome made up of single-stranded RNA, just as coronavirus is made. In the infected cell, the virus releases the genome to induce it to synthesize the proteins necessary to create new viral RNAs. At the time of replication, double helix molecules are formed, in practice a copy of the genome that serves as a "template" to replicate the new RNA and the new genome. Here, at that precise moment, the plant "feels" the attack and reacts with some enzymes that "cut" the RNA into small pieces and make it harmless. Of course, viruses have also developed strategies to protect against the plant's defenses, and in plant cells there is an uneventful struggle between pathogen and host ».
So the only solution is to have strong plants, able to implement their protective mechanisms.
«The only defense we have is to obtain greater resistance of the plant. In nature this happens randomly, but in agriculture they can be obtained plants more resistant to viruses using classical genetics, that is the old system of crosses. Plants of the same species cross which we have seen to be less affected by the disease. We will get stronger plant seeds that we can cross again until we have a resistant variety. We must not forget the modern methods of genetic engineering, even if not very applied, which allow to accelerate the time of traditional genetics very much. With cisgenesis it is possible to transfer a resistance gene from an interfertile species, and not from a completely foreign plant as in the case of transgenesis, thus imitating the technique of crossing ».
How do viruses spread to plants?
«They can enter through microperitis on the plant. Or they are transmitted by vectors, i.e. virus-carrying insects (aphids, buzzers, bedbugs for example) which, by pricking the plant to suck the sap, inoculate the virions in the plant. The most dangerous viruses that we know are transmitted with insects: for this reason, the insect is fought not only to reduce the damage that it does, but also to hinder the dispersion of the viruses which it could carry. Where insect nets are placed or where insecticide cultivation is concerned, we see a lower incidence of viruses. But another possible mechanism for spreading viruses is transmission through human intervention. A cut made on an infected plant followed by another cut on a healthy plant with the same infected tool spreads the disease through the tool blade ".
Intensive agriculture is naturally more exposed to viruses ... «Yes, of course. One seed of one million lettuce infected is enough to attack the whole field. Farmers who grow in a greenhouse know something about it: indoors, you need to be very careful. A classic case is the ban on smoking: a cigarette can be a vector of the tobacco mosaic virus if you touch the plants of a greenhouse with your hands, and you can infect the entire cultivation if you cultivate a species attackable by the same virus . It is a good practice to remove and destroy infected plants to prevent them from acting as sources of inoculation. "
For this reason, prevention is fundamental in the fight against plant viruses. The defense against the spread of viruses in Italy is at the forefront. Our legislation has often been a pioneer. In general, it is forbidden to import plants from non-EU countries that are not certified. Seeds must also be certified as immune to viruses. And it's a good idea to limit generations of DIY seeds unless they are produced in less vulnerable environments.
But Italian agriculture always introduces new varieties and always new species, due to market demands but also to adapt to climate change. Therefore new plants arrive and we often plant species in environments that are not suitable for them, bringing specimens into the field that weaken and are more subject to diseases.
«Of course, planting mango instead of wheat is not a good idea if you want strong plants; nor do ancient varieties help us, which have been selected above all to resist other factors than viruses. I would say that the best defenses are these: seeds certified immune, planting materials as much as possible from good general fitness and above all virus-free, defense from vector insects. Otherwise, if the right virus arrives in the field or in the orchard, the entire crop is lost and the subsequent ones can be lost. "
By the way, here are the most famous viroses present in our crops. Those able to make us lose large quantities of food in a few days.
For Pomodoro: the most classic viruses are the tomato yellow leaf curl virus and the tomato spotted wilt virus, to which cucumber mosaic virus can be added virus) that brought tomato cultivation to its knees in southern Italy in 1988. Among the emerging viruses, the wrinkled browning virus, the tomato brown or wrinkled fruit virus is important.
For the Vine: among the oldest viruses there is the main agent that causes the disease of infectious degeneration of the vine (grapevine fan leaf virus, virus of the leaf curl of the vine). Two emerging viruses that are assuming some importance are the vine red leaf spot virus and the Pinot gris virus (grapevine red blotch virus and grapevine Pinot gris virus).
For the Citrus fruits: the "sadness" virus of citrus fruits (Citrus tristeza virus), "sadly" famous in Sicily.
for Stone fruit: the plum pox virus (plum pox virus), a real scourge
