Increased Soybean Nitrogen Fixation: A Potential Game-Changer?

In the agricultural science field, as in all scientific fields, new discoveries are rather rare and oftentimes offer promise that in fact may not be realized. However, most scientific discoveries are exciting in the beginning, and their potential impact to their particular discipline can be surmised.

A recent discovery regarding nitrogen (N) fixation by soybean has been reported in an article entitled “Increasing Nitrogen Fixation and Seed Development in Soybean Requires Complex Adjustments of Nodule Nitrogen Metabolism and Partitioning Processes” that appears in Current Biology, Vol. 26, p. 2044-2051, 2016. The authors are Amanda Carter and Mechthild Tegeder of Washington State University.

Below are some points to remember when assessing the results from the study.

•    In the legume–rhizobia symbiotic relationship, bacteroids (rhizobia) in the root nodules “fix” atmospheric N₂. After a series of chemical transformations, this N is mobilized and used by the plant to supply its nitrogen needs in developing tissue and seeds. Seed yield in nodulated legume plants is linked to this bacterial N₂ fixation.

•    A source-sink relationship in a plant involves the movement of various substances from their region of supply (the source; e.g. nodules) to their region of use or storage (the sink; e.g. soybean seed). Sink capacity is the capacity of a plant’s tissues or organs to import and store compounds from the source.

Below is a summary of the results from the study.

•    Two transgenic soybean lines–UPS1-OE1 and UPS1-OE2 (OE stands for over-expressor)–were produced, and nodulated OE plants were used in a greenhouse study.

•    In the study, the authors increased the expression of importers necessary to enhance N flow out of the nodules, and measured the consequence on N nutrition in the shoot and on sink development and nodule function.

•    To determine the effects of this improved N availability on the plant, they analyzed the development of reproductive sink organs; i.e. number of pods and seeds per pod, and seed yield.

•    Number of pods was increased and seeds per pod were increased, which resulted in an increase in total number of seeds and an overall increase in seed yield from OE plants. Weight of individual seed was not changed. Thus, they concluded that this pointed to increased N acquisition from the atmosphere, followed by increased N delivery to the shoot, which then increased the development of reproductive sinks in the OE soybean plants.

Interestingly, previous research conducted for 3 years in the field at Stoneville, Miss. provided results [see Crop Sci. 46:52-60 (2016)] showing that using fertilizer N to replace fixed N in soybean resulted in a significant increase in seed yield, and this increase was largely attributable to an increase in the number of seed. Also, the results led to the conclusion that N₂ deficiencies that limit soybean yield occur before the beginning of the processes that determine the number of seed. These earlier findings regarding how increased N supply to the soybean plant affects the components that affect yield support the above results.

•    Nodule numbers were greatly increased in OE plants, which led to an increase in nodule biomass. The N₂ fixation rate per nodule increased in OE plants. Thus, the increase in N₂ fixation resulted from both nodule development and activity.

There is anecdotal evidence that the number and mass of nodules per se has little to do with N₂ fixation activity. Thus, the above research results may in fact be the result of increased efficiency of the N₂ fixation/mobilization process in soybean.

•    The authors concluded that enhancing N export from soybean nodules leads to increased N₂ fixation, nodule metabolism, and shoot N nutrition in the OE plants, and promotes increased seed development which resulted in increased seed yield.

As stated above, these results are from a greenhouse study. Further research will need to focus on: 1) imparting this OE characteristic to proven agronomic soybean types/varieties; 2) determining if these results can be duplicated with these OE varieties grown in the field; and 3) determining whether or not this potential increase in N fixation/availability/metabolism can be supported by the energy resources available from the growing plant since the increased nodule activity would become a larger sink for the photosynthetically-derived energy produced by the plant.

I am not a plant physiologist or biochemist, but I find this discovery very exciting, and a potential game-changer if it can be transferred to the soybean genome in general, and then can be translated into increased yield of newly developed varieties. However, this is the type of work that will require long-term support and resources that may only be available from sources outside public institutions. Hopefully, future research based on the above finding can be achieved with such forthcoming resources.

Composed by Larry G. Heatherly, Oct. 2016, larryheatherly@bellsouth.net. Thanks to Dr. Tom Sinclair, North Carolina State Univ., and Dr. Jeff Ray, USDA-ARS, Stoneville, MS for their input for this article.