Day 48: Newly discovered protein ‘missing link’ in plant clocks

This mustard seedling is stained blue wherever the newly discovered clock protein CHE occurs. Credit: Jose Pruneda-Paz

Scientists have found a key protein that’s been missing from their understanding of how plants tell time.

The discovery came from studies of the tiny mustard plant (Arabidopsis genus), which is often used as a laboratory model.  Researchers have long known about a protein that senses fading light and kicks in in the evening (called TOC1), and two others that respond to sunlight in the morning (CCA1 and LHY). They knew that the morning proteins were good at shutting off TOC1. But TOC1 had no known way of telling the morning proteins to pipe down at bedtime.

Now, Jose Pruneda-Paz, a biologist at the University of California at San Diego, and his colleagues have found the missing link.

Pruneda-Paz and his team grew various plant proteins in laboratory yeast, and then screened them for the ability to influence CCA1 and LHY. In the transcription factor called CCA1 Hiking Expedition, or CHE, they found a winner. 

Transcription factors are proteins that bind to DNA and regulate its genetic expression. Specifically, CHE attaches to CCA1′s promoter region and forces it to shut down. The researchers also found a link between CHE and TOC1, but that will take more time to describe. 

The new discovery appears in this week’s issue of the journal Science, and represents an important piece of the puzzle for describing circadian rhythms, used by both plants and animals to separate biological activities according to when sunlight is available.

Evidence increasingly points to the plant circadian clock as a critical component regulating growth and the timing of flowering. Another recent paper published in the journal Nature, by a team out of the University of Texas at Austin, reports that an altered clock contributes to hybrid vigor, suggesting that targeting clock genes may be a way to improve the growth of crops. 

The Science study is part of a larger effort to build a complete library of all the transcription factors in the mustard plant.

Pruneda-Paz and his co-authors “solve a major puzzle in our understanding of the plant clock,” wrote C. Robertson McClung, a biologist at Dartmouth College, in a commentary that will appear in the same issue of Science. But he said the work isn’t done yet.

“The identification of CHE represents an important advance, but our understanding of CCA1 regulation remains incomplete,” he wrote. “Similarly, the picture of the regulation of the other genes constituting the clock remains fragmentary. Full understanding of the plant circadian clock mechanism needs more time.”

Source: Eurekalert and Science