Feb 19 2021

CRISPR-Edited Bananas

In the British Drama, Years and Years, they imagine the very near future. I do wonder what someone from 2010 would have thought about a tv show accurately depicting 2020. In any case, one of the throw-away lines of the show was that there are no more bananas. The writers did their research – that the Cavendish banana will disappear sometime in the 2020’s is extremely likely. It is being threatened by a fungus called Tropical Race 4 (TR4), which a century ago wiped out the previous commercial dessert banana, the Gros Michel (it’s not extinct, but cannot be grown commercially anymore).

TR4 is now on every continent that grows bananas. It is literally just a matter of time before the entire commercial Cavendish market is wiped out. TR4 and similar funguses also threaten other banana varieties (more like plantains) that provide a staple source of nutrition for large segments of the world (about 400 million people). So this is not just about no longer having access to a favorite dessert fruit – this can create a serious threat to food security in parts of the world.

Part of the problem is that all Cavendish banana plants are clones. The plants are triploid hybrids, which is why they don’t produce seeds. This also makes them sterile. They are reproduced by taking new shoots that grow off the underground bulb (or corm). For this reason the entire Cavendish industry is basically comprised of clones. This is the ultimate monoculture – which leaves them particularly susceptible to disease, such as TR4.

One solution is to do what they did after the Gros Michel crop was lost – cultivate a new banana that is resistant to TR4. But few cultivars have all the traits necessary for a commercial banana. It has to ripen slowly, so it ships well, and has to taste good. Cavendish bananas, apparently, are a pale comparison to their tastier predecessor, and many were concerned they would not be accepted. There are many local varieties of dessert bananas, some are quite tasty (I have tried a few, like apple bananas, which are good). But these varieties don’t ship as well as the Cavendish, which is why they are mostly available locally.

The other solution is to breed or genetically engineer the Cavendish (or the Gros Michel, for that matter) to be highly resistant to TR4. Then we could at least get another century out of the banana, hopefully. A research group in Australia is working on a CRISPR edited version of the Cavendish that is resistant to TR4. Fortunately, nature already has a partial solution – some wild type bananas have a gene, RGA2, that provides resistance to TR4. In fact, the RGA2 gene is already present in the Cavendish, it is just expressed 10% of that of the resistant varieties. So all the scientists have to do is activate the expression of the already present RGA2 genes in the Cavendish. They have done this and are already in field trials, which are showing enhanced resistance.

There is also a gene from a nematode, the Ced9, which confers resistance to TR4. This is not being used by the Australian group, but is another path to resistant bananas. We may end up needing this gene as well at some point.

There is a bit of news in that Del Monte has agreed to fund the Australian group developing the Cavendish with enhanced RGA2. Hopefully this will help bring the resistant banana to market in time to save it. And of course, there are anti-GMO activists who against this banana – for no legitimate reason. They just oppose everything GMO for misguided ideological reasons.

Clearly we are in an arms race against infectious disease. This extends beyond the banana – our entire farming infrastructure is a constant battle with pests and diseases. We need to use a multi-pronged defense, something called “integrated pest management”, for example. We need to maintain output to feed the world, but at the same time adjust our practices to minimize the threat of crop disease. This will be a never-ending battle. Also, we will need every tool we have, including genetic engineering, in order to keep up. Without genetic modification, the banana industry is going away. Other crops are similarly threatened – genetic engineering has already saved the papaya industry in Hawaii. Citrus greening threatens our citrus industries.

I do wonder what this will be like in 50, 100, or 500 years. Can we get into a sustainable loop, where we abandon certain crops for resistant ones, but eventually can return to them once the diseases that were specific to them wane (similar to antibiotic resistance)? Or perhaps we will need to develop farmland on the Moon or Mars, or in orbiting stations, that are entirely sterile except for the crops themselves and beneficial bacteria in the soil – but no pests. They could be run by robots, and entirely sealed off from any possible contamination.

Another benefit to off-worlding our food production is that more of the Earth’s surface can be returned to natural ecosystems. Right now about half of the world’s habitable land is used for agriculture. This is perhaps the biggest negative effect on other species, the loss of habitat due to agriculture. This, of course, is a far future (at least a few centuries) solution, but I do wonder if this will be an ultimate solution.

In the meantime, genetic engineering is probably our best chance to keep one step ahead of the worst pathogens like TR4.

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