Biological plant-virus ‘arms race’ uncovered

How tiny geminiviruses that measure a mere 20 nanometres can enter large plants and break through their defences

Updated - October 11, 2020 11:39 am IST

Published - October 10, 2020 08:48 pm IST

Fatal attraction:  When the virus attacks the plant, it produces vein-clearing symptoms which make the plant look beautiful, but hinders flowering and fruiting.

Fatal attraction: When the virus attacks the plant, it produces vein-clearing symptoms which make the plant look beautiful, but hinders flowering and fruiting.

Plants and viruses are constantly involved in a race to outdo one another, and their lives literally depend on this. A new study with researchers from National Centre of Biological Sciences (NCBS-TIFR), Bengaluru, has discovered a new step in this arms race between the virus called Synedrella Yellow Vein Clearing Virus and the plants it attacks. The virus was isolated by the researchers from a plant named Synedrella nodiflora, and it was able to infect tobacco and tomato plant in their studies.

Large family

This virus is a representative of the Begomovirus family of viruses. “Begomoviruses are a large family with about 400 members. They infect economically important plants and are a major reason for crop loss,” explains Ashwin Nair, from NCBS-TIFR, who is the first author of a paper on the work published in BMC Biology. “We think SyYVCV can infect many more host plants.”

Attacks, counter-attacks

The arms race typically happens like this: The virus first attacks the plant, and the plant has defences that are actually counter-attacks – mechanisms that seek to destroy the virus. In turn, the virus develops a counter-counter-attack by trying to escape being destroyed by the plant’s mechanisms. In the case of the Synedrella Yellow Vein Clearing Virus, it happens this way: When the virus attacks the plant, it produces vein-clearing symptoms which make the plant look beautiful.

The fact, however, is that this does not make it better for the plant. It actually makes it difficult for the plant to produce flowers and fruits. “Without BetaC1, a viral protein, the virus will not be able to defeat the host attacks and also will not be able to completely infect the plant, as the virus will not be able to move through the veins of the plant,” says P.V. Shivaprasad, in whose lab at NCBS-TIFR the work was carried out.

In turn, the plant develops defence mechanisms to destroy the virus. It targets the protein called BetaC1 made by the virus which helps in successful infection and intracellular movement within the plant. Plants degrade BetaC1 protein of virus by tagging this protein with another smaller protein called ubiquitin.

Viral response

In their study, the researchers found that, in response, the virus uses the plant’s machinery to create a small modification of the BetaC1 protein. It adds a tiny protein called SUMO to the betaC1 protein in a process termed SUMOylation. “BetaC1 hijacks the SUMO pathway machinery of the plants and makes itself a substrate for SUMOylation. Essentially, BetaC1 mimics or tricks the host SUMOylation machinery as if it is one of the host plant protein requiring SUMOylation,” explains Prof. Shivaprasad.

Kiran Chatterjee and Ranabir Das, also at NCBS-TIFR, collaborated in understanding the nano-scale SUMOylation process, because such small interactions can only be studied by special protein-protein structure determining techniques

Tiny virus

The study used tobacco plants. The virus is fairly new. Says Dr Shivaprasad, “We isolated it in 2018 and have not studied its prevalence in other crops. Viruses very similar to this virus are the biggest threat to crop production throughout world.” Apparently, in infected fields, up to 60% of horticultural crops are lost due to begomoviral infection.

Viruses can range in size from 5 nanometre to 300 nanometre. The studied virus, which falls under the category geminivirus, is among the smaller ones, measuring about 20 nanometres. The SARS-CoV-2 virus, for instance is about five times larger than these. Within this small size, they make proteins that are comparable in size with those of the plants that help them function.

The difference comes in the number of proteins they make. While the rice plant makes about 35,000 proteins, and we humans make about 20,000 proteins, geminiviruses code for just 8-10 proteins. The larger SARS-CoV-2 virus codes for about 25 proteins.

New results

“These concepts are new to plant–pathogen interactions. Previously, researchers did find the significance of other protein modifications, but not the ones we have found in this study,” says Dr Shivaprasad. “Our results also provide newer tools to identify and generate plants that can resist viruses.”

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