Fungus Fuels Tree Growth
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 U.S. Department of Energy’s (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 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 does not 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.
University of Florida