Note : This cover story was published in CSIR-NISCAIR’s prestigious magazine Science Reporter in July 2020.
2020 is the International Year of Plant Health.
In true terms 2020 has become a year of flourishing plants. Want to know how, just look out of your window or walk to your balcony and feel the green. The leaves that were covered with layer of dust are now smiling wide. The trees that were dying because of pollution and heat are now breathing tight. The depressing surroundings have become lively. We can make out the difference with our naked eyes. But, these plants are not so relaxed. As we can watch them, they are also watching us. They can make out the fear in our heads and the panic on our faces. And they are wondering what if they also get attacked by some virus like coronavirus. Although the plants are used to virus attack, and very well know the pain and loss caused by a virus.
Yes, you heard it right. Just like a virus has set the reset button of our lives, the viruses attack these plants too. We need to know about these plant viruses because now we are all the more cautious about the term ‘Virus’. Also, we are surrounded by plants, eat these plants, use them and abuse them to the peak. But are the virus attacking humans and plants the same? Let’s start from the basic and proceed to the complex.
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. Similar lipid envelop 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. Plant virus range from 17 nanometer to 2000 nanometer in size. Shapes of viruses are predominantly of two kinds: rods or filaments… like rigid rod, flexous rod, isometric, bullet shape, twin icosahedral etc. Majority of the plant viruses have single stranded positive sense RNA genome. Besides that, there are plant viruses with genome composed of single stranded negative sense RNA, double stranded RNA, single stranded DNA and double stranded DNA. Many of the plant viruses have multi-segmented genome, which either encapsidated in different particles or within a single particle. Plant virus genome codes for proteins essential for their replication, cell-to-cell movement, and encapsidation. Besides these essential protein coding genes, genome of many plant viruses additionally codes for proteins necessary for insect transmission and suppressors of RNA silencing to overcome the plant defense. Many plant viruses are associated with satellite DNA or RNA molecules, which increases their virulence and host adaptability.
How virus attack plants ?
Plants are just as susceptible to viral infections as other living organisms are. 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 into plant cell. The wound may be created by weather, insects, animals, or even human activities during agricultural operations. Once a plant gets viral infection, then it spreads through different vectors like insects, nematode, fungus which attack plants. More than 70 percent of known plant viruses are transmitted by insects, majority belong to order Hemiptera, which includes aphids, plant hoppers, leaf hoppers and bugs etc. Additionally, 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 development of abnormalities or symptoms.
What happens in a Virus attack ?
One of the characteristic of Virus is that they mutate very fast but not on their own, but into the host body. They multiply fast thus infecting not only one cell or one plant but the emerging viruses infect newer hosts and lead to 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 virus act friendly with plants, equilibrate themselves with plant ecosystems and ruin agriculture. The tospovirus of tomato is an example of the 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 plant’s respiratory system badly and also lead to tension in plants.
Virus attack on plants can be as deadly as Corona Virus ?
Just like all other viruses, plant viruses are obligate pathogen 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 quality of the produce. If the severe virus infection occurs in an early stage of plant growth then virtually the plant could not be able to produce any product and due to weak immunity, many times attacked by other pathogen and 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 could be obtained from the entire field.
How to tackle the viruses?
We are now well experienced 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. As we know breaking the chain and social distancing are the keys to prevent virus attack, so remove or burn 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. What least can be done is changing agricultural practices, which most of the times is not very feasible.
But these are not long term solutions… Mr. Sunil Mukherjee, a very Senior scientist in the field of Plant Molecular Biology, Indian National Science Academy emeritus scientist retired from International Centre for Genetic Engineering and Biotechnology, made tomato transgenics to resist tomato leaf curl viruses, is of the opinion that the traditional methods are not effective and most of the times turn out to be ecologically unfriendly. He emphasised that plants can sense pain, death-pang etc. but at the same time, plants also recover from viral infection because of RNAi-considerations. It is one of the recent therapeutic techniques.
In terms of saving plants from virus attack, RNAi and CRISPR-Cas methodologies are the most preferred ones. In terms of silencing a gene, the primary difference is that RNAi means it ‘knocks-down’ a gene. Whereas CRISPR/Cas means it ‘knocks-out’ a gene. RNAi is widely used by researchers to silence genes in order to learn something about their function. But, if you want to completely silence a gene, CRISPR technology is better.
