Australian researchers have developed and grown modified Cavendish bananas resistant to the devastating soil-borne fungus Fusarium wilt tropical race 4 (TR4), also known as Panama disease.
In a world-first GM field trial conducted in heavily TR4-infested soil, one Cavendish line transformed with a gene taken from a wild banana and the other with a nematode-derived gene, Ced9, remain disease free. The results have been published recently in Nature Communications.
In the first half of last century, Fusarium wilt (Panama disease) caused one of the most serious plant disease epidemics in history in South and Central America. It was caused by Fusarium oxysporum f. sp. cubense (Foc) race 1 and led to the almost complete replacement of the then dominant export cultivar Gros Michel with Cavendish, which is resistant to Foc race 1. Cavendish now accounts for >40% of world’s banana production and completely dominates the banana export market.
In the early 1990s, another form of Foc, tropical race 4 (TR4) was recognized in South East Asia, which differed from Foc race 1 in that it infects and kills Cavendish as well as a number of other important race 1-resistant cultivars. TR4 can remain in the soil for more than 40 years and there is no effective chemical control for it. It has devastated Cavendish plantations in many parts of the world and it is spreading rapidly across Asia.
Cavendish Grand Nain were modified with the RGA2 gene, taken from the TR4-resistant wild, south-east Asian banana subspecies, Musa acuminata ssp malaccensis and with a nematode-derived gene Ced9.
One modified Cavendish line (RGA2-3) and one line Ced9-21 were TR4-free for the three years of the trial.
The other modified lines showed strong resistance, with 30% or fewer plants exhibiting disease symptoms in three years.
By contrast, 67%-100% of control banana plants after three years were either dead or TR4-infected, including a Giant Cavendish variant 218 generated through tissue culturing in Taiwan and reported to be tolerant to TR4.
Researchers found RGA2 gene activity level in the modified bananas was ‘strongly correlated’ with TR4 resistance.
Cavendish bananas have been found to also have this RGA2 gene naturally, but it is not very active-it is expressed tenfold lower than that in our most resistant transgenic line. New research is looking at how to ‘switch on’ the gene in Cavendish to make them TR4 resistant.
James Dale et al, Transgenic Cavendish bananas with resistance to Fusarium wilt tropical race 4, Nature Communications (2017). DOI: 10.1038/s41467-017-01670-6