Deep into deserts: A letter from Carnegie President Matthew Scott
In 1903 the Carnegie Institution established a Desert Laboratory to explore the properties of desert plants. From that humble stone building in Tucson, Arizona, eventually emerged our spectacular Department of Plant Biology on the Stanford University campus and, by descent, our Department of Global Ecology at the same site.
The Carnegie scientists who came to Tucson had a central goal of understanding how desert plants manage in seemingly impossible conditions. Tucson provided an ideal environment because the Sonoran Desert has stunningly rich biota in spite of its extreme heat and desiccation. As documented in Patricia Craig’s centennial history of Carnegie Plant Biology, an early Tucson scientist—William Cannon—wrote “even the most hardened liar among the natives will not defend this summer climate.” About 30 centimeters (12 inches) of water lands on Tucson in an average year, though a single storm last January delivered more than 13 centimeters (5 inches ) … and dramatic floods. The most familiar and dramatic plant of the Tucson desert is the Saguaro cactus, a prickly giant so emblematic of cowboy movies. The cactus has the scientific name Carnegiea gigantea in honor of none other than our founder, Andrew Carnegie. Perhaps he was viewed as prickly.
To cope with desert life, Tucson people have done an exceptional job of responsible gardening, planting native plants that are perfectly happy in baking hot gravel. Carnegie’s present day plant and ecology laboratories lie in what, at first glance, looks like a completely different environment. Lush gardens of Palo Alto incorporate many plants that are far from drought-tolerant, including moisture-loving redwoods. Yet the Stanford area receives scarcely more water than Tucson, about 35 centimeters (14 inches) each year. That blooming of the Bay Area desert, fed by a pipeline from Yosemite National Park, has been severely challenged by years of intense drought that affect much of the American West. The Sierra Nevada mountains have been deprived of their normal snowpack for years, with the lowest level of snowpack on record this past spring. The snowpack serves as a water storage system, slowly releasing melt water during the summertime drought. Similar water storage occurs in most mountain ranges of the world and dwindling glaciers, even among the world’s highest peaks, are causing changes in what farmers can produce, and when.
Fieldwork by Carnegie scientists has been a constant source of discovery for understanding how plants survive extreme heat and desiccation. In the 1960s Olle Björkman arrived at Carnegie from Sweden and, exploring a climate that could hardly have been more unlike his home, began work in Death Valley in 1968. He and colleagues found a plant called Arizona honeysweet (Tidestromia suffruticosa v. oblongifolia) that was beautifully adapted to heat. In a paper published in Science in 1972, he reported that Tidestromia performed photosynthesis best in 120°F heat conditions that kill most plants. We still do not know how Tidestromia works its miracle. The Björkman group also showed that certain desert shrubs changed the composition of their membranes as the springtime temperature rose so that they would remain stable at Death Valley's summertime temperature extremes. Investigating plants that have such abilities may reveal ways to transfer certain of those abilities to crop plants to increase their resilience.
The start of World War II reduced endowment income for Carnegie, a drought of another sort, and the Desert Laboratory in Tucson was closed. However seeds had spread and by 1929 Carnegie had established a Division of Plant Biology on the campus of Stanford University.To this day, the original laboratory and office building remains occupied by the Department of Plant Biology. The original lease from Stanford required that Carnegie scientists shoot all the squirrels on the property. This has not been enforced, and the property now has an ample population of squirrels, and jackrabbits.
Carnegie Plant Biology department scientists continue to be at the forefront of learning the molecular, cellular, and developmental mechanisms that underlie the wonders of plant physiology, development, and evolution that shape our geologic, climatic, and human world. After years of the current drought in the American West, understanding the basis of plant resilience in arid conditions may extend growing seasons, allow survival of crops through short periods of highly extreme weather, and provide ideas about how to protect plants from drought vulnerability.
Most plants obtain water and other resources from a normally hidden network that can extend deep underground: their roots. Carnegie’s José Dinneny investigates root growth, behavior, and sensing. How do roots find their way to water and nutrients? How do they respond to salinity or toxins? He and his coworkers have made spectacular progress in developing new methods to watch living roots behave. They find that root branching and growth patterns respond to signals with exquisite precision, sensing even slight differences in moisture over distances as small as 100 microns.
What happens below ground is half the story. Water travels up through plants and is released through leaf pores called stomata. These have two “guard” cells that swell or shrink in response to conditions to open or close stomata. How much water is retained or released by a desert plant will affect how much must be found in the first place. Thus stomata have elaborate systems of opening and closing that allow CO2 to enter the plant for photosynthesis while controlling water loss. Carnegie’s Kathy Barton studies the genes that control growth and patterning of plant stem cells, the cells that give rise to all the specialized cells of the plant including leaf structures like stomata. She and her colleagues have identified genes that control whether stem cells stay active during drought stress. Changing the threshold at which plant stem cells enter a dormant state in response to dry conditions may allow plants to remain productive in periods of moderate drought.
The growth and patterning of plants, as well as their daily physiological and biochemical functioning states, are controlled by elaborate signaling networks. Hormones are signals that can work locally or globally to tell cells what to do. The molecular biology of responses to signals is a major area of study in multiple Carnegie labs, such as those of Zhiyong Wang and Seung (Sue) Rhee. This work requires multidisciplinary approaches including genetics, biochemistry, and computational science. Knowing how to manipulate such control networks will be key to exploiting new understanding of how plants cope with stress including dehydration. Dinneny comments, “A future challenge is to broaden this understanding, to place it in an ecological, evolutionary and physiological context to ultimately understand the adaptive mechanisms plants use to grow in a desert, be it the cactus of Arizona or the almond tree of the central valley.” We at Carnegie see every prospect for substantial advances in, first, understanding how plants can and do cope with stress and, second, using that information to create more resilient crop plants and improved agricultural methods.
Let’s drink (water) to that!