The humble but fibre- and nutrient-rich banana is a staple food for 400-plus million people worldwide. And more than 40 percent of global banana production, including virtually all those exported, are of the large, crescent-shaped Cavendish variety.
The hugely popular Cavendish is under worldwide threat, however, from a potentially devastating, virulent fungus, Fusarium oxysporum f. sp. Cubense tropical race 4 (TR4, or Panama disease), for which “no acceptable resistant replacement has been identified”.
In a world-first field trial, a team of QUT scientists led by Distinguished Professor James Dale at QUT’s Centre for Tropical Crops and Biocommodities grew two lines of transgenic Cavendish Grand Nain banana cultivars – one modified with RGA2, a gene isolated from a TR4-resistant South-East Asian banana subspecies; the other with a nematode-derived gene, Ced9 – in soil heavily infested with TR4.
Over the three-year trial period, one line from each transgenic cultivar remained completely disease-free.
The researchers also noticed that activity levels of the RGA2 gene correlated “strongly” with resistance to TR4. As the RGA2 gene is naturally present in Cavendish varieties, albeit weakly expressed, this finding opens avenues for boosting natural TR4 resistance in Cavendish varieties through gene editing, a line of research Prof. Dale and his team are pursuing.
The team published its findings online in Nature Communications in mid-November 2017.
Game-changing global implications
According to Professor Dale, the trial’s findings are “a major step” towards protecting the Cavendish global export business, worth around US$12 billion and currently under “serious threat” from the TR4 virus.
“TR4 can remain in the soil for more than 40 years and there is no effective chemical control for it. It is a huge problem,” says Prof. Dale.
“It has devastated Cavendish plantations in many parts of the world and it is spreading rapidly across Asia. It is a very significant threat to commercial banana production worldwide.
“We have a Cavendish banana that is resistant to this fungus that could be deployed, after deregulation, for growing in soils that have been infested with TR4.”
“These results are very exciting because it means we have a solution that can be used for controlling this disease.”
Key field-trial findings
The QUT Cavendish field trial was conducted from 2012 to 2015 on a commercial banana plantation near the Northern Territory town of Humpty Doo that had been impacted previously by TR4 and was “heavily reinfested” with the disease prior to the trial.
Ten lines of Cavendish Grand Nain cultivars were grown over a period of three years.
One had been transformed with a previously isolated gene dubbed RGA2, taken from Musa acuminata ssp malaccensis, a wild banana subspecies native to South-East Asia that is known to be resistant to Panama disease. Nine lines of Cavendish Grand Nain modified with the nematode-derived Ced9 gene were also trialled.
Over the three-year trial period, one of the RGA2-modified cultivars, RGA2-3, stayed completely TR4-free, while three others displayed “robust” resistance, with no more than 20 percent of the resulting plants showing Panama disease symptoms over the trial period.
One of the Ced9-modified Cavendish lines also remained TR4-free for the full three years.
In contrast, between two-thirds and 100 percent of the ‘control’ Cavendish plants were either dead or TR4-infected by the time the trial ended – including a Giant Cavendish variety known as variant 218 that had been generated in Taiwan by tissue culturing and reported to be TR4-tolerant.
Happily, there was no discernable difference in mature bunch size between the transgenic bananas and healthy Cavendish controls.Commercialising TR4-resistant Cavendish varieties
Professor Dale and his team have now embarked on an expanded field trial on the same Humpty Doo plantation, growing the four RGA2 lines that displayed TR4 resistance in their first trial, along with newly-developed lines of GM Cavendish Grand Nain and Williams cultivars.
In the new trial, up to 9,000 plants will be grown and crop yields quantified over a five-year period.
“The aim is to select the best Grand Nain line and the best Williams line to take through to commercial release,” Prof. Dale explains. “While in Australia we primarily grow Williams, in other parts of the world Grand Nain is very popular.”
Further research into Cavendish TR4 resistance
In the Cavendish bananas grown in the first field trial, the researchers noticed that activity levels of the RGA2 gene correlated “strongly” with resistance to TR4.
This correlation opens up new avenues of potentially productive research, Prof. Dale notes.
“We can’t make the assertion that the RGA2 gene is the gene responsible for the resistance in the original wild diploid banana, because in the modified Cavendish we significantly increased the gene’s expression – the level of its activity – over its activity in the wild banana,” he explains. “But we’ve established a correlation, and we’ve found that the RGA2 gene occurs naturally in Cavendish – it just isn’t very active.
New QUT research is investigating potential ways to ‘switch on’ RGA2 in popular varieties of Cavendish banana so as to increase their natural resistance to TR4. While RGA2 is only weakly expressed in non-modified Cavendish varieties – at levels around tenfold less than in the team’s most TR4-resistant cultivars – Prof. Dale and his team are hoping to boost Cavendish plants’ own natural resistance to the fungus.
But naturally-resistant Cavendish bananas are a way off yet.
“We are aiming to find a way to switch that gene on in the Cavendish through gene editing,” says Prof. Dale. “We’ve started that project. It is not easy; it’s a complex process that is a way off, with four or five years of lab work.
Meanwhile, the QUT team is screening wild-banana varieties for other useful genes, says Prof. Dale.
Read the published article: James Dale et al, Transgenic Cavendish bananas with resistance to Fusarium wilt tropical race 4, Nature Communications (November 2017). DOI: 10.1038/s41467-017-01670-6
Read this Phys.org article on the global Panama disease threat: The Cavendish banana could be wiped out