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It is more than 100 years since Charles Darwin discovered that hybrid plants such as maize (corn) generally grow to be stronger and larger than their parents. The same is true of allopolyploid plants — those formed by hybridizing two or more types of plant, for example wheat and rye. What scientists didn't know was what caused this phenomenon, known as growth vigour. Jeffrey Chen at the University of Texas at Austin and his colleagues discovered that, in both cases, increased growth occurs because many genes for photosynthesis and starch metabolism are more active during the day in such plants. On page 327, Chen and his team explain that these plants' vigour is linked directly to circadian-clock regulators — which control growth, metabolism and fitness. Chen tells Nature how clock genes are key to vitality.

Was the study physically demanding?

We had to collect tissue samples every three hours around the clock for several days at a stretch because the expression of genes changes over time according to the circadian clock. One of my co-authors, Zhongfu Ni, a visiting postdoc from China Agricultural University in Beijing, stayed in the lab for three weeks to harvest the samples. He was so exhausted he was almost walking around on his knees.

What were the technical challenges?

Controlling the expression of clock genes is not easy. If these genes are turned off completely, plants and animals lose their fitness and their growth vigour. This can be avoided by lowering the expression of clock genes only during the day. To control the expression of these genes over time, we used a custom-made clock-gene promoter, a DNA fragment that amplifies the expression of a particular gene.

Are these genes the only thing controlling the increased growth of hybrids and allopolyploids?

No. There are certain to be other causes for growth vigour, but these genes are a major factor. We can now consider developing tools to help crops grow better and to increase yield.

Are there any implications for humans?

The circadian clock is a universal mechanism for controlling growth vigour and metabolism in animals as well as plants. In humans, obvious effects of the circadian clock can be seen in jet lag and sleep disorders. If we had a means of resetting the clock, we might be able to overcome these problems.