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In tomato breeding, resistance has not always had an easy reputation. Some resistance mechanisms genuinely place a burden on the plant, and some resistance genes have historically arrived together with unwanted traits from wild relatives. Those experiences created the idea that resistance and performance cannot fully coexist.
It is a reasonable concern. In certain crops and certain resistance types, the resistance itself can be heavy. And in many breeding programs, resistance has indeed come as a package deal with side effects that reduce yield or vigor. These two realities shaped the belief that resistance must always come with a trade off.
Understanding why HREZ behaves differently starts with understanding these two types of cost.
Some resistance mechanisms operate like a permanent emergency system. They keep defense pathways active or alter basic cellular processes in ways that drain energy from growth and production. In those cases, the resistance itself is the burden. The plant pays for it every day, whether the pathogen is present or not.
HREZ does not work that way.
HREZ is a classic R gene. It remains silent until the plant detects ToBRFV. Only then does it trigger a precise, local response: the plant sacrifices the infected cell to protect the rest of the plant. A useful metaphor is Napoleon’s invasion of Russia in 1812. Napoleon’s army was enormous, but it had a critical weakness: it relied heavily on plundering every town along the way to resupply. Logistics were difficult, winter was harsh, and the French supply lines stretched thin across thousands of kilometers. The Russian strategy took advantage of that. As the French advanced, Russian forces retreated deeper into the country and burned their own food stores, barns and villages. They denied Napoleon’s troops the resources they depended on. Small towns were sacrificed so the nation could survive. Painful locally, but effective overall.
Napoleon’s army enters Russia in 1812, only to find villages burned and supplies destroyed — a scorched earth tactic that denied the invaders the resources they depended on. The strategy was brutal but effective: sacrifice the local to protect the whole. At the cellular level, HREZ resistance works the same way.
At the cellular level, HREZ enables something very similar. When ToBRFV enters a cell, the plant shuts that cell down. It walls it off, destroys the resources the virus needs and prevents it from moving further. It is a microscopic scorched earth tactic. The plant loses a few cells, but it protects the entire plant.
And in a clean greenhouse, where the virus is absent, HREZ simply waits. It does not drain energy, reduce vigor or affect yield. It only acts when it is needed.
The second type of cost is genetic. For decades breeders sourced resistance genes from wild tomato relatives. Those wild species were invaluable for their resilience, but they also carried traits no grower wants. When a resistance gene was introgressed, it often arrived with a large block of wild DNA around it. That block could reduce yield, weaken vigor or affect fruit quality.
This is the breeding cost of resistance. The gene itself may be useful, but the package it comes in is not.
Whether that package can be reduced depends on where the gene sits in the genome. In regions with low recombination, it is difficult to trim away the surrounding wild DNA. Tm 22 is a well known example of a gene in such a region. In regions with high recombination, breeders can gradually reduce the introgression until only a tiny fragment remains.
Here HREZ had a crucial advantage.
Walter Verweij, Senior Researcher Molecular Marker Development: “The identification of the HREZ gene came with a lot of dedication of the whole team, but also with a bit of luck. The gene was located in a recombinant hotspot, enabling us to quickly identify the gene and introgress it in our elite tomato varieties without negative side effects.”
Because HREZ sits in a recombination hotspot, the introgression around it could be made very small. Molecular markers allowed the team to follow that fragment precisely in every generation and select only the cleanest versions. The result is a resistance gene embedded in elite genetics, without the usual breeding cost.
Once HREZ was secured as a clean, stable introgression, the breeding program gained something extremely valuable: freedom. With resistance already in place, breeders no longer had to spend cycles searching for ToBRFV resistance or compensating for the agronomic penalties that often accompany large introgressions. The foundation was resistant by default.
That shift allowed the team to focus fully on the traits that matter most in commercial production. Yield, fruit quality, plant type, shelf life and post harvest performance could be optimized without worrying about losing resistance along the way. As a result, agronomic traits are improving across the board. The breeding effort is no longer split between “finding resistance” and “building performance.” It is now entirely focused on performance, because the resistance is already secured.
Martijn van Stee, Crop Breeding Manager Tomato: “At this moment we added HREZ in all our elite lines. That gives us a basis of resistant material from which we focus on making the best varieties for our growers. In yield, quality and post-harvest.”
Resistance is no longer a compromise. It is the platform on which the next generation of elite varieties is being built.
Multiple independent studies support this picture. Reviews in Trends in Plant Science describe how R‑gene–mediated resistance against plant viruses relies on a highly localized hypersensitive response, where only the infected cells are sacrificed to stop the pathogen from spreading (Sett et al., 2022). Crucially, these R genes remain suppressed in the absence of infection, avoiding unnecessary energy costs and maintaining normal growth — exactly the behaviour expected from a clean, silent resistance gene like HREZ.
A second review in Trends in Plant Science highlights how modern resistance‑breeding strategies increasingly focus on minimizing the physiological burden of resistance itself (Wang et al., 2022). The authors contrast heavy resistance mechanisms, such as S‑gene knockouts that can affect plant development, with NLR‑type R genes that activate only when triggered. This distinction reinforces why HREZ, as a classic R gene, does not impose a biological cost in clean greenhouse conditions.
Finally, work published in Molecular Biology and Evolution maps recombination hotspots and coldspots across the tomato genome and shows how these patterns shape introgression breeding (Fuentes et al., 2022). Regions with high recombination allow breeders to trim introgressions down to very small, clean fragments, while coldspots trap large blocks of wild DNA and increase the risk of linkage drag. This explains why some resistance genes, such as Tm‑22, are difficult to clean and why HREZ, located in a recombination‑friendly region, could be reduced to a tiny, well‑behaved introgression without agronomic penalties.
Together, these findings show that when resistance genes are biologically light and genetically clean, they do not compromise yield or plant performance. HREZ fits precisely into that category.
The belief that resistance always costs yield comes from real experiences with heavy resistance mechanisms and with large, wild introgressions. HREZ avoids both. It is a precise R gene that only acts when needed, placed in a tiny, well behaved introgression that does not drag along unwanted traits.
In practice that means strong resistance and elite performance can coexist in the same plant, without hidden trade offs. HREZ shows that when biology and breeding align, resistance becomes a strength, not a sacrifice.