Mr. Mukherjee explained in detail the RNAi and SRISPR/Cas methodologies. He continues saying plants possess the immunological-factors like proteins called RNAi factors which help plants defend themselves against plant viruses. In places like Hawaii, USA, China, latin America etc., RNAi-based transgenic plants are in commercial cultivation to minimise viral pathogen induced losses. As transgenics are not liked by regulatory considerations, application of dsRNA (of viral sequences) spray is a popular method to adopt for plants’ protection. However the stability of the externally applied dsRNA is a big question and nano-carrier-tagged dsRNAs are used for sustained and prolonged release of dsRNA. Thus research is focused on finding the nano-carriers which are cheap and non-toxic to plants. CRISPR-Cas is the most recent technique in the armoury of antiviral strategies. The guided Cas9 enzyme is generally used against DNA viruses like geminiviruses, caulimoviruses, banana streak viruses etc. Similarly Cas13 is used against RNA viruses like Turnip Mosaic virus ( TuMV), Potato Virus Y ( PVY) ,Rice stripe Mosaic Virus ( RSMV), Southern Rice black-streaked dwarf virus ( SRBSV) etc. These are applications of CRISPR-Cas on viral genome directly. Conversely, hosts can also be manipulated as plants harbour genes known as the sensitive (S) genes which sensitize the plants towards the pathogens. The elimination of S genes paves the way for achieving virus resistant plants. Papaya can be cured of Papaya Ring spot virus (PRSV) in this manner. Both RNAi and CRISPR-Cas methodologies suffer from attack on unwanted neutral or healthy genes, technically called as “off-target” effects. However, a great deal of research is being carried out to get rid of such effects and to make plants virus free for safe human consumption.
Nucleic acid based or antisera based detection kits are known for a few of the viruses. But we lack the large scale screening systems of the viruses. DNA-microarray is a technique which has the ability to do this kind of massive screening. Using this technology, we will be able to know the nature of emerging viruses in short time with relatively moderate resources. This technology thus gives us the surveillance power over the prevailing viruses and adding to this about the virulence information, we can build mathematical models to predict which crop will be in danger by what kind of viral pathogens and when. Metagenomics is also a recent technique for large scale screening but the functionality of the viruses remains unknown by the method.
Can plant viruses be cured ?
Once a plant gets infected by a virus there is no way to cure that plant. Sometimes due to plant’s own defense mechanism symptom recovery is observed but such situation occurs rarely. Though plants do not have an antibody mediated immune response system like animals and humans, they possess a general defense system, which is called RNA silencing, that detects and degrades viral RNAs or viral transcripts. To overcome such defense, majority of plant viruses encode a protein called suppressor of RNA silencing, which inactivate host defense. Depending upon the arm race between plant defense and viral pathogenicity plant’s response to 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 symptom can be reduced but under severely infected condition no cure is possible. However, prophylactic application of different molecules (e.g. dsRNA) showed promising result to prevent plant virus infection.
How to manage plant virus ?
The best way to manage plant virus infection is to protect the plant from getting infected as there are no antiviral compounds available to cure plants with viral diseases. First step for management of the virus is to identify the virus, understand it’s 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, 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 source or may be derived from the virus through transgenic development. Recently topical application of dsRNA and gene editing technology showed 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 there is great specificity exists with the virus and their hosts. However, for viruses it is not uncommon to switch the host kingdom. As human being depends upon agricultural crops, it is definitely plant virus can enter human body through virus infected foods, which is evidenced from the fact that many stable plant viruses have been detected in human faces. If a plant virus could breakdown the host specificity and able to multiply in 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 are causative agents of disease in humans and other animals. There is one report which showed that a plant virus, pepper mild mottle virus (PMMoV), which is found in many pepper-based products could be detected in human body but the symptom associated with the illness can not be correlated. Upon experimental exposure plant viruses can be detected in mammals and humans samples, and there are 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 human or animal. There are many plant viruses, like tospoviruses, reoviruses, rhabdoviruses, which can replicate in their insect host. So, if a plant virus can break the kingdom to infect insect host it may also be not impossible to infect human. Though till now there is no such evidence. Many human infecting viruses and plant infecting viruses belong to same family with similar genome organization. Like under the family Reoviridae, Phytoreovirus, Fijivirus and Oryzavirus infect plant while the rotavirus, a major cause of gastroenteritis in humans also belong to this family. Similarly, under the family Rhabdoviridae, which includes rabies virus infecting human and animals also have the plant and insect infecting viruses like Cytorhabdovirus, Nucleorhabdovirus, Dichorhabdovirus and Tenuivirus. Plant and insects infecting genus Tospovirus belong to the family Bunyaviridae, which also possess Hantaan virus and Toscana virus infecting human. Though in those cases viruses have similar genome structure and the genes involved in viral genome replication and expression are conserved between plant and animal viruses, but the genes concerned in virus-host interactions are not same. Thousands of year humans are eating plant products but till now there is no evidence to suggest a plant virus as a human pathogen.
