By Cana Yao
In a harsh, hyperarid desert, there grows a plant that “cannot die.” Scientists have been uncovering its genetic secrets to gain a better understanding of how the plant, known as Welwitschia, can survive for millenia in the unforgiving Namib desert--which receives less than two inches of precipitation per year.
The plant’s name in Afrikaans is “tweeblaarkanniedood,” meaning “two leaves that cannot die.” The naming is apt: the Welwitschia only grows two leaves in a lifetime that can last for thousands of years. “Most plants develop a leaf, and that’s it,” said Andrew Leitch, a plant geneticist at Queen Mary University of London. “This plant can live thousands of years, and it never stops growing. When it does stop growing, it’s dead.”
Some of the oldest Welwitschia are estimated to be over 3,000 years old--having steadily grown the same two leaves since the iron age.
Welwitschia’s two fibrous leaves become shredded and curled over time as it withstands desert winds and is fed on by thirsty animals, giving it a distinctly octopus-like look. One 19th-century director of Kew Gardens in London called it, “the most wonderful plant ever brought to this country and one of the ugliest.”
Though it might not be much to look at, Welwitschia has captivated biologists including Charles Darwin since its discovery. It is said that when the plant’s namesake, botanist Friedrich Welwitsch first came across the plant in 1859, “he could do nothing but kneel down on the burning soil and gaze at it, half in fear lest a touch should prove it a figment of the imagination.”
In a study published this month in Nature Communications, scientists have turned to the plant’s genome to gain a “better understanding how Welwitschia does all the crazy stuff that it does,” says Jim Leebens-Mack, a plant biologist at the University of Georgia. Reflecting the plant’s arid and nutrient-poor surroundings, the Welwitschia genetic history seems to correspond with environmental history.
Approximately 86 million years ago, during a period of prolonged drought in the region--and possibly the formation of the Namib Desert itself-- the entire Welwitschia genome doubled after a mistake in cell division. Tao Wan, a botanist at the Fairy Lake Botanical Garden in Shenzhen, China, and lead author of the Welwitschia study said that “extreme stress,” such as aridity, is often associated with similar genome duplication events. Dr. Leitch, a co-author of the study, added that duplicated genes can take on new functions, as they are released from their original ones. However, Dr. Wan said that having more genetic material comes with the cost of DNA replication, or “the most basic activity for life.” A big genome makes it “really energy consuming to maintain life,” Wan and Leitch said, especially since a large amount of Welwitschia’s genome is “junk” self-replicating DNA, called retrotransposons.
Most likely due to increased temperature stress, the researchers detected a “burst” of retrotransposons activity one to two million years ago. To counteract this, the Welwitschia genome underwent DNA methylation, or widespread epigenetic changes that silenced these junk DNA sequences. Paired with other selective forces, methylation drastically pared down the size and energetic maintenance cost of Welwitschia’s duplicated library of DNA, Dr. Wan said, giving it “a very efficient, low-cost genome.”
The study also found that Welwitschia had other genetic tricks up its leaves. Whereas the average plant leaf grows from the plant’s apexes, or the tippy-tops of its stem and branches, Welwitschia leaves instead grow out of the basal meristem, which supplies fresh cells to the growing plant, Dr. Wan said.
A large number of copies or increased activity of some of its genes may help Welwitschia continue to grow under extreme environmental stress. In a warming world, the genetic lessons Welwitschia has to offer may help humans breed hardier, less thirsty crops, added Dr. Leebens-Mack.
The study also underscores the importance of curiosity-driven research--the importance of kneeling down in burning soil to take a closer look when you encounter two leaves growing in a desert against all odds. “From weird things, you discover weird things that help you understand things that you didn’t know you didn’t understand,” Dr. Leitch said.