Are legumes more than the sum of their nodules?
marie schaedel
PhD Candidate, University of Minnesota
Production-oriented research has reduced the diversity of legumes on agricultural landscapes and narrowed our criteria for evaluating ‘successful’ legume-microbe interactions. Metrics like total nodule number, nodule dry weight, and biomass nitrogen (N) have become prolific and are often focused on just one service: the provision of fixed atmospheric N. Legumes perform biological nitrogen fixation (BNF) with symbiotic rhizobia bacteria, which is inarguably a defining feature of legume-microbe interactions. However, the end goal of most legume research has been to increase biomass production and total fixed N in pounds per acre. This narrow focus has limited the number of cultivated legume species and varieties, the diversity of nodule endosymbionts, and the ecosystem benefits we can derive from legumes beyond BNF.
The study of legume microbiomes is a small but rapidly growing field. We reviewed articles published within the past fifteen years that investigated legume-microbe interactions, with a specific focus on associations with non-rhizobia bacteria. We found strong evidence suggesting that legume root zones enhance microbial abundance and diversity compared to non-legume plants such as grasses. Many of these interactions are associated with other benefits such as disease suppression and abiotic stress adaptation. There are several possibilities to explain why legumes are so efficient at recruiting microorganisms, including unique flavonoid profiles, N rhizodeposition, and organic acid exudates. Legumes differ from non-legumes in nearly all these features, yet scientists have not yet determined their role in recruiting beneficial rhizobacteria.
Recent research has also rejected the long-held belief that BNF involves only two symbiotic partners: the legume host and a compatible rhizobia strain. We now know that other bacteria, known as non-rhizobia endophytes, co-inhabit legume root nodules alongside rhizobia. Studies have found that unmanaged, wild legumes have a higher diversity of non-rhizobia endophytes than domesticated legumes. Because nodulation is an energetically costly symbiosis for the host plant, the prevalence of non-rhizobia in root nodules suggests that they provide another important service to ‘earn their keep’. Many, but not all, of these non-rhizobia have a genetic capacity to fix nitrogen. Other non-rhizobia endophytes such as Proteus have putative roles in fighting bacterial and fungal pathogens. Still others can solubilize phosphate, a crucial nutrient required for BNF, on solid media. Although scientists now recognize the diversity and prevalence of non-rhizobia nodule inhabitants, understanding their functional role in planta remains a major research priority.
Research advances in legumes have lagged far behind cereal and grain crops for the past fifty years. At a time when there is mounting pressure to stall anthropogenic climate change through more sustainable land use, legumes have a central role to play in efforts to conserve soil and reduce agricultural inputs. The legume microbiome is a largely untapped research frontier that will allow us to take full advantage of all the potential agronomic and ecosystem services that legumes have to offer.
Publication: Schaedel, M., Hidrobo, G., Grossman, J. (2021). From microns to meters: exploring advances in legume microbiome diversity for agroecosystem benefits. Front. Sustain. Food Syst. 5:668195. DOI: https://doi.org/10.3389/fsufs.2021.668195