New organ found in world’s best-studied plant

The machinery of life is dazzlingly complex. To try to make sense of it, researchers have spent decades focusing on so-called model organisms: creatures that are easy to study in the lab and share key features with many other forms of life. This model group includes the lab mouse, the fruit fly, and an unassuming weed called thale cress, or Arabidopsis thaliana.

Model organisms are among the best understood lifeforms in the world. So imagine scientists’ surprise when, after a decade of work, a dogged plant biologist found a brand-new organ on thale cress, hiding in plain sight.

The newfound organ, described today in the journal Development, is a horizontal arm that juts off the main stem of Arabidopsis and acts as a support for the pedicel, the small stalk that leads to the base of a flower. Extending from the main stem of the plant, the part is reminiscent of the structural support known as a cantilever, leading researchers to name the organs “cantils.”

“There are still things to find out there!” says lead study author Timothy Gookin, a former postdoctoral researcher at Pennsylvania State University.

A surprising appendage

The small flowering plant Arabidopsis thaliana is native to Europe and Asia. The weed has no doubt been observed by humans for millennia, sprouting up along the Silk Road and peeking out from untended patches of Roman farmland. The plant was scientifically described in the 1570s, and scientists began sequencing its genome in the 1990s. Because the plant has a small genome and is easy to grow, plant biologists have used it for decades to study the cellular secrets of all plant life.

By 2015, scientists had written more than 54,000 papers about Arabidopsis, and since then, about 4,000 more have been published each year. We now know the plant’s DNA so well, researchers can shop online for specific mutant strains of the species. So how could an entire organ of Arabidopsis evade researchers for so long?

In part, because cantils are finicky. In Arabidopsis, pedicels normally sprout directly from a stem, with no cantil acting as an intermediary. Cantils form only when the plants grow up in an extended “springtime,” growing conditions where the plants get just eight to 10 hours of sunlight per day, as opposed to the 12 or more as they’d see in summer.

In addition, cantils form only near a particular zone on the stem: the dividing line between leafy growth on the lower stem and the upper end where flowers blossom. Finding the cantil is like discovering a whole new organ in a mouse or fruit fly that forms only in certain conditions when the animals hit puberty.

“I think this is going to be one of those papers where people will look at it and go, Oh, I saw that once! I didn’t know what it meant!” says Dominique Bergmann, a plant biologist at Stanford University in California who wasn’t involved with the study.

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Deconstructing the cantil

Gookin first noticed cantils in 2008 while working in the laboratory of Sarah Assmann, a plant biologist at Pennsylvania State University. Then a postdoctoral researcher, Gookin was studying the genetics of how leaves age, using mutants of Arabidopsis that withered when forced to grow with fewer hours of light.

During these experiments, he noticed something peculiar: On some of his plants, horizontal arms jutted several millimeters off from the stems. Gookin filed the observation away, chalking it up to a fluke caused by mutations in the lab plants.

But as his research progressed, Gookin saw the feature again and again—including in some wild strains of Arabidopsis that had not been manipulated for lab work. He checked every source of possible contamination, even growing plants in different labs to be sure that substances wafting through his first lab’s air supply hadn’t triggered the structures’ growth.

Again and again, the structures appeared. Only then did he accept that the cantils were no fluke, but a part of Arabidopsis’s biology that nobody had ever described before.

For years, Gookin worked on nights and weekends to figure out what made cantils tick, genetically speaking. He doesn’t have the whole picture yet, but so far, he’s made three key discoveries. For one, a gene called FLOWERING LOCUS T, which plays a vital role in plants’ flower formation, triggers cantils to form. Second, suppressing molecules called G-proteins, which help cells receive outside signals, can boost the odds of cantils forming.

Finally, Gookin figured out that a key plant gene called ERECTA acts as a master switch for cantils. Disable the ERECTA gene, and cantils can’t form regardless of what the other genes are doing.

ERECTA’s role in stunting cantils is part of the reason the structures haven’t been discovered before. Arabidopsis plants with broken ERECTA genes behave particularly well in the lab, growing strong and straight without the need for extra support, hence the gene’s name. These sturdy strains are mainstays of Arabidopsis research, which means many scientists were working with plants that couldn’t make cantils at all.

Why grow a cantil?

The finding underscores just how flexible flora can be in their evolutionary development. Because plants cannot get up and move to a new environment, they must grow new organs constantly, changing their structure on the fly.

“You were born, and you got the arms you were born with,” Bergmann says. “For a plant, it’d be like, I need six arms over here.” This sort of flexibility—and biochemical chaos—is at its peak as plants prepare to flower, so it’s telling that cantils only form on Arabidopsis during this time.

For now, it’s unclear whether cantils help the thale cress survive in the wild, or if the appendages are a harmless side effect of other changes within the plant. The structures may be vestigial, meaning they are evolutionary leftovers from an ancient ancestor in the plant’s lineage, no longer serving a practical purpose.

University of Toronto biologist Nicholas Provart, who wasn’t involved with the study, suspects that cantils don’t confer a clear benefit to Arabidopsis since the organ appears only in special conditions. But even if the cantil’s function remains obscure, Provart is astounded by the lengths Gookin went to to chase down the structure—and define a new organ in the world’s best-studied plant. “It was just kind of this labor of love,” he says.