What Wild and Parasitic Plants Can Tell Us About Adapting Crops to Climate Change
To feed an estimated 9.8 billion people by 2050, global food production needs to grow by about 70 percent. Yet rising patterns of extreme weather are challenging food security. Glaciers are melting, sea levels are rising, wildfires are blazing, and droughts are intensifying. Earth is in an alarming state of flux.
To adapt and feed the world, we need stronger crops. A key to resilience may lie in plant genetics.
Over a few thousand years, human beings have selected varieties with traits that have allowed agriculture to flourish and human civilizations to grow and evolve.
– Neelima Sinha
“Over a few thousand years, human beings have selected varieties with traits that have allowed agriculture to flourish and human civilizations to grow and evolve,” says Neelima Sinha, a UC Davis professor of plant biology.
This has led to a highly productive food system, with California agriculture alone generating $45 billion in 2016. According to the Department of Food and Agriculture, the Golden State provides one-third of the country’s vegetables and two-thirds of its fruits and nuts.
While this human assistance has allowed many crops to thrive, some crops have lost adaptive traits that once helped them flourish in environments rife with stressors, like droughts and parasites.
A study this year from UC Davis and UC Merced found that California’s intensified natural disasters and droughts spell trouble for the state’s crops. It found that climate change is expected to impact eight of the 20 major crops grown in the state: almonds, wine and table grapes, strawberries, hay, walnuts, freestone peaches and cherries.
Amid such changes, UC Davis plant biologists are looking to wild relatives and parasitic plants for hints to strengthen crops for a changing future.
The arid Andean region of South America is home to one tough wild tomato species. Able to tolerate drought, salt and pathogens, Solanum pennellii is an ideal study in extreme plant adaptations. Strong roots and waxy-skinned fruits and leaves help it cope with desert challenges.
Associate Professor Siobhan Brady in the UC Davis College of Biological Sciences studies this wild tomato and its domesticated relative Solanum lycopersium, the garden tomato. Unlike garden varieties, the desert tomato’s roots grow best in drought conditions. Brady and her team are using genomics tools to search for the key to a water-retaining compound in the desert tomato present in the desert tomato’s roots.
“If it can be expanded to other crops by breeding for a particular factor, then it could provide the garden tomato’s roots with more waterproofing and a better ability to withstand drought,” Brady said.
A shifting climate isn’t the only threat to producing bountiful harvests. More than 4,000 different species of parasitic plants leech nutrients from host plants, leaving destruction in their wake.
“Parasitic plants are hugely significant, especially in the lesser developed parts of the world,” Sinha said. “We have things like Striga, or witchweed, which causes incredible devastation.”
Witchweed has been known to cause up to 90 percent loss in crop yields. The weed affects more than 49 million acres of cropland in sub-Saharan Africa, according to the International Maize and Wheat Improvement Center. It attaches to cereals like maize and sorghum, costing an estimated $1 billion in losses annually.
Witchweed actively hunts its prey. When sorghum roots are low in phosphorous, they secrete a molecule that tells the roots to take up more of the nutrient. But that molecule, a class of plant hormones called strigolactone, acts like a smoke signal for witchweed.
“When the witchweed senses it, it will grow towards the sorghum root, penetrate it and hijack all its resources,” Brady said.
Since witchweed evolved alongside sorghum, about 70 percent of sorghum crops have developed some degree of tolerance or resistance to the parasite.
Brady and her colleagues recently started a project to genetically define how sorghum roots interact with the soil microbiome to either enable or suppress witchweed growth. One sorghum line has been shown to be defective in producing strigolactone.
“Without this hormone, the witchweed roots can’t emerge from their seeds, invade the host plant and take it over,” Brady said.
Like Brady, Sinha is trying to understand at the genetic level what makes certain crops resistant or susceptible to a parasite. She studies the genetic interactions between the parasitic plant Cuscuta, also known as dodder, and its host plants. Common in California, dodder grows like cobwebs of orange spaghetti as it envelops its host.
“This plant has no roots, and it doesn’t photosynthesize. It doesn’t have any leaves, so everything it needs to grow it gets from the host plant,” says Sinha. “It’s a significant problem for California agriculture.”
Brady and Sinha hope that, by investigating plant genomes, they will be able to find genetic triggers that enable such plant defenses. This may help encourage the growth of a microbe that could help some plants survive better in a shifting climate.
Understanding the language of plants
To me, the chemistry in plants is much like a language. They use chemicals to communicate, where we use words.
– Philipp Zerbe
Plants produce an array of chemical compounds to cope with environmental challenges. From defending against predators to attracting pollinators, these chemicals, called metabolites, number in the hundreds of thousands and vary from species to species. Each plant is unique in its chemical repertoire, making them the envy of organic chemists.
“To me, the chemistry in plants is much like a language,” said Philipp Zerbe, assistant professor of plant biology. “They use chemicals to communicate, where we use words.”
Zerbe works to decode the diversity of plants by mapping the genes, enzymes and pathways that form their complex metabolic machinery. He’s particularly interested in terpenoids, the largest and most diverse class of metabolites.
Terpenoids perform many functions, from regulating growth and development to protecting plants from environmental stresses like drought and salinity.
To Zerbe and other biologists, the advent of targeted genome editing with technologies like CRISPR/Cas9 has been a game-changer. It’s now possible for plant biologists to pinpoint the genes and pathways involved in specific metabolite production, which could then be introduced into crop varieties.
“If we can find the genes that are involved in this chemical defense, we can target these genes very precisely,” says Zerbe. “If we listen carefully and understand plants’ language, we may translate this knowledge into new avenues to improve crop resistance and ultimately provide for our burgeoning population.”