These mosses dry out so completely that it’s as if they had been placed in an oven, said Brent Mishler, who directs the University and Jepson Herbaria at UC Berkeley and the lab where Caswell-Levy studies.

“Isn’t it amazing?” said Mishler. “We humans die without water way before we completely dry out.”

When there’s no rain, Tortula mosses dry out completely and stop photosynthesizing. That is, they stop using carbon dioxide and the light of the sun to grow. They’re virtually dead, reduced to a pile of chemicals, and can stay that way for years. Researchers have found dry, 100-year-old moss samples in a museum that came back to life when water was added.

These mosses are very good at repairing their damaged cells, and that’s a skill that would serve crops well, experts say. Right before they dry out, the mosses write themselves a set of genetic instructions, so that if they ever get water again they can start growing right away. Their ability to prepare themselves beforehand and repair themselves after a dry spell reminds Mishler of what humans do to prepare for natural disasters.

Brent Mishler examines a moss sample under the microscope at the University of California, Berkeley. He studies mosses that have the ability to live without water for decades and spring back to life when it rains. (Gabriela Quirós/KQED Science)

“If people know there’s a hurricane coming, there are things they can do to prepare ahead of time, like boarding up the windows,” said Mishler. “But you also have to be able to survive after the storm and fix any damage, so you buy disaster supplies ahead of time. The moss likewise tries both to limit the damage during drying and does other things to repair itself upon rewetting.”

Trying to make crops that can survive a drought

Mel Oliver, a research geneticist with the U.S. Department of Agriculture and the University of Missouri, Columbia, has identified close to 80 genes from Tortula that allow the mosses to write genetic instructions and repair themselves. He calls these genes rehydrins.

“We have some very good gene candidates,” said Oliver. “By studying how the moss handles losing water and how it repairs damage, if we can understand those processes, we can look at new ways to improve drought tolerance in crops.”

Researchers have looked inside the crops themselves to see if they don’t already contain some of these moss genes, left over from 450 million years ago, when a common ancestor of mosses and crop plants moved onto land and acquired the ability to live without water.

But using moss genes to make crop plants better able to survive dry spells is a big challenge, in part because this ability comes not from a single gene, but likely from a group of genes, said Mishler.

Another challenge is balancing drought-protection with the need for high yield. It turns out that there really is no free lunch: mosses’ useful ability to live without water makes it hard for them to grow very big.

“What happens in evolution is trade-offs,” said Mishler. “If you do one thing well, you can’t do another. It’s hard for one organism to be able to do everything.”

The cellular mechanism that a plant needs in order to live without water slows down its productivity, measured by the amount of new green tissue a plant can grow. That’s why many plants shed their ability to live without water in favor of an increased ability to grow big.

When there’s no rain, mosses like this Orthotricum dry out completely and stop photosynthesizing. (Josh Cassidy/KQED Science)

Mosses, such as this Orthotricum, don’t have roots to transport water. Instead, moss’ porous cells absorb water like a sponge whenever it’s available. (Josh Cassidy/KQED Science)

“Productivity is a good thing in evolution,” said Mishler. “The plants that grow faster outgrow the ones that grow slower.”

Plants that can hold water inside their bodies are able to grow big. So if you try to make a crop plant more like a moss its yield will decrease. That’s why researchers’ goal isn’t to make a crop that can live entirely without water, like a moss, but rather a plant that could repair itself after a dry period, even if some yield were lost in the process, said Oliver.

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