When you see a field of wheat in summer, the spikes of grain rippling gracefully in the breeze, you probably won't have guessed that the plants are fat. Yet, compared to the wild grasses they are bred from, the ears of modern cereal plants are grotesquely obese. They have larger and more numerous grain, laden with vast reserves of starch, way in excess of what they actually need. This excess weight is our food.
With year-on-year gains from conventional breeding beginning to peter out and an ever-expanding human population to feed, the race is on to find new ways to persuade plants to put on even more weight. And it turns out that an effective way to do this is to interfere with the signaling systems that control the rate at which plants synthesize their food.
For plants, "food" means carbon dioxide from the atmosphere which they turn into sugars by photosynthesis, and nitrates in the soil which are metabolized to form amino acids. Plants then monitor the concentration of sugars and amino acids in their tissues and grow more rapidly when they "sense" that food is available. This is a "feed-forward" control system.
But that's not the whole story. Plants also have genetically programmed limits on growth. These limits ensure they produce the right tissues, of the right size, at the right time. They also stop the plant trying to grow when it is damaging to do so, for example when the weather turns bad.
When a plant comes up against its growth limits, food begins to accumulate and this generates a "feedback" signal causing the plant to turn down the food production systems. Effectively the plant realizes it is full and stops eating.
But what if we could tweak the controls? Could we then make crops even more obese? Experiments with the sugar control system suggests that the answer is a resounding yes.
A team of researchers from agrochemists Syngenta and Rothamsted Research made a single genetic modification to maize plants to prevent the accumulation of trehalose-6-phosphate, a key sugar monitored by the plant. Essentially, the plants were tricked into "thinking" that they were not producing enough sugar and as a result they increased production. This, in turn, seems to have triggered the feed-forward system because the genetically-modified plants produced up to 50% more grain in well-watered conditions and outperformed unmodified plants by 123% in drought conditions.
Gorging on nitrogen
If the same changes could be engineered for the nitrogen control system, then not only might we achieve even higher yields, but we could also address the agricultural run-off problem at the same time. Millions of tonnes of nitrate fertilizer are applied to fields every year but much of it remains unconsumed by crops. And when it rains, the excess runs off the fields, polluting nearby rivers and lakes.
The difficulty is that, despite decades of research, the signaling system that underpins nitrogen appetite control has remained something of a mystery.
Until now. In a study recently published in Plant Cell, a Swiss-German team describe how they uncovered part of the system lurking in a surprising place.
Quite by accident, they found out that a specific form of vitamin B6 (known as a vitamer) tells the plant when it is full of nitrogen. The first clue was that the vitamer accumulates in plants in parallel with ammonium, one of the immediate products of nitrate metabolism. The second was that plants with unusually high amounts of the vitamer had impaired growth that could be overcome by supplying ammonium.
Although not all the details are yet clear, the most telling observation was that the accumulation of the specific B6 vitamer led to the nitrate metabolism system being turned down -- it works as an appetite control system.
Perhaps the main reason we are having to retune the settings on the appetite systems of crop plants is that they are held back by their evolutionary history. The grass species that were domesticated to form cereal crops such as maize, rice and wheat are likely to have grown in poor soils -- and plants that have adapted to such soils generally have conservative food strategies. This means they take up only as much as they need to grow and produce seed for the next generation. So it's not surprising that when we throw nitrogen fertilizer at their cultivated descendants, they don't gorge themselves on the unexpected feast.
A mismatch between evolutionary history and modern conditions is also behind the human obesity epidemic. Just as with crops, the solution could lie in tweaking appetite systems; we just need to work out how to go in the opposite direction.