International Journal of Molecular Sciences Review The Regulation of Nodule Number in Legumes Is a Balance of Three Signal Transduction Pathways Diptee Chaulagain and Julia Frugoli *   Citation: Chaulagain, D.; Frugoli, J. The Regulation of Nodule Number in Legumes Is a Balance of Three Signal Transduction Pathways. Int. J. Mol. Sci. 2021, 22, 1117. https://doi.org/ 10.3390/ijms22031117 Academic Editor: Jen-Tsung Chen Received: 31 December 2020 Accepted: 21 January 2021 Published: 23 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Genetics & Biochemistry, Clemson University, Clemson, SC 29634, USA; dchaula@g.clemson.edu * Correspondence: jfrugol@clemson.edu; Tel.: +1-864-656-1859 Abstract: Nitrogen is a major determinant of plant growth and productivity and the ability of legumes to form a symbiotic relationship with nitrogen-fixing rhizobia bacteria allows legumes to exploit nitrogen-poor niches in the biosphere. But hosting nitrogen-fixing bacteria comes with a metabolic cost, and the process requires regulation. The symbiosis is regulated through three signal transduction pathways: in response to available nitrogen, at the initiation of contact between the organisms, and during the development of the nodules that will host the rhizobia. Here we provide an overview of our knowledge of how the three signaling pathways operate in space and time, and what we know about the cross-talk between symbiotic signaling for nodule initiation and organogenesis, nitrate dependent signaling, and autoregulation of nodulation. Identification of common components and points of intersection suggest directions for research on the fine-tuning of the plant’s response to rhizobia. Keywords: autoregulation of nodulation; nodulation; nitrogen response in nodulation; Medicago truncatula 1. Introduction Nitrogen (N) is a major determinant of plant growth and productivity. In addition, N is required as a constituent of nitric oxide (NO) and polyamines that influence constitutive and induced plant defense [1]. While N is the most abundant gas in the atmosphere, it is unusable as a direct source of plant nutrients because of the inability of plants and most organisms to enzymatically break the triple bond of N 2 and convert it into the main forms that plant roots can take up: NO 3 - and NH 4 + . Thus, N as a plant nutrient must be obtained from decomposition products in the soil or added to soil in plant-absorbable forms. The largest natural source of N input to the biosphere is biological nitrogen fixation, adding approximately 50–70 Tg of N globally to agricultural systems [2]. Biological nitrogen fixation is the conversion of N 2 to NH 3 catalyzed by nitrogenase enzyme in diazotrophs. These diazotrophs are both free-living and in symbiotic associations between plants and nitrogen-fixing bacteria (legume-rhizobia, Azolla-cyanobacteria, nonlegume-Frankia). A smaller amount of N input to the biosphere is contributed by nitrates in the rainwater and by organic nitrogen through manure. Non-legume plants take up on average 20–50 g of N per1 Kg of dry biomass produced [3]. In contrast, soybean, a widely cultivated legume for human consumption and animal feed, grown in unfertilized soil contains 55–70% of fixed nitrogen in its aboveground parts during the nodulation period [4]. Thus, symbiotic nitrogen fixation (SNF) is of intense interest as an alternative to chemical fertilizer. Because only a small proportion of commercial legume crop production relies on biological nitrogen fixation, a better understanding of the legume-rhizobia symbiosis could enable the efficient use of the natural process of SNF and reduce dependence on chemical N fertilizer. SNF is the result of a mutualistic interaction between a compatible plant and dia- zotrophs in which the plant provides a niche and fixed carbon to bacteria in exchange for fixed nitrogen. The plant family Fabaceae (Legumes) is the third-largest family of flowering plants consisting of ~19,000 known species, 88% of which form nitrogen-fixing Int. J. Mol. Sci. 2021, 22, 1117. https://doi.org/10.3390/ijms22031117 https://www.mdpi.com/journal/ijms