A newly developed tool is set to simplify the study of rare plant molecules. Researchers have created microbial cell factories using E. coli and yeast to produce a specific group of plant hormones, referred to as strigolactones, in unprecedented amounts. By increasing the output of these hormones, which are typically found in minimal quantities in plants, scientists can now explore these elusive molecules in much greater detail than was previously possible. This research has the potential to enhance sustainable farming practices by providing deeper insights into how plants produce and utilize their natural hormones for adaptation and survival.
Sometimes, leveraging a different tool can significantly ease a challenging task. A team of researchers, co-led by the University of California San Diego, has developed a novel method that generates a particular class of plant hormones, strigolactones, in unprecedented quantities using microbial cell factories. This advancement allows the researchers to study these rare and intriguing plant molecules far more comprehensively than before.
The findings, published in the January 17 edition of Science, could bolster sustainable agricultural methods by providing deeper knowledge about how plants create and utilize their natural hormones to adapt and thrive.
Globally, scientists have been intrigued by strigolactones due to their role in regulating plant growth, managing the plant’s symbiotic interactions with surrounding soil microbes, and initiating the sprouting of parasitic plants.
Nevertheless, progress in fully grasping the function of strigolactones has slowed, mainly because these molecules are present in such minute quantities in plants. Consequently, researchers often resorted to tedious procedures that required large amounts of plant material just to gather enough substance for identification.
Now, a team from the UC San Diego Jacobs School of Engineering, in partnership with UC Riverside and Utsunomiya University in Japan, has embraced a genomics-focused approach using microbial cell factories to tackle this production challenge.
“We have implemented an engineering strategy that significantly eases the process, turning previously impossible tasks into achievable ones,” explained Yanran Li, a professor at the UC San Diego Jacobs School of Engineering with a specialization in synthetic biology and metabolic engineering.
This innovative strategy utilized E. coli and Baker’s yeast, resulting in a microbial cell factory that produces strigolactones at levels over 125 times greater than earlier microbial setups. In contrast, traditional methods for investigating strigolactones required the extraction of a minimum of 340 liters of xylem sap — roughly equivalent to what you would get from 7 or 8 poplar trees. In reality, to account for extraction losses, this volume needs to approach 1000 liters, Li noted.
“Utilizing this microbial cell factory means you can avoid extracting large volumes of xylem sap and thus prevent ecological harm by destroying numerous trees just to discover important plant physiology molecules,” Li stated.
Engineering Approach
The discovery of the first strigolactone dates back to the 1960s, but it wasn’t until 2008 that the hormonal roles of this group were acknowledged. As hormones, strigolactones influence plant growth and their responses to stress factors like lack of water or nutrients. Since this foundational understanding in 2008, plant biologists have been eager to uncover the chemistry and functions of strigolactones and similar compounds. However, findings have been more theoretical than definitive, partly due to these hormones’ extremely low concentrations in plants.
So far, about 30 strigolactones have been identified, all traced back to a common ancestor. The transformation of this precursor into various strigolactones is driven by a specific protein-coding gene (CPY722C) found in most flowering plants. Since related genes, designated CYP722A and CYP722B, are prevalent among seed plants, Li and her team hypothesized that these genes might also synthesize strigolactones with vital biological functions.
To explore this, the researchers evaluated the effects of the sister genes within their previously established microbial cell factory, which was formed by co-culturing E. coli and Baker’s yeast. They expressed the CYP722A and CYP722B genes from 16 diverse plant species, including poplar, pepper, pea, and peach. Through further metabolic engineering, Anqi Zhou, a Ph.D. student in Li’s lab, discovered effective methods to boost strigolactone concentrations by over 125-fold.
This enhanced productivity provides enough material for the researchers to elucidate the structures of the resulting compounds, which could be pivotal to plant physiology.
One significant compound that emerged from the CYP722A or CYP722B pathways is a strigolactone called 16-hydroxy-carlactonic acid (16-OH-CLA).
Shoots, Not Roots
Although 16-OH-CLA had been identified before, its precise structure and significance were not thoroughly understood. The capability to produce ample quantities of 16-OH-CLA through the microbial cell factory enabled the team to determine its exact structure for the first time.
Interestingly, when searching for 16-OH-CLA in plants, the researchers found it only in the shoots and not in the roots, contrasting with all other documented strigolactones. Furthermore, the compound isn’t always present; for annual plants like pepper or common pea, it vanishes once the plant matures. Conversely, in trees like poplar, it appears only seasonally.
While the specific role of 16-OH-CLA remains unclear, its frequent detection in seed plants and its unusual localization suggests it may play a vital, yet underappreciated role in plant signaling or adaptation to environmental stressors. Thanks to the new engineering approach, researchers will have adequate supplies to pursue further investigation — exactly what Li and her team are currently working on.
*These authors contributed equally to this work.
This research was supported by the National Science Foundation (including CAREER Award CBET-2144626, IOS-1856741, and IOS-2329271, plus CAREER Award 2047396 and Research Traineeship Program Grant DGE-1922642 “Plants3D”), USDA-NIFA (AFRI Predoctoral Fellowship 2023-67011-40396), Japan Science and Technology Agency (FOREST, JPMJFR220F), and JSPS (KAKENHI, 21H02125).
