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(The Gist of Science Reporter) International Year of Plant Health

[July-2020]

International Year of Plant Health

2020 is the International Year of Plant Health. By definition, a virus is an infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host. A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria.

What is a Plant Virus?

Like other viruses, plant viruses are also composed of a nucleic acid (genome), which is encapsidated by a protein coat. In addition, particles of some plant viruses, like members of the genus Tospovirus, are enveloped by an outer membrane containing lipids and proteins. A similar lipid envelop is also found in Coronaviruses, though they belong to different families. In India, two families of viruses, namely Geminiviridae and Potyviridae, are the major killers of agricultural crops.
The amount and arrangement of the proteins and nucleic acid of viruses determine their size and shape. There is a great variation in the size and shape of the plant viruses, ranging from 17 nanometres to 2000 nanometres in size. Shapes of viruses are predominantly of two kinds: rods or Filaments, like a rigid rod, flexuous rod, isometric, bullet shape, twin icosahedral, etc. Majority of the plant viruses have single stranded positive-sense RNA genome. Besides, there are plant viruses with a genome composed of single-stranded negative-sense RNA, double-stranded RNA, single-stranded DNA and double-stranded DNA.
Many of the plant viruses have a multi-segmented genome, which is either encapsidated in different particles or within a single particle. The plant virus genome codes for proteins essential for their replication, cell-to-cell movement, and encapsidation.
Apart from these essential protein-coding genes, genomes of many plant viruses additionally code for proteins necessary for insect transmission and suppressors of RNA silencing to overcome the plant defence. Many plant viruses are associated with satellite DNA or RNA molecules, which increases their virulence and host adaptability.

How do Viruses Attack Plants?

Plants are just as susceptible to viral infections as other living organisms. Plant viruses can not directly penetrate the host cell as plants have a cell wall to protect their cells. Till now there is no evidence that a receptor of cell wall allows the plant virus to enter host cell through endocytosis, which is very common for human and animal viruses.
So, a wound is necessary for a plant virus to enter a plant cell. The wound may be created by weather, insects, animals, or even human activities during agricultural operations. Once a plant gets a viral infection, it spreads through different vectors like insects, nematodes, fungi which attack plants. More than 70 per cent of known plant viruses are transmitted by insects, the majority belong to the order Hemiptera, which includes aphids, planthoppers, leafhoppers, bugs, etc.
Many viruses may pass to new offspring through propagating materials of plants, pollen, etc. Upon entering into the plant cell, plant viruses translate their proteins, replicate
their genome and move from one cell to another cell. During these processes plant viruses continuously exploit different host proteins, interact with different proteins and thus regulate different cellular pathways. Such intense interaction ultimately leads to the development of abnormalities or symptoms.

What Happens in a Virus Attack?

One of the characteristics of viruses is that they mutate very fast, not on their own, but inside the host body, infecting not only one cell or one plant – the emerging viruses infect newer hosts and lead to an epidemic like situation. The viruses can turn out to be deadly for any living form. They have the negative ability of destroying complete crop production, species and even genres of plants.
Some viruses act friendly with plants, equilibrate themselves with plant ecosystems and ruin agriculture. The Tospovirus of tomato is an example of a friendly virus destroying the tomato crop completely. Viruses, in general, affect plant growth, production and productivity. In a virus attack, plants don’t get proper nutrition as it is all taken by the virus. The plants feel stranded and unassisted. Just like in humans and animals, viruses affect the plant’s respiratory system badly and also lead to tension in plants.

Can Virus Attack on Plants be as Deadly as Coronavirus?

Just like all other viruses, plant viruses are obligate pathogens that depend on their hosts to survive. So, no viruses deliberately kill their host as that will not be beneficial for their own survival. Except a few plant viruses like Tospovirus, which are believed to be of insect origin, plant viruses generally do not kill any plants. They weaken the plant immunity, reduce the plant growth, vigour and thus reduce yield and deteriorate the quality of the produce.
If a severe virus infection occurs in an early stage of plant growth the plant may not be able to produce any product. Due to weak immunity, or when attacked by other pathogens too, the plants ultimately get killed. There are many plant viruses belonging to the genus Begomovirus, Tospovirus, Illarvirus, which are as severe as coronavirus, where the infection spreads like wildfire through insects within weeks and almost no yield can be obtained from the entire field.

How to Tackle the Viruses?

With our recent experience, we can fathom that a virus can’t be handled even with care. For any farmer, the only solution to get rid of viruses in plants is to burn and remove the affected plant – breaking the chain and establishing social distancing are the keys to prevent a virus attack. And so, removing or burning the affected plant for saving other plants is the idea. Else, farmers can spray insecticides or pesticides to save the plants from any kind of attack by insects and pests. The least that can be done is changing agricultural practices, which most of the times is not very feasible.

Can Plant Viruses be Cured?

