Key points
- Resistance genes protecting Australian cotton for decades failed in the United States without warning, sending researchers back into the genome looking for a backup plan if this were to happen in Australia.
- Our researchers used genomic sequencing to find backup resistance genes hiding deep within the plant's DNA.
- Mapping genetic diversity across many cotton plants, not just one, could reshape how future varieties are bred.
As a bioinformatician with the cotton breeding team at CSIRO, Dr Angel Popa-Báez can read the genome of a cotton plant like a book. Yet, he's the first to admit the plant keeps its secrets.
"There's a little bit of mystery to it," Dr Popa-Báez said, adding, "some of it good, some not so good."
Consider the case of bacterial blight: the first hint that bacterial blight has claimed a cotton plant is small, water-soaked lesions on the leaves. These spread, until they turn brown and angular. Once entrenched, the disease can take whole leaves and bolls with it.
Even knowing this, Australian cotton growers weren't usually troubled by the disease. Their confidence dated back to the 1980s, when an outbreak hit crops hard enough that breeders went looking for a permanent fix. They thought they found one: breeders elsewhere had faced the same disease and bred their way past it. Those genes were then selected and adapted to Australian conditions.
For decades that resistance held. Then it didn't. News broke in 2011 that bacterial blight had resurfaced in the United States. The genes that had defended against it for so long had stopped working – and no one knew why. It was exactly the kind of surprise Dr Popa-Báez was talking about – one of those 'not so good' ones.
Rather than wait to see whether the failure would translate into fresh outbreaks, CSIRO's cotton breeding team set about finding a backup. Before the disease could return to our shores, they wanted a second line of defence ready to go.
"We needed to find a different resistance mechanism – something we could have in our system, so that if the first one broke down, we'd have a backup ready to release without negative impact for the farmers," Dr Popa-Báez said.
Why Australia has so much to protect
Cotton is one of Australia's most valuable crop exports, and Australian growers produce the highest cotton yields of any country in the world. CSIRO's researchers have bred the varieties behind that success for more than 50 years: today, all Australian cotton – along with much of the dryland cotton grown in the United States, and a substantial share of crops in Brazil, Turkey and Greece – carries CSIRO-bred genetics. Independent economic analysis puts the return on that breeding investment at roughly $AU80 dollars for every dollar spent.
This isn't a position CSIRO can win once – it has to be defended, year after year. A resistance gene that quietly stops working is exactly the kind of threat that can erode industry success, no matter how minor the disease appears to be.
Hunting for a backup
CSIRO breeders, led by Dr Warwick Stiller, used a clever breeding strategy – crossing an ancestral cotton species with modern, susceptible commercial lines, a long process with a low success rate. Once those crosses were established, Dr Popa-Báez was brought in to find exactly where in the genome the new resistance was hiding.
Commercial cotton, as Dr Popa-Báez explained, carries the genetic legacy of two wild species that merged millions of years ago, and still, in effect, share the one plant – each contributing its own set of chromosomes, known as a subgenome. The new resistance gene, it turned out, sits on a chromosome from an entirely different subgenome to the original – in effect, on the "other" species' half of the plant's genome.
"You have two genomes – a collection of all the chromosomes in a species – that are living together inside a cotton plant," he said.
Those two genomes appear to take turns switching genes on and off: "It's like they're siblings," Dr Popa-Báez said. "One will start acting up early, while the other does nothing. Then when the first one settles down, the other pops up."
Cotton is a plant whose biology resists simple fixes. Push for one trait and another can quietly suffer.
"You don't want to push too much in one direction, because you may end up with negative effects somewhere else," Dr Popa-Báez said.
Chase fibre length too hard, for instance, and yield can drop – affecting farm profit. Without a model that can predict those trade-offs in advance, breeders are still relying on extensive trials like a tug-of-war game, and on the plant's own capacity to surprise them, for better or worse.
"Sometimes the plants surprise us and do even better than we expected," Dr Popa-Báez said. "But sometimes the opposite happens – you're confident you'll get a result, then you put it in the field, and it doesn't happen. There's a constant reality check, both ways."
Why a single reference genome isn't enough
To improve the odds, Dr Popa-Báez and the wider team are rethinking the genetic map that cotton breeding relies on.
He explained that most breeding programs compare new lines against a single reference genome, a kind of genetic blueprint built from one representative plant. It works well for common traits, but it has a blind spot.
"Even in a field of cotton, not all the plants are the same as that single reference," Dr Popa-Báez said. "If you try to build everything around one plant, you may be missing subtleties – overlooking unique differences."
Dr Popa-Báez said some of those structural changes are directly tied to the traits that matter most commercially.
"Some of these structural changes are important for fibre traits like length and strength – measurements that matter to farmers, because they affect the quality of the fibre and value of the cotton."
The fix is developing a pangenome: a composite map built from many cotton plants rather than one, designed to hold onto the structural variation that currently slips through the cracks.
"We're increasing the diversity," Dr Popa-Báez said, "so that when we're mapping and selecting, we're accounting for structural changes that may be important for fibre traits, and other more complex traits, like yield."
An industry under pressure
Genomics – which involves mapping an organism's complete set of genes – is a key tool for an industry which is also fighting a reputational battle. Water use has long been a flashpoint, and the European Union has ruled cotton out as one of its recognised sustainable fibre sources, favouring synthetics instead.
"Cotton has a reputation for being too water hungry," Dr Popa-Báez said, explaining this has led to it being excluded from lists of sustainable fibre sources, and tipped the balance towards synthetic fibres.
Dr Popa-Baez believes part of the response is scientific, and part is simply making the case for what cotton already does well.
"Australian cotton is significantly more water efficient than it was in the past, maybe 50 per cent more efficient, but there are other advantages aside from fibre quality – like a reduction in microplastics, that you don't get from other fibres."
Artificial intelligence, he suspects, will help close the gap between ambition and success rate, particularly in predicting which genes are worth editing in the first place. Gene editing – making precise, targeted changes to an organism's DNA – is one area he expects AI to help with. "There's a real push now to use AI to predict the impact of gene editing, before you even make it," he said.
Dr Popa-Báez is drawn to where the technology could eventually lead.
"You can start dreaming a little," he said – imagining cotton that is easier to dye and doesn't fade away, cotton clothing engineered to sense sweat and wick it away or cotton wound dressings that are able to detect infection early and alert medical staff to its presence, or even cotton fibres engineered to make more powerful electrical batteries.
For now, that's still a hunch and a research question. But so, a few years ago, was a backup gene for bacterial blight.
