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Populus sp. is a developing feedstock for second-generation biofuel production. To ensure its success as a sustainable biofuel source, it is essential to capitalize on the ability of Populus sp. to associate with beneficial plant-associated microbes (e.g., mycorrhizal fungi) and possibly engineer Populus sp. to associate with non-native symbionts (e.g., rhizobia). We found that, in addition to rhizobia and arbuscular mycorrhizal fungi, many fungi produce lipo-chitooligosaccharides (LCOs) and that Populus sp. responds to these LCOs with strong root developmental responses, nuclear calcium spiking (a hallmark of symbiotic responses), and changes in gene expression. Some of the genes up-regulated by LCOs in Populus sp. are direct orthologs of legume Nodule INception (NIN), which is induced by LCOs in legumes and a central regulator of nodule organogenesis. One such ortholog (PtNIN2b), in particular, strongly enhances lateral root development and can even overcome the inhibition of lateral root development by cytokinins. We also showed that various species of rhizobia can induce nuclear calcium spiking in the roots of Populus sp. Given that Populus sp. already possesses the molecular machinery necessary for perceiving rhizobia, and connections between LCO signaling to NIN, Populus sp. seems a very promising model to engineer root nodule symbiosis in non-leguminous crops.

Complete list of authors:
Thomas B. Irving^1,2, Sanhita Chakraborty^1,2, Daniel Conde^3,4, Kevin R. Cope^1,2, Sara A. Knaack⁴, Lucas Gontijo Silva Maia^1,2, Henry W. Schmidt³, Sushmita Roy⁴, Matias Kirst^3,5, Jean-Michel Ané^1,2

¹Department of Bacteriology, University of Wisconsin, Madison, WI 53706, USA
²Department of Agronomy, University of Wisconsin, Madison, WI 53706, USA
³School of Forest, Fisheries and Geomatics Sciences, University of Florida, Gainesville, FL 32611, USA
⁴Genetics Institute, University of Florida, Gainesville, FL 32611, USA
⁵Department of Biostatistics and Medical Informatics, University of Wisconsin, Madison, WI 53715