Los Angeles: Plants must be adaptable in order to withstand environmental changes, and the adaptive strategies they employ must frequently be as variable as the changes in climate and conditions to which they adapt.
Plant roots create a water-repellent polymer called suberin in response to drought, which prevents water from moving up towards the leaves, where it would soon evaporate. The consequent water loss would be equivalent to leaving the tap running without suberin.
Endodermal cells that line the capillaries inside the roots of some plants produce suberin. Suberin is produced in epidermal cells directly below the epidermis of the root in some plants, such as tomatoes.
The purpose of exodermal suberin has long been unknown, but a new study published in Nature Plants by researchers at the University of California, Davis demonstrates that it fulfils the same function as endodermal suberin and that tomato plants are less able to cope with water stress without it. This knowledge could aid scientists in developing drought-resistant crops.
"This adds exodermal suberin to our toolbox of ways to help plants survive for longer and cope with drought," said Siobhan Brady, professor in the UC Davis Department of Plant Biology and Genome Center, and senior author on the paper. "It's almost like a jigsaw puzzle--if you can figure out which cells have modifications that protect the plant during difficult environmental conditions, you can start to ask questions like, if you build those defenses up one upon the other, does it make the plant stronger?"
In the new study, postdoctoral scholar Alex Canto-Pastor worked with Brady and an international team of collaborators to uncover the role of exodermal suberin and map the genetic pathways that regulate its production.
"It's the merging of classical and cutting-edge methodology that lets us look at both the process that's happening in an individual cell and what you see in the whole plant," said Brady.
"So going from super small to big."
Brady, Canto-Pastor and colleagues started by identifying all of the genes that are actively used by root exodermal cells. Then they performed gene editing to create mutant strains of tomato plants that lacked functional versions of several genes they suspected might be involved in suberin production. They discovered seven genes that were necessary for suberin deposition.
Next, the researchers tested exodermal suberin's role in drought tolerance by exposing some of the mutant tomato plants to a ten-day drought. For these experiments, the researchers focused on two genes: SIASFT, an enzyme involved in suberin production and SlMYB92, a transcription factor that controls the expression of other genes involved in suberin production.
The experiments confirmed that both genes are necessary for suberin production and that without them, tomato plants are less able to cope with water stress. The mutant plants grew as well as normal plants when they were well-watered but became significantly more wilted after ten days with no water.
"In both of those cases where you have mutations in those genes, the plants are more stressed and they're not able to respond to drought conditions," Brady said.
Having shown suberin's worth in a greenhouse setting, the researchers now plan to test suberin's drought-proofing potential in the field.
"We've been working on taking this finding and putting it into the field to try and make tomatoes more drought tolerant," Brady said.