When looking at a steam traction engine, one of the most prominent features is a pair of heavy metallic balls spinning around a vertically mounted axis. This is the 'governor', a device dedicated to preventing the engine from running dangerously fast. As pressure builds the balls spin faster, forcing them further from their axis and thus opening a valve which lets excess steam harmlessly escape.

Such safety devices are integral to all well-designed machines and the natural world is no exception. But only recently have scientists begun to elucidate the cut-outs and safety valves employed in the biochemical world. Now, in Nature1, Krishina Niyogi and colleagues of the University of California at Berkeley report their investigations of the safety systems of perhaps the most important biological engine of all.

Plants harvest the power of the sun, converting it into chemical energy initially through the activity of 'photosystem II', a large protein complex. At the heart of photosystem II is chlorophyll, which absorbs light to convert this green pigment into an activated form. The energy contained in this activated chlorophyll is used to make all the complex chemicals on which life depends.

But the sun is not a constant energy source (as all of us currently in the grip of the Northern Hemisphere's winter are acutely aware). Plants are at their most efficient in average levels of sunlight. In full sunlight, especially at noon, they absorb much more energy than they can use. If this excess is not disposed of safely, chlorophyll forms a hyper-excited state that can pass its energy on to any nearby oxygen molecules. The resulting activated oxygen, called 'singlet' oxygen, causes indiscriminate damage and ultimately the death of the plant's cells in a process similar to the bleaching of carpets or paintings in direct sunlight.

So plants have developed a way of quickly dissipating excess energy as heat, which is technically known as 'nonphotochemical quenching'. For a system so crucial to the continuance of life on Earth, it is very poorly understood.

Much of the attention so far has been focused on the role of a class of organic compounds called 'carotenoids'. These are found throughout plant photosystems and, among other things, are able to detoxify singlet oxygen by accepting its energy and dissipating it as heat. Plants unable to synthesize certain carotenoids have impaired nonphotochemical quenching.

But Niyogi and colleagues attacked the problem from a different angle. They made a large number of mutants of the model plant 'thale cress' (Arabidopsis thaliana) and looked for those unable to deal with excess light energy. This screen threw up a number of plants with mutations in a single gene for a protein known as 'PsbS'. This protein had previously been identified as a component of photosystem II, capable of directly binding chlorophyll but of no known function.

PsbS is similar to, although larger and more complex than, many proteins found in plant and bacterial photosystems. Apparently, its sole function is in nonphotochemical quenching, as mutant plants lacking PsbS show no other deficiencies and can photosynthesize as efficiently as their normal cousins.

This work opens up a second front in the campaign to understand the mechanisms of nonphotochemical quenching. Where exactly in photosystem II does PsbS reside? Does it interact with carotenoids, or are the two systems entirely separate? What is clear, however, is that in plants, as in any good machine in this uncertain world, safety is so important that separate, dedicated safety systems have arisen to ensure it.