The fungus Mortierella elongata enjoys a dual lifestyle. It can thrive in the soil as a saprophyte, an organism that lives off decaying organic matter. It can also live as an endophyte, an organism that lives inside a plant between root cells. The fungus is almost always found among and within poplar trees. In an effort to understand the fungus’s influence on the plant, a team of scientists studied what happens to the tree’s physical traits and gene expression when the fungus is present.
Black cottonwood, or poplar (Populus trichocarpa), is the fastest growing hardwood tree in the western United States. This characteristic makes it a promising feedstock for bioenergy. By better understanding how poplar responds to endophytes, scientists can improve their work on both plant and root microbiomes to grow energy crops more efficiently.
To interrogate the close partnership of endophyte M. elongata and poplar, a team collected forest samples of poplar and soil from Washington and Oregon. The cuttings included genotypes from the DOE BioEnergy Science Center (BESC), predecessor of DOE’s Center for Bioenergy Innovation (CBI) at Oak Ridge National Laboratory. To see how the fungus affected poplar growth, the team compared poplar cuttings grown with and without an inoculation of the M. elongata strain PM193 that was added to a diluted soil mixture. The results were striking. Adding PM193 caused poplar cuttings to grow about 30 percent larger by dry weight than without PM193. By contrast, a different endophytic fungus, Ilyonectria europaea, had no effect on growth. The team partnered with the U.S. Department of Energy (DOE) Joint Genome Institute (JGI), a DOE Office of Science user facility, through its Community Science Program to sequence and annotate the M. elongata and I. europaea genomes for this study. The team found that, unlike pathogenic or mycorrhizal fungi (mutualist symbionts that induce structural changes in plant roots), M. elongata doesn’t have as many gene products that directly influence plant phenotype, such as secreted proteins. However, M. elongata seems to encourage the plant to have leakier cell walls and weaker defenses in general; the fungus decreased the expression of poplar genes associated with plant defense (e.g. jasmonic acid and salicylic acid). The team also observed that the plants instead put more energy into growth, noting an increased expression of genes involved in signaling of gibberellin, one of the best-known plant growth hormones. One other tidbit that caught the researchers’ attention is that the poplar cuttings had increased expression of lipid signaling genes when they were inoculated with M. elongata. Poplar might be detecting lipids from M. elongata; the fungus produces them so prolifically it oozes. The team hypothesizes that lipids could act as a bridge of interkingdom communication between the plant and fungus. Discovering how microbes can influence plant physiology helps scientists better understand how to optimize characteristics like growth rate. Harnessing that power could help usher widespread use of biofuel as a replacement to fossil fuel.
Contacts (BER PM)
Hui-Ling Liao, Ph.D.
University of Florida
This work was supported by the Plant-Microbe Interfaces program at the Oak Ridge National Laboratory (ORNL) sponsored by the Office of Biological and Environmental Research at the United States Department of Energy (DOE) Office of Science. ORNL is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the DOE. The work conducted by the DOE Joint Genome Institute, a DOE Office of Science user facility, is supported by the Office of Science of the United States Department of Energy under contract DE-AC02-05CH11231. Support was also provided by the NSF Zygolife (NSF-DEB1441715), the United States National Science Foundation (NSF) (DEB 1737898 to G. Bonito), the Laboratory of Excellence Advanced Research on the Biology of Tree and Forest Ecosystems (ARBRE) (ANR-11-LABX 0002 01 to F. Martin), JGI Community sequencing program project (CSP570, DE-AC02-05CH11231), and National Science Foundation (concept ID 10.13039/100000001; grant number DEB 1737898).
Liao H et al. “Fungal Endophytes of Populus trichocarpa Alter Host Phenotype, Gene Expression, and Rhizobiome Composition.” Molecular Plant-Microbe Interactions. 32, 853 (2019) July. [DOI: 1094/MPMI-05-18-0133-R]
- JGI Community Science Program
- JGI News Release on Poplar Sequencing Effort: The Book Opens on the First Tree Genome
- JGI News Release on Poplar Genome: The First Tree Genome is Published
- JGI News Release: The Poplar Genome at 10 Populus trichocarpa genome on JGI Plant Portal Phytozome Mycocosm, the JGI fungus genome portal, where Mortierella elongata and Ilyonectria europaea genomes can be found: here and here, respectively