Dr. Anirban Roy, Principal Scientist in Advanced Center for Plant Virology, Division of Plant Pathology at Indian Agricultural Research Institute, firmly states that plant viruses are not less important than a human virus. In case of human we are looking the impact of a virus on an individual basis, while in case of plant we measure the impact in population. Thousands of plants in a field are getting infected, hundreds get killed, so if you see on the individual impact scale, any plant virus cause more damage to its host than a human virus. Human can protect themselves by quarantine, can plant do so ? Being immobile it has to get infected in a field if infection starts.
Plant virus disease has many fold impact starting from quality and quantity of yield, to business trade. Estimating the loss caused due to plant viruses are very complicated and often are not available for all the diseases. Some reports indicated that a range of viruses are responsible for an estimated $60 billion in crop losses worldwide each year. A few specific examples, like yield loss caused by barley yellow dwarf virus and potato leaf roll virus to a value of £40-60 million per year in the United Kingdom, and yield losses of moong bean, urad bean, cowpea, and soyabean by yellow mosaic viruses in the tune of US$300 million in India, definitely show the importance of plant viruses in human society. If you see in historical perspective, a virus disease which cause a colour variegation symptom in tulip flower cause the Dutch economy to collapse during 17th century as such flowers were considered as a gift of god and abnormal high prices were paid for them.
An initiative to flourish plants
The story of IYPH began in 2012 in Stockholm conference which decided that a year will be devoted to plants. The United Nations General Assembly took the 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.
Trilochan Mohapatra, Director General of Indian Council of Agricultural Research believes that investing in plant health research is necessary to withstand plant diseases, reduce food insecurity, feed the growing population, and make our planet more sustainable. All national governments and international organisations should join hands to create massive awareness across the globe. He strongly recommends for massive awareness campaigns to educate travellers moving in and out of the country to be cautious while bringing plants or plant products across borders and to spread the key messages of healthy plants responsible for a healthy living.
Healthy plants are vital to sustainable and profitable crop production and to the quality and cost of the nation’s supply of food, fuel, and fibre. According to the Food and Agriculture Organization of the United Nations (FAO), 40% of food crops are lost to plant pests and diseases annually. This leaves millions of people without enough food to eat and seriously damages agriculture – the primary source of income for poor rural communities.
Health is also critical to plants used for ornamentals, natural resources, and animal feed. Plant health is increasingly under threat. Climate change and human activities have altered ecosystems, reducing biodiversity and creating new niches where pests can thrive. At the same time, international travel and trade, which have tripled in volume in the last decade, can quickly spread pests and diseases around the world causing great damage to native plants and the environment.
What We The Humans mean for Plants ?
A plant’s saga
“I am dying. Please save me. You cherish my beauty but don’t bother to shield it. You enjoy my company, but don’t care to salvage me. You appreciate my patience, but don’t stand up for my well being. WHY is it so ? If you want to deserve every part of me, you have to protect me, preserve me.”
We study since we enter school that plants need water, light, temperature and nutrients to survive. These four things affect the growth hormones of the plant, which will either make the plant grow quickly or slowly. But as we grow we realise that plants depend on us as well for their emotions. So, it is the right time that we should analyse our behaviour as humans. It’s not that we don’t understand that health of a plant is crucial. Yet, they are under constant and increasing threat from us the humans, climate change, pests and diseases. We need to go step by step to understand the problem and then find solutions.
Senior Science Communicator Mr. Kuldeep Sharma opines with an expression – “We are the guests of the plants on the earth.”
Associated with the oldest and longest running show devoted to agriculture and focussed on welfare of our farmers, Krishi Darshan, since 1982, Mr. Sharma continues that plants give us air we breathe, the food we eat, the homes we live. He stressed 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, we get the fruits and vegetables in ample amount. From farmers to plant health authorities are on their toes to support plant health issues from virus to pests and insects to climatic conditions to humans. They make arrangements for smooth and effective transport and market for healthy plant products so that consumers have enough to eat and producers and sellers can earn their income.
After all this analysis, mind still boggles around the term virus as it has two faces. We have known ‘The Bad Virus’ in detail, but do you have any idea about ‘The Good Virus’. We already talked about the Tospovirus in tomato. Similarly, Brome mosaic virus, family Bromoviridae, Cucumber mosaic virus, family Bromoviridae, Tobacco rattle virus, family Virgaviridae, and Tobacco mosaic virus, family Virgaviridae are examples of good or friendly virus.
Also, we can find viruses everywhere, even in our own intestines : bacteriophages. These viruses infest bacteria and eliminate harmful ones in the process. This ability makes it possible to use them as an alternative to antibiotics, which is greatly needed.
A million dollar question is – Do plants also need to live with viruses like we are forced to do today ? May be not, if we come forward and be a part of the global IYPH Campaign in our own unique way.