The Last of the Bananas

                     Bananas around the world are dying.

                  There’s an article in the Jan 10, 2011 issue of New Yorker magazine by Mike Peed entitled “We Have No Bananas.”  It happens that 99% of all bananas produced for export are of a single cultivar—the Cavendish.  And a fungus which attacks and kills this variety, called Tropical Race Fungus Four (TRF-4) is spreading worldwide. This pathogen has already wiped out the plantations of Asia and has spread to Australia.  Most experts agree that it’s only a matter of time before it becomes established in Latin America.  (Technically, a “banana tree” is not actually a tree—but I will call it a tree for this post.)

                  How did the industry become dependent on a single cultivar?  There are over 1000 varieties of banana.  But most are wild bananas whose fruit is tiny, filled with seeds, and in most cases inedible.  And most of the domesticated varieties are sterile, which makes cross breeding nearly impossible, and most are either too fragile to ship, too small, or can be eaten only if cooked.  The Cavendish is the only game in town.

                  No chemical spray or antibiotic has any effect on this fungus.   TRF-4 kills banana trees by entering the root system and then invading cells and switching on a gene already present within the cell, causing the cell to commit suicide, through apoptosis (programmed cell death).  The fungus then feasts on the dead cells, continuing until the tree trunk is consumed from within.  Yet apoptosis is a common defense mechanism which most organisms possess.  If something goes wrong with a cell, the cell is supposed to remove itself and make room for its replacement, or at least remove itself before the problem spreads to the rest of the organism.  Yet TRF-4 turns this defense against its owner.

                  The only way to stop the disease would be to start growing varieties that have natural resistance to it. To do this would mean finding a wild plant, preferably a banana plant, that has resistance, and then cross this plant with a Cavendish or some other marketable variety—to somehow insert a gene that tells the cells not to kill themselves.  But crossing anything with the Cavendish might not be possible because the Cavendish is sterile.  It bears fruit without fertilization, and it produces no seeds.  (In fact, its lack of seeds is what makes it marketable).  It is sterile because it has three sets of chromosomes. It is a triploid.  It originated from an accidental cross between two normal, seed producing wild varieties.  New Cavendish trees are grown from suckers which remain when a banana tree is cut down.

                  Although Alexander the Great introduced some kind of banana to Europe in 327 BC, the banana as we know it was brought to the United States in 1870.  The cultivar was from Jamaica and was called the Gros Michel. It was well received and by 1910 Americans were eating 40 million bunches a year, and vast tracts of South American jungle had been planted with Gros Michel bananas.  But in the mid 1920s, these plantations were being attacked by a fungus, TRF-1, and in about thirty years they were mostly destroyed.   What saved the industry was the Cavendish.  This plant, growing in the private greenhouse of the Duke of Devonshire, was from a cutting in a nineteenth century garden in China. 

                  The Cavendish was in most ways less desirable than Gros Michel.  The fruit was harder to ship, spoiled more quickly, was less tasty, required artificial ripening with ethylene gas, and the plants required heavy pesticide use.  But it was resistant to Tropical Race Fungus One, so it was used.  If only the Cavendish were also resistant to Tropical Race Fungus Four.   Eighty-seven percent of all bananas grown are not for export.  They are eaten locally, and they are of many different cultivars, so fungus is not much of a problem. 

                  In 1960, United Fruit Company, seeing that a replacement for the Cavendish would eventually be needed, opened a research center in Honduras, headed by Phil Rowe.  They hoped to develop a cultivar that would be marketable, shippable,  could be grown efficiently, and would be fungus resistant.  After 40 years of effort, Mr. Rowe had made little progress, and in 2001, he hanged himself.  The center he founded is now a non-profit and is continuing his work, headed by Juan Fernando Aguilar. They are still using conventional plant breeding techniques and have not lost hope.  But plant breeding with plants presumed to be sterile is not easy.  About one Cavendish banana out of every 10,000 will actually produce a seed, if you expose Cavendish plants to massive amounts of wild pollen.  So you must sort through tons of banana pulp, running it all through a sieve to find that one seed, and then get it to grow.

                  In Brisbane, Australia, a team led by James Dale of Queensland University of Technology is trying to solve the problem with genetic engineering. They hope to find a plant with resistance to TRF-4, and splice its resistance gene into a common soil bacterium. Then they will allow the bacterium to invade banana cells in a culture, so as to place the gene within the banana cell. They will then kill the bacterium with an antibiotic.   If a live plant could then be cultured from this altered cell, they will have what they want.

                  But even if they succeed, they will still have two problems:  First, present technology would only allow a transgenic plant in which the foreign genes would be expressed throughout the whole plant, not just in the roots.  So it’s not clear that the resulting fruit would be demonstrably safe to eat. (And even if it could be proven safe, most consumers might wish to avoid it.)  Second, we would still have the main problem, which is monoculture.  These fungi have been around for thousands of years.  As long as banana trees were scattered throughout the jungle, no one fungus that specialized in attacking one kind of plant could ever become a pandemic. But when the same cultivar is planted for thousands of square miles, then once a pathogen gets started there is nothing in its path to stop it.