Fungal Protein Allows Beneficial Colonization in Populus

Disease-causing microbes use proteins to alter host JA signaling to aid their growth in plant tissues.

The Science

Ectomycorrhizal fungi, such as Laccaria bicolor, support forest growth and sustainability by providing growth-limiting nutrients to their plant host through a mutualistic symbiotic relationship with host roots. Researchers have previously shown that the effector protein MiSSP7 (Mycorrhiza-induced Small Secreted Protein 7) encoded by L. bicolor is necessary for the establishment of symbiosis with host trees. Researchers demonstrate here that MiSSP7 interacts with the host protein PtJAZ6, a negative regulator of jasmonic acid (JA)-induced gene regulation in Populus. As with other characterized JAZ proteins, PtJAZ6 interacts with PtCOI1 in the presence of the JA mimic coronatine, and PtJAZ6 is degraded in plant tissues after JA treatment. The association between MiSSP7 and PtJAZ6 is able to protect PtJAZ6 from this JA-induced degradation. Furthermore, MiSSP7 is able to block—or mitigate—the impact of JA on L. bicolor colonization of host roots. Researchers show that the loss of MiSSP7 production by L. bicolor can be complemented by transgenically varying the transcription of PtJAZ6 or through inhibition of JA-induced gene regulation. This study concludes that L. bicolor, in contrast to arbuscular mycorrhizal fungi and biotrophic pathogens, promotes mutualism by blocking JA action through the interaction of MiSSP7 with PtJAZ6.


The soil environment surrounding plant roots is filled with bacteria and fungi, both harmful and beneficial, many of which attempt to colonize root tissues to gain access to and use plant nutrients. In response, plant hormones such as jasmonic acid (JA) mediate the plant’s defense signaling system. By altering this pathway, some microorganisms can gain entry into the plant root cells and promote colonization. Investigating the symbiotic relationship between the bioenergy feedstock tree Populus trichocarpa and the beneficial fungus Laccaria bicolor, researchers at Oak Ridge National Laboratory found that a fungal protein essential for root establishment (called MiSSP7; Mycorrhiza-induced Small Secreted Protein 7) interacts with a plant-produced protein within the host plant nuclei to promote symbiosis. While both pathogenic and mutualistic fungi use fungal “effector” proteins to facilitate colonization, the results suggest how the mechanisms used to overcome the plant’s defenses differ between these two types of organisms, furthering understanding of how L. bicolor alters the plant’s response to JA and allows formation of symbiotic relationships.

BER Program Manager

Kari Perez

U.S. Department of Energy, Biological and Environmental Research (SC-33)
Biological Systems Science Division
[email protected]


Plett, J. M., Y. Daguerre, S. Wittulsky, A. Vayssières, A. Deveau, S. J. Melton, A. Kohler, J. L. Morrell-Falvey, A. Brun, C. Veneault-Fourrey, and F. Martin. 2014. “Effector MiSSP7 of the Mutualistic Fungus Laccaria bicolor Stabilizes the Populus JAZ6 Protein and Represses Jasmonic Acid (JA) Responsive Genes,” Proceedings of the National Academy of Sciences (USA) 111(22), 8299-304. DOI:10.1073/pnas.1322671111.