How light-farming chloroplasts morph into defensive warriors

Chloroplasts may seem like docile farmers of light. But inside these microscopic plant and algal cell structures lurks the spirit of a warrior.

When a pathogen attacks a plant, chloroplasts stop making food from sunlight and rush to the site of infection to help fend off the invader. Now, researchers have identified the protein that mobilizes these organelles into a defensive army.

Plant pathologist Tolga Bozkurt of Imperial College London and colleagues infected a tobacco relative (Nicotiana benthamiana) with the Irish potato famine pathogen (Phytophthora infestans), a funguslike microbe. The team suspected that a protein called chloroplast unusual positioning 1, or CHUP1, plays a role in rallying chloroplasts for defense. The protein is better known for helping move chloroplasts to where light enters the cell and then attaching the organelles to the cell membrane there.

In experiments, when researchers silenced the gene for CHUP1, chloroplasts mostly didn’t respond to an infection. But when the gene was active, chloroplasts shut off their photosynthetic machinery and formed a defensive onslaught, the researchers report January 9 at bioRxiv.org.

Invasion triggered a cascade of molecular signals that prompted CHUP1 to mobilize chloroplasts. The chloroplasts grew tentacle-like projections called stromules that occasionally linked to other chloroplasts, possibly to transfer information to one another. CHUP1 then directed these chloroplast clusters to the infection site. There, the chloroplast army clasped the microbe’s invading fingerlike appendages, called haustoria, and released toxic photosynthesis by-products. Chloroplasts even seemed to squeeze until haustoria collapsed, helping to counteract the infection.

Bozkurt says he was shocked by how dynamic the chloroplasts’ reaction was. “They look as if they’re going back to their ancestral origins when they were free-living microbes,” Bozkurt says, referring to the theory that chloroplasts were once photosynthetic bacteria that were captured millions of years ago by the common ancestor of plants and algae. It’s possible, he says, that these responses are vestiges of chloroplasts’ primitive defense system that plants have co-opted for their own immunity.