For Plants, Endophytes Promote Phosphorus Uptake

Remember that feeling as a kid, standing on the kitchen floor staring at the cookie jar placed just out of reach? Now imagine you’re a plant, but that cookie jar is the life-giving phosphorus you need, bound up in metal complexes—right there, but so far away.

New findings recently published in Frontiers in Plant Science have unlocked the critical role that endophytes—bacteria, as was used in this study, or fungi that live inside a plant for at least some of their life cycle—play in solubilizing phosphorus and making it accessible to plants. Born of a collaboration between researchers from the Environmental Molecular Sciences Laboratory (EMSL), the University of Washington (UW), Pacific Northwest National Laboratory, Lawrence Berkeley National Laboratory, and Argonne National Laboratory, the work provides new direct, molecular-level evidence of the symbiotic relationship between plants and endophytes. The work also suggests that this may occur not just outside the plant, but internally as well.

This groundbreaking work began materializing in 2018, when EMSL’s Tamas Varga, a materials scientist and expert in synchrotron X-ray techniques, was awarded funding to study plant mechanisms using EMSL’s capabilities. The funding came with one condition—contribute to an ongoing study.

Thus began a key collaboration between Varga and UW’s Sharon Doty, a plant microbiome researcher. Doty’s laboratory primarily studies the microbiome in pioneering plant species, such as the poplar and willow, that are able to colonize “young” and nutrient-poor substrates, especially in Washington State river systems.

Symbiosis is vital for plants in nutrient-poor environments

Both poplar and willow have long been known to thrive in environments that lack nutrients and contain little organic matter. Scientists previously thought that plants growing in these conditions were simply super-efficient at taking up and processing nutrients. However, Doty’s work demonstrated that their hardiness is largely due to their dependence on microbes.

Some of these microbes can solubilize phosphate, which is usually found in soils as a complex with metals like iron, aluminum, or calcium. Strategic plants will therefore associate with these microbes to better acquire phosphorus. While the ancient symbiosis between plants and mycorrhizal fungi has long been understood, molecular evidence that endophytes serve a similar role has been lacking.

Endophytes—small but mighty

Although scientists already widely agreed that endophytes help plants absorb nutrients, Doty and Varga wanted to illuminate on a molecular level microbes’ ability to solubilize phosphates.

To accomplish this, Doty identified two different endophyte strains—known as WP5 and WP42—that exhibited a strong ability to solubilize tricalcium phosphate. Host plant samples were then inoculated with these strains and compared with uninoculated controls.

Using the advanced technologies available at EMSL, including X-ray microtomography proteomics, as well as those at other U.S. Department of Energy (DOE) national laboratories, such as X-ray fluorescence microscopy and X-ray absorption near edge structure spectroscopy, Varga and Doty observed clear evidence that the endophyte strains were solubilizing phosphorus, which ended up inside the plant. While previous work in Doty’s laboratory demonstrated that endophytes can fixate nitrogen from the air, such direct evidence of phosphate solubilization was previously unavailable.

The study also turned up an intriguing side point—Varga’s X-ray analyses showed the presence of tricalcium phosphate inside the plants. This indicated that although phosphorus had been made accessible outside the plant before uptake, it was somehow recomplexed inside the tissues, once again making the essential nutrient inaccessible. Even more interesting was that the endophytes may have played a role in resolubilizing the phosphates within the plant tissues. Such a phenomenon is “an extra finding… and has implications that would warrant further research,” said Varga.

Broad applications yield cross-discipline benefits

This new knowledge can be applied across a range of areas, from fundamental science to industry. One especially promising use of endophytes is making agriculture more environmentally friendly.

Current agricultural practices use fertilizers that provide readily available phosphorus and nitrogen directly into the soil. Any excess nutrients not absorbed by crops are flushed into the environment, which has a negative effect on nutrient cycling, water quality, and ecological health. Reducing the use of fertilizers and employing endophytes—such as those studied by Doty and Varga—opens the door for a more sustainable food future.

UW has already licensed the endophyte strains used in this study to IntrinsyxBio, a California-based company working to commercialize a collection of endophyte microbes. The direct evidence provided by Doty and Varga’s research of endophyte-promoted phosphorus uptake is “game-changing for our research on crops,” said John Freeman, Chief Science Officer of IntrinsyxBio. “The finding that the solubilized phosphate may react and become insoluble once inside the plant tissue and that endophytes may aid in the re-release of phosphate is critical to our understanding of nutrient uptake in plants,” Freeman added.

A multi-institution effort

Close collaboration between experts at multiple DOE user facilities was integral to the success of this study. X-ray-based tomography and proteomics characterization of the plants were performed at EMSL. Researchers at Lawrence Berkeley National Laboratory’s Advanced Light Source provided key micro X-ray fluorescence, X-ray absorption near edge structure, and synchrotron X-ray computed tomography instruments. Further micro X-ray fluorescence mapping was enabled by collaboration with Argonne National Laboratory’s Advanced Photon Source.

Acknowledgments

The DOE Office of Science, Biological and Environmental Research program sponsored the research performed at EMSL, which took place under EMSL’s Strategic Science Area intramural program. The DOE Office of Science, Basic Energy Sciences program supports the operation of the Advanced Light Source and the Advanced Photon Source. Key funding for Sharon Doty’s research comes from the Byron and Alice Lockwood Foundation, providing Doty’s laboratory the freedom to be curious and actively pursue new research directions.