One of the first things that happens when the plant detects light is that these phytochromes move from the cell's cytoplasm to its nucleus, where the genes are kept. The photoreceptors gather in discrete spots known as phytochrome nuclear bodies. In an earlier study, Chen had found that the size and number of phytochrome nuclear bodies was directly related to light intensity.
Chen, who started this line of work as a postdoctoral researcher in Chory's lab at the Salk Institute, ran genetic screens for mutants with abnormal phytochrome nuclear bodies. He identified a new gene, hemera, that seems to be required for both the localization and the signaling of phytochrome.
He named the gene for the Greek goddess of daylight, Hemera. It is present in all land plants studied so far.
Mutant plants without hemera were found to have dramatically reduced sensitivity to red and far-red light, they failed to develop chloroplasts, were albino, and died while still only seedlings. Without Hemera, "a plant is blind to light and the chloroplasts can't develop," Chen said.
The prevailing model of chloroplast development involves signaling molecules called PIFs that hold chloroplast development back when the plant senses darkness. But when phytochromes are activated by light, they destroy the PIFs, clearing the way for chloroplast development. PIFs tended to aggregate around these phytochrome nuclear bodies before being destroyed. Hemera also tended to be found near the nuclear bodies, suggesting that the nuclear bodies were the site of PIF destruction.
In a series of experiments with hemera mutants, the team found these plants tended to have smaller phytochrome nuclear bodies and were unable to remove PIFs in light, which could explain why the mutants weren't making chloroplasts.
The team also found that hemera
|Contact: Karl Leif Bates|