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By Manus Thoen, Sr. Researcher Phythopathology
Plants are sessile organisms. They can’t run away from their predators, pathogens, or environmental stress. But that doesn’t mean they are passive bystanders. For billions of years, plants have evolved sophisticated resistance mechanisms to defend themselves against viruses, fungi, bacteria, and insects. And for billions of years, these attackers have evolved new ways to overcome those defenses.
This ancient arms race is still ongoing today. Every time we deploy a new resistance gene in commercial agriculture, we are adding a new chapter to a story that began long before humans existed. So, it’s natural and sensible to ask the question:
If we deploy HREZ at scale, won’t ToBRFV quickly evolve mutants that overcome recognition by HREZ?
I have to admit something: whenever I hear the word “mutant”, my mind still flashes back to my childhood obsession with the Teenage Mutant Ninja Turtles. For years, the word carried this sense of awesomeness; mysterious powers, glowing ooze, and heroes emerging from the sewer to save the day.
But in plant virology, mutants are far less glamorous.
A “mutant” is the result of a tiny copying error. Every time ToBRFV replicates its RNA, small typos can slip in. Most of these changes do nothing. Some make the virus weaker. And a very small number might slightly alter the viral proteins that resistance genes normally recognize.
For growers, these mutants don’t feel like comic‑book heroes. They are natural, yes. But they feel like threats. Anything that might slip past resistance understandably raises concern. And that’s why we study them so closely.
ToBRFV is a tobamovirus, and like all RNA viruses, it accumulates mutations over time. Some of these mutations can alter the viral proteins that resistance genes normally recognize. This is what happened with Tm‑2², which protected global tomato production for decades. And even in the early deployment of Tm-22, the industry was faced with ToMV mutants that avoided recognition by this resistance gene. But Tm-22 stayed relevant for decades, because the newly emerging mutants never became a global problem.
The similarities between HREZ and Tm-22 are numerous. On the molecular level, and in their functions. Both genes trigger a hypersensitive response (HR). HR is the rapid, localized cell‑death reaction that stops the virus from replicating and prevents systemic spread. Where Tm-22 and HREZ differ, is in its broadness of recognition. Whereas HREZ recognizes TMV, ToMV and ToBRFV, Tm-22 recognizes only TMV and ToMV. ToBRFV has thus evolved to avoid the recognition by Tm-22.
In the blog for Myth 1, we used Napoleon’s 1812 invasion into the Russian winter to explain resistance costs[/link]. The metaphor also helps explain mutation and durability. Imagine ToBRFV as Napoleon’s army marching into Russia. HREZ as the Russian soldiers sounding the alarm upon spotting Napoleon’s forces in the distance. Deploying the scorched earth tactic, locally burning down resources in order for the whole country to survive. A decisive force that stops the invasion. Now imagine a mutant strain trying to escape HREZ. It wraps itself in a huge camouflage blanket so the plant no longer recognizes it. But that blanket is heavy. The army moves slower. Supplies run out. Soldiers weaken.
Biologically, this is what we call a fitness penalty.
A mutation may help the virus hide, but it often slows replication, reduces movement, or weakens transmission. The virus survives, but it struggles. And in real greenhouse conditions, weak mutants rarely outcompete the original virus.
Imagine ToBRFV resistance‑breaking mutants as Napoleon’s army trudging into Russia under a massive camouflage net. Hidden from sight, but slowed to a crawl. They’ve escaped recognition, yes, but at a steep cost in speed and strength. There’s a reason this scene belongs to fiction.
When a virus mutates to avoid recognition, it often sacrifices something essential:
These trade‑offs are well documented in plant virology. A mutant may escape detection, but it becomes less competitive. At Enza Zaden, we monitor ongoing ToBRFV diversity closely. Whenever new mutants are detected through our global diagnostics efforts, we always ask two crucial question;
“We monitor the diversity of ToBRFV globally by analysing thousands of samples that come in through our diagnostics process. This gives us unique insight into the mutations that are out there. If we find new mutants, we always study their impact on HREZ — including their virulence and their ability to move from plant to plant. In other words: are there fitness costs involved?”
So far, no ToBRFV mutant has been found that escapes HREZ without suffering severe fitness penalties.
Resistance breakdown is not automatic, it is pressure‑dependent. High virus pressure increases the chance of mutation. Poor hygiene accelerates this process.
Good hygiene reduces:
This is why hygiene remains essential even when growing resistant varieties. It protects the plants, and it protects the resistance (something we will explore in depth in Myth 5 of the series).
“Currently there are thousands of HREZ varieties grown all around the world. We see in the vast majority of cases that the resistance is holding up very well, especially when HREZ is combined with good hygiene practices.”
In Myth 3, we explained why a strong dominant R gene is the foundation of durable resistance. HREZ plays that role today. But durability is not about relying on one gene forever. It’s about building layers.
Our long‑term strategy includes:
ToBRFV didn’t arise from a single mutation in ToMV or TMV. Genetic studies show it is a distinct tobamovirus lineage that diverged long ago and later gained the ability to infect tomato. Its genome even contains recombination‑derived regions, meaning parts of the virus originated from exchanges with other tobamoviruses.
In other words, ToBRFV is not a “ToMV with a typo,” but a separate species that gradually evolved the right combination of traits to infect modern tomato varieties (Salem et al. 2015). Public research confirms that ToBRFV mutants do occur, but they are rare, and most show reduced fitness.
Salem et al. (2022) identified natural ToBRFV variants with amino‑acid substitutions in the movement protein. These mutants infected susceptible tomatoes but showed reduced systemic movement and lower viral accumulation.
Yan et al. (2023) generated experimental ToBRFV mutants that escaped Tm‑2² recognition but suffered significant replication penalties.
These findings align with what we observe in our internal diagnostics workflow: mutants exist, but they are weak and none have overcome HREZ without severe drawbacks.
HREZ is not easily broken. Durability depends on strong genetics and low virus pressure. Even when mutations occur, fitness penalties often prevent them from becoming a real threat.
With HREZ as the foundation, complementary mechanisms on the horizon, and good hygiene practices in place, we can stay ahead of ToBRFV’s evolution and protect tomato production for the long term.
And in myth 5, we’ll explore why hygiene is not just a supporting measure. It’s one of the most powerful tools.