For many years, scientists used the length of the embryonic stem, or hypocotyl, to quantify a plant's response to light. But scientists had still come up short in understanding the mechanisms involved in phytochrome's earliest signaling events, explains Chen.
Other studies had shown that light directly regulates the relocation of phytochrome A (PHYA) and phytochrome B (PHYB) from the cytoplasm to the nucleus, where they congregate in distinct foci that are commonly referred to as phytochrome nuclear bodies. Sitting at the microscope for hundreds of hours allowed Chen to identify a new gene, HEMERA-named after the Greek goddess of day-based on his observation that PHYB could not be found in large nuclear bodies. When HEMERA was missing, plants were unable to respond to light, Chen explains, and grew into spindly, albino seedlings that died before they could make flowers.
Chen and his collaborators then found that in hemera mutants, key proteins that must be degraded by cells responding to light were still present, thereby making the seedling believe it was still in the dark. Digging deeper, they found striking structural similarities between HEMERA and the yeast protein RAD23, whose job it is to shuttle proteins flagged for destruction to the cellular junkyard. Thus, these nuclear bodies appear to be sites where key regulatory proteins go when they need to be removed from cells.
But Chen discovered that HEMERA does something else: It also slips into chloroplasts, small compartments that contain chlorophyll and are in charge of photosynthesis, which could explain the pale appearance of hemera mutants. "People had known for a long time that one of the things light does is to trigger the differentiation of chloroplasts afte
|Contact: Gina Kirchweger|