A plant molecule that was once considered biologically irrelevant, now offers new leads into the development of hardier plants

Desert landscape. Monument valley, Arizona Credit: Pixabay

Anyone who knows a bit of plant biology, knows the importance of a molecule called Abscisic acid or ABA as it is popularly called. It is a big deal in plant research. ABA is a plant hormone that is present in all the land plants and helps in alerting the plants to various environmental stressors like drought conditions and pathogens. ABA is also known for its connection to the good stuff in fruits and vegetables: carotenoids, such as beta-carotene and lycopene.

ABA is produced through an intricate metabolic pathway that transforms a simple carbon- based building block to something so complex, which in turn helps in carrying out various functions in the plant.  In the plants, this hormone is degraded by an enzyme into another molecule called phaseic acid which has historically been cast aside as an inactive byproduct- a metabolic dead end of sorts.

However, researchers from the Salk Institute have published a new paper in the journal Cell that reveals an unexpected role for this small and often overlooked molecule – phaseic acid.

Their new findings suggest that phaseic acid and its receptors probably co-evolved to become crucial for drought resistance and other survival traits and may help in the development of new, hardier crops that can weather natural disasters wrought by climate change.

“There had been some hints that phaseic acid was not just an inactive bystander but a plant hormone with an important role,” says the study’s senior investigator Joseph Noel, holder of the Arthur and Julie Woodrow Chair and professor in the Jack H. Skirball Center for Chemical Biology and Proteomics at Salk. “But now, by using an array of cutting-edge biological approaches, we’ve shown more convincingly, that phaseic acid is likely important for survival.”


Seed plants evolved a dedicated enzyme to degrade phaseic acid and keep its level in check. A plant lacking this enzyme accumulates more phaseic acid, and is more resistant to long-term drought conditions (right) compared with a normal plant (left). Credit: Salk Institute

The researchers studied a commonly studied plant called Arabidopsis thaliana and obtained varieties of this plant that lacked the enzyme to degrade phaseic acid into smaller molecules, in effect accumulating a large amount of phaseic acid.

This had an interesting effect on the plants, as they could now survive without water for a longer time period (becoming drought-resistant) and showed changes to the timing of seed germination.

Says Noel “This suggested to us that maybe we should be thinking of phaseic acid not as an inactive degradation product but actually as a molecule that had its own capacity to cause changes like other plant hormones.”

Using a high-resolution imaging technique called x-ray crystallography, the group also showed that phaseic acid binds to ABA’s receptors, solidifying evidence that ABA’s receptors can sense more than just ABA.

This study just goes on to show how much of hidden complexity is there in organisms and how they make use or repurpose small molecules to bring about a variety of responses in them. Maybe, it is evolution in play and it surely does look like the plants are continuing to evolve since they began to inhabit land roughly 450 million years ago.

Noel’s team will study the role of the hormones in the roots of plants. There is evidence that the phaseic acid and ABA play a role in moving carbon from a plant’s leaves and stems into its roots to protect them during periods of drought, overwintering and during pathogen attacks in the soil, Noel says. “Understanding this area of plant biology offers new ways to think about how we might mitigate climate change.”

This research was supported by the National Science Foundation, the Pioneer Foundation, the Pew Charitable Trusts, the Searle Scholars Program and the Howard Hughes Medical Institute.