RHIZOBIAL GENES THAT CONTRIBUTE TO EFFECTIVE SYMBIOSIS WITH LEGUMES

Katie Adolphsen

Rhizobia are soil bacteria that can form a symbiotic relationship with legumes in which the rhizobia fix atmospheric dinitrogen and contribute the resulting ammonia to plant nutrition. One laboratory model for studying symbiotic nitrogen fixation is a partially effective interaction between Sinorhizobium meliloti and Medicago truncatula in which the M. truncatula still shows signs of nitrogen deficiency. A closely related bacterium, Sinorhizobium medicae, forms a more efficient symbiotic relationship with M. truncatula. Using proteomic data from free-living and symbiotic S. medicae, we identified a subset of 83 proteins that did not share high homology with S. meliloti proteins. We speculated that some of the genes encoding these proteins might contribute to the more effective symbiotic relationship between S. medicae and M. truncatula and chose four candidates based on their annotations and/or relative abundance. Heterologous expression of three of the four candidate genes (Smed_6456, Smed_5985, and Smed_3503) in S. meliloti resulted in improved symbiotic efficiency with M. truncatula. S. meliloti expressing Smed_3503 (iseA) formed a symbiosis that showed few signs of nitrogen deficiency and nodulation was increased. The role of iseA in symbiosis is currently unknown although when this gene is deleted from S. medicae, nodulation was significantly decreased. Constitutive expression of iseA also increased nodulation of S. medicae with M. truncatula. Introduction of iseA into Rhizobium leguminosarum increased nodulation by 50% in pea plants and also increased nodulation on lentils, indicating the iseA may have broader applications in understanding the interaction of host-symbiont regulation of nodulation. Split root experiments in pea as well as in M. truncatula indicate that iseA can interfere with or overcome plant induced resistance to rhizobial infection. RNA-seq experiments in S. meliloti revealed significant upregulation of 42 genes with 2 genes upregulated over 100-fold (SMa0089 and SMa0091) and a large, 300-gene region downregulated on pSymB. Deleting SMa0089 and SMa0091 abolishes the ability of iseA to increase S. meliloti Rm1021 nodulation of M. truncatula. Overexpression of SMa0089 increases nodulation of Rm1021 with M. truncatula but not to the same extent as iseA alone. iseA is annotated as a glyoxalase/dioxygenase/ bleomycin resistance gene and may be involved in aromatic carbon metabolism.

RHIZOBIAL GENES THAT CONTRIBUTE TO EFFECTIVE SYMBIOSIS WITH LEGUMES

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