As sessile organisms, plants are directly challenged by fluctuating environments that usually lead to limited growth. As all living organisms, plants require for growth the basic chemical elements on top of which appear: carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). Plants are capable of acquiring C, H and O elements via photosynthesis while N availability is one of the crucial factors limiting plant yield in agriculture. Consequently, external supply of N fertilizers has been key to improve plant yield in extensive farming systems. However, two main costs arise from this practice: (i) the significant and high energetic demand of the industrial synthesis of N fertilizers, and (ii) the environmental costs of releasing N compounds into the environment resulting in e.g. water pollution, algal bloom, and eutrophication.
To reduce the impact of agricultural fertilization on ecosystems, naturally occurring processes such as biological nitrogen fixation could be viewed as a promising and sustainable strategy for plant N supply. Indeed, a limited portion of flowering plants can symbiotically interact with nitrogen-fixing soil bacteria collectively known as rhizobia. This interaction takes place in specialized, relatively recently evolved and facultative plant organs termed as “symbiotic nodules”. In these symbiotic organs, a bacterial enzyme, the nitrogenase, is converting gaseous dinitrogen (N2) into ammonium that is provided to the host plant. As N2 accounts for approximately 78% of atmospheric gases and is recycled, this form of nitrogen represents a huge and potentially unlimited pool available to plants forming a nitrogen-fixing symbiosis. In particular, this group of plants encompasses the agronomical relevant Legume family (e.g. garden pea, common bean, soybean, chickpea, alfalfa and lentil). Soybean (Glycine max [L.] Merrill) is the most extensively cultivated legume worldwide and accounts for 8.77% of world total harvested area in 2016, ranking it fourth after wheat, maize and rice (fao.org).
Understanding the molecular mechanisms controlling the robustness of nodule development and functioning in soybean is crucial to improve symbiotic efficiency and to alleviate environmental costs of chemical N fertilisation. Exploiting the potential of several documented examples of nodule-to-root conversion, we propose to identify plant and bacterial factors required for an efficient symbiosis and nodule maintenance. It has been shown that both the bacterial general stress response (GSR) system and the plant NBCL gene products are required for nodule maintenance. However, this has been done in two different biological systems, namely the G. max - Bradyrhizobium diazoefficiens and Medicago truncatula - Sinorhizobium meliloti models. Using the B. diazoefficiens - soybean system, we propose to decipher (i) the symbiotic roles of the three NBCL proteins in soybean, (ii) which cells are involved in nodule to root conversion in soybean nodules induced by B. diazoefficiens mutants impaired in the GSR, and (iii) the molecular determinants and mechanisms which respond to the lack of the bacterial GSR-dependant signals and subsequently lead to nodule-to-root conversion.
