More than 100 years ago, botanists first observed an interesting phenomenon under the microscope: when you shine light on a leaf, the chloroplasts inside the cells start moving around. They seek areas of weak light and avoid areas of strong, burning light by hiding close to the cell walls. Now researchers from Japan have uncovered part of the molecular mechanism behind this behavior.

In last Fridays Science, they described a simple test they used to screen over 100,000 mustard weed plants, each of them having a mutation somewhere in their DNA. They took a leaf, covered it with a black plate with a slit in it and switched on a strong lamp. In those plants capable of a normal light reaction, the chloroplasts in the light-exposed area moved to the side, making part of the leaf look pale. But in plants in which one of the molecules responsible for the movement was defective, the chloroplasts stayed where they were. Ten of the leaves remained completely green, and in four of them, the scientists tracked this lack of a reaction down to a mutation in a gene called NPL1.

The protein that NPL1 encodes is very similar to phototropin, a plant photoreceptor that senses weak blue light and makes the leaves turn toward it (a behavior called "phototropism"). NPL1, on the other hand, seems to detect strong blue light: the chloroplasts of normal plants having the receptor avoided parts of the cell that were under strong blue light, but chloroplasts from plants with a defective NPL1 actually entered these areas, failing to detect the light. What is not yet known is how the signal gets from the photoreceptor, which is localized in the plasma membrane, to the chloroplasts inside the cell. Several other molecules are most likely involved.