Once a plant gets infected by a virus there is no way to cure that plant. Sometimes due to the plant’s own defence mechanism symptom recovery is observed but this happens rarely. Though plants do not have an antibody mediated immune response system like animals and humans, they possess a general defense system called RNA silencing, which detects and degrades viral RNAs or viral transcripts.
To overcome this defence, the majority of plant viruses encode a protein called suppressor of RNA silencing, which inactivates the host defence. Depending on the plant defence and viral pathogenicity the plant’s response to the infection may range from a symptomless condition to severe disease. There is no chemical or drug available till now to cure an infected plant. With proper nutrition and supplement of some essential minerals, in few cases severity of symptoms can be reduced but under severely infected condition no cure is possible. However, prophylactic application of different molecules (e.g. dsRNA) has shown promising results in preventing plant virus infection.

How are Plant Viruses Managed?

The best way to manage a plant virus infection is to protect the plant from getting infected as there are no antiviral compounds available to cure plants with viral diseases. The first step for management of the virus is to identify the virus, understand its transmission behaviour and survival.
Different preventive measures like use of certified virus-free seed or vegetative propagules, elimination of the weeds and other surrounding plants that may harbour virus, modification of planting and harvesting practices, and intercultural operations are important cultural practices those can greatly reduce the risk of viral infection in plant. If the virus is transmitted by a vector, control of the vector through judicious use of chemical or mulching may be effective.
The most important aspect of plant virus management is to develop crop varieties with resistance. The resistance may be incorporated through conventional breeding from natural plant sources or may be derived from the virus through transgenic development. Recently topical application of dsRNA and geneediting technology has shown promise for management of viral diseases but these studies are still in their infancy.

Can Plant Viruses Jump to Humans?

Generally, viruses evolve with their host and a great specificity exists with the virus and their hosts. However, for viruses, it is not uncommon to switch the host kingdom. Since humans depend on agricultural crops, plant viruses can enter the human body through virus-infected foods, which is evidenced from the fact that many stable plant viruses have been detected in human feces.
If a plant virus could break down the host specificity and is able to multiply in an animal, this multiplication could remain unnoticed if it is not associated with a specific symptom and if there is no further transmission to other animal. Certainly, till now there is no evidence that plant viruses cause diseases in humans and other animals. A report has shown that a plant virus, Pepper Mild Mottle Virus (PMMoV), found in many pepper-based products could be detected in human body but the symptom associated with the illness cannot be correlated.
Upon experimental exposure plant viruses can be detected in mammals and human samples, and there is also evidence of immune responses to plant viruses in animals and humans.
There is no rigid rule that plant virus can not break the barrier of their host kingdom and invade humans or animals. There are many plant viruses, like tospoviruses, reoviruses, rhabdoviruses, which can replicate in their insect host. So, if a plant virus can infect insects it may also be not impossible to infect humans. Although till now there is no such evidence.
Many human-infecting viruses and plant-infecting viruses belong to the same family with similar genome organisation. For instance, viruses of the family Reoviridae, Phytoreovirus, Fijivirus and Oryzavirus infect plants while the rotavirus, a major cause of gastroenteritis in humans, also belongs to this family. Similarly, under the family Rhabdoviridae, which includes rabies virus infecting humans and animals, there are also plant and insect infecting viruses like Cytorhabdovirus, Nucleorhabdovirus, Dichorhabdovirus, Tenuivirus.

An Initiative for Plants to Flourish:

The story of the International Year of Plant Health (IYPH) began in 2012 at the Stockholm Conference which decided that a year would be devoted to plants. The United Nations General Assembly took charge and raised this issue globally.
It can be marked as a golden opportunity to raise global awareness on protecting plants. Considering plant health as priority can help end hunger, reduce poverty, protect the environment, and boost economic development. All these are interlinked.
The theme of the International Year of Plant Health 2020 is ‘Protecting Plants, Protecting Life’.
IYPH is a once in a lifetime opportunity to raise global awareness on how protecting plant health can help end hunger, reduce poverty, protect the environment, and boost economic development.
The focus is on preventing the spread of pests and diseases because they have the greatest impact on our crops, our environment and our way of life.

What We Mean for Plants?

Plants need water, light, temperature and nutrients to survive. But today there is increasing threat from us the humans – climate change, pests and diseases – are all outcomes of the kinds of lifestyles we have chosen to adopt.
Associated with the oldest and longest-running show devoted to agriculture and focussed on welfare of our farmers, Krishi Darshan, since 1982, Mr Sharma stresses that IYPH is important because we have misused plants over decades and don’t understand the importance even after continuous reminders.
Even during the COVID-19 pandemic, and the subsequent lockdown, it is the supply of fruits and vegetables that is keeping us going. It is incumbent on us therefore that in the International Year of Plant Health we pledge to keep the plants and crops around us healthy and free of diseases.