NSP1 and NLP9 mediate a beneficial, non-canonical interaction between Arabidopsis thaliana and Sinorhizