Science News, writing

Fat Mutant Plants

Your Random Science News Story for the Month of September

Sugar is delicious. I don’t think I’m revealing some great surprise in saying that. Humans crave sugar because we need it to survive – it’s the best source of energy for us out there. But it was only very recently in our history on this planet that sugar became so readily available. Despite any Big Sugar assurances to the contrary, it’s contributing to our obesity epidemic. Part of that’s simply due to the fact that – since sugar’s in just about every processed food, ever – it’s way too easy to consume a bunch of calories. And if our old-school bodies don’t use them, they don’t just pee out the excess – they store that extra energy source as fat.

You are somehow delicious and disgusting at the exact same time…

But research out of Long Island’s Brookhaven National Laboratory has shown that excess sugar doesn’t just make humans fat. It makes plants fat, too.

The paper was published on Plant Physiology‘s website originally back in August, but it’s hitting the actual journal this month. Unlike quite a few of the papers I’ve covered, it’s actually free to read.

What happened?

Since plants make their own food, the research team couldn’t just feed their victims bowls full of Snickers until they plumped up; rather, they had to find a way to get the plants to stop transporting the sugar made in their leaves (via photosynthesis) to other parts of their bodies, and to stop them from turning the sugar into starch for storage.

To do this, they used a form of genetic manipulation known as selective breeding. Specifically, they bred Arabidopsis thaliana plants1 with very specific mutations together to combine genetic traits in offspring that would hinder both sugar transport and starch synthesis.

Arabidopsis thaliana
Figure 1A from Zhai et al. 2017: From left to right, wild Arabidopsis thaliana, mutant adg1 (disables starch synthesis), mutant suc2 (disables sugar transport), and crossbreed of adg1 and suc2

After producing plants with sugar-increasing traits, they also bred those with other mutant Arabidopsis, including one which struggled to break down lipids, i.e. the category of molecules comprising fats, oils, and waxes.

After growing the plants in soil for 6 weeks, they measured leaf sugar content and two types of lipids  – triacylglycerols (TAG) and fatty acids (FA).

This work builds upon the team’s previous research, which identified how higher sugar levels in tobacco leaves prevented a protein (named KIN10) from breaking down2 another protein (named WRINKLED1), whose job it is to trigger oil-production.

What’d they find?

mutant plants
Figure 6A from Zhai et al. 2017: From left to right, mutant suc2, double mutant adg1suc2, triple mutant adg1suc2tt4, triple mutant adg1suc2sdp1, and quadruple mutant adg1suc2tt4tdg1. Top row: top side of leaves, Bottom row: bottom side of leaves. The reddish/purpleish hue is due to the production of anthocyanin

Plants with two mutations disabling both starch synthesis and transport from the leaves (double mutant adg1suc2) increased the amount of sugar (i.e. both glucose and sucrose) in their leaves by a factor of 80 relative to their wild counterparts. Fatty acid content increased by 180%, and TAG increased by 1000% – that is, a factor of 10.

Plant offspring with both those traits plus a limited ability to break down plant lipids (triple mutant adg1suc2sdp1) increased the accumulation of TAG by an additional 66%. Quadruple mutant adg1suc2tt4tdg1 – the most mutated plant tested – had the highest amount of both TAG and FA by weight (once the plant had been dried out)

Lead researcher John Shanklin summarizes the results of his team’s work thusly:

“Combining genetic mutations that decrease the transport of sugar out of leaves and the conversion of sugars to starch increases sugar levels in leaves. That excess sugar drives increased oil production by stabilizing the oil on-switch, and also by supplying the carbon building blocks needed to make more oil in leaves.”

They also found that combining different mutations helped balance out one mutation’s negative side effect. Plants unable to transport sugar have severely stunted growth, because they can’t deliver the carbon atoms (as part of the sugar molecules) to places the plant needs in order to grow; but, crossing the plants with those that can’t turn their sugars into starch produces offspring that aren’t as stunted, though you can obviously see they’re smaller than their wild counterparts. The researchers aren’t sure why this helps exactly, but it somehow gets more sugar to the rest of the plant.

So what?

These fattened plants are full of lipids that could be used for biofuel and a variety of other chemicals. Regular plants, if they have any useable amount of lipids, store them in their seeds. Getting them to accumulate in leaves, instead, would make the them more accessible.

Whether or not people are going to take this research and try to breed super-sugary/fatty lettuce in some weird attempt to make people buy more salad is a question for another day.

1. Otherwise known as the thale cress, it is a small flowering plant related to mustard and all cultivars of Brassica oleracea, which include cabbage, broccoli, cauliflower, brussel sprouts, kale, and collard greens. Yes. All those veggies come from the exact same species of plant. Selective breeding at its finest. Well, maybe not finest. ‘Cause, ya know, dogs… 
2. The protein doesn’t directly destroy the other – it sicks a molecular ‘mark’ on it to be destroyed by some sort of molecular hit-man. 

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