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“O Christmas tree, O Christmas tree, how steadfast are thy branches.” So goes the age-old marketing song of Christmas tree sellers. Well, you would be forgiven for thinking that, because one thing their customers dream of is a real tree that clings resolutely to its needles, despite being hacked from its roots, dragged inside and propped up in a warm living room for a month or more.
For years, producers have been trying to breed trees with this coveted trait – as well as a pretty shape, bushy green branches and resistance to plant diseases. Breeders of Christmas trees are increasingly turning to genetics to help them but the process is far trickier than you might think. The genomes of most tree species used at Christmas are huge, many times bigger than the human genome, and most have never been fully mapped. Relatively little is known about which genes perform what function in these trees.
But Ulrik Braüner Nielsen, a forestry expert at the University of Copenhagen, knows that Christmas trees are big business in Denmark. The country produces about 11 million Christmas trees annually – about a fifth of the total in Europe, where real Christmas trees are more popular than anywhere else in the world. And yet, 20-30 per cent of farmed trees don’t meet growers’ expectations and fail to make it to market, Nielsen says.
Nielsen and his colleagues are refining breeding techniques to help ensure the uniformity of Christmas trees. Their secret weapon is a small orchard where trees and their offspring have been allowed to develop. By analysing the genetic material in those trees, Nielsen and his team can work out which parent trees produced the most desirable offspring.
The great benefit of this approach is that the team doesn’t have to wait decades for the trees to reproduce – that part has already taken place. “We call it ‘backwards selection’,” he explains. “We can figure out who’s related to who.”
Lately, Nielsen and his colleagues have been using the backwards selection approach to investigate the properties of Abies bornmuelleriana, a species of fir tree originally from Turkey. Abies nordmanniana is a far more common Christmas tree in Europe, but bornmuelleriana might appeal to producers because it has more rigid needles and upward-bending branches, which give it a fuller appearance.
“There’s a good possibility to improve these traits also, [including] post-harvest needle retention,” he adds.
Once they have a good idea of what tree stock has the sought-after attributes, the next phase involves extracting the embryos and multiplying them in a lab. It’s a form of cloning known as somatic embryogenesis.
“They’re copies, genetically, of the seedling that would have developed, or the tree that would have developed, from that embryo in the seed,” says Gary Chastagner, a Christmas tree expert at Washington State University who is not involved with the research in Denmark.
It means that many thousands, even hundreds of thousands, of seedlings can be produced at once in a breeding, with producers reasonably confident that they will all grow up to be beautiful Christmas trees.
Such work has been carried out with a species of spruce tree by researchers in Finland – but it’s not exactly easy. It’s taken seven years in that case just to get to the first commercial trails.
The process carries biological risks, too. A population of cloned trees might turn out to have inherited a genetic defect making them all susceptible to disease or an invasive insect. Growers, having bet on a mini forest of clones, might later discover that a huge swathe of their stock is doomed. Chastagner explains that cloned trees’ resilience can be tested in trials once they grow, though that adds more time and cost to the process. It’s also possible to rely on multiple clones, rather than just one.
Speaking of genetic diversity, that’s something that is actually at risk in at least one Christmas tree species, according to research from the United States Department of Agriculture (USDA). A major analysis of plant diversity published earlier this year by Colin Khoury at the USDA Agricultural Research Service and colleagues found that the genetic diversity in Abies nordmanniana specimens held in gene banks, botanical gardens and wild populations was poor – scoring just 13 out of 100.
Jill Wegrzyn, a plant genomics expert at the University of Connecticut, says this is a worrying state of affairs. If some new pathogen arises that threatens farmed Christmas trees, or if breeders become desperate for new genetic material to improve needle retention or other aspects of tree quality, they might eventually find themselves stuck without a suitably diverse wild population to turn to.
That’s why there’s all the more pressure to unravel the vast and murky Christmas tree genome so that breeders better understand what they’re working with. Wegrzyn and her colleagues have spent years searching for one particular Holy Grail – the cluster of genes that determines exactly how and when Christmas trees shed their needles after they’ve been cut down.
In Christmas tree branches, there is a small bit of tissue that connects to the needles. The cells in that tissue eventually rupture, causing the needles to fall off. But this process is triggered by certain genes and some families of trees are known to have better needle retention than others.
In 2017, Wegrzyn told WIRED that she felt the hunt to find out exactly which genes were in play here was nearing its zenith. Two years later, she says there’s been progress – but the work is far from over.
“We have actually seen that there are genes that are mediating good and poor needle retention,” she says. And yet it’s still not possible to confirm for sure which ones, precisely, are involved. In the end, it may turn out to be a complex cluster of genes that is difficult to modify with genetic engineering techniques.
A GM super-tree that never sheds its needles still feels like a long way off.
Ole Kim Hansen, one of Nielsen’s colleagues at the University of Copenhagen, says that Christmas tree growers may find that the benefits of genetic analysis are limited in the near-future. He works with both genomics and traditional breeding approaches and says the latter is still the most useful at present.
But traditional breeding is, as explained above, becoming more sophisticated all the time. Backwards selection, clonal propagation, the potential for genetic analysis to inform crosses – this is the gritty reality of plant breeding today. Fewer than 100 million real Christmas trees are sold in North America and Europe every year, not exactly a bumper crop given that the population of all those countries combined easily exceeds a billion. If trees were more reliably beautiful and if their needles were less likely to fall off around Christmas day, the market might be much bigger.
That’s the hope of the tree sellers, anyway. Listen carefully at this time of year and you may hear the charming ditty. “O Christmas tree, O Christmas tree, of all the trees most lovely. Come get ‘em! We’ll do a good price! Seriously, they’re great.”
This article was originally published by WIRED UK
