Communicating with chloroplasts

© hybrid medical animation / Photo Researchers, Inc.

The paper:

S. Koussevitzky et al., "Signals from chloroplasts converge to regulate nuclear gene expression," Science, 316:715–9, 2007. (Cited in 58 papers)

The finding:

Joanne Chory, a Salk Institute molecular biologist, used Arabidopsis thaliana mutant screens to identify two key molecular players in the communications between chloroplasts and plant cell nuclei: GUN1, a protein found in chloroplasts that regulates RNA molecules, and ABI4, a nuclear transcription factor. The findings indicated that the few known chloroplast-to-nucleus signals converge on ABI4 and are integrated by GUN1 within the organelle and not in the cytoplasm or nucleus. "That was a big conceptual leap," Chory says.

The significance:

A robust understanding of retrograde signaling—where chloroplasts alter the expression of nuclear genes that encode chloroplast-bound proteins—may help biologists reconstruct cellular endosymbiosis. "How did these organelles evolve communication?" asks Peter McCourt, a University of Toronto plant biologist. "That's a very big, interesting question to all biologists."

The interim:

Although GUN1 and ABI4 are important, they don't tell the whole retrograde signaling story. Research teams around the world are now focusing on the chemical nature of the retrograde pathway's master go-between molecules (see box below).

The future:

Chory's lab is describing the mechanism whereby GUN1 links organellar and nuclear genomes. Meanwhile, Rachel Green of the Hebrew University of Jerusalem found that GUN1 is also involved in regulating circadian rhythms (Plant J, 51:551–62, 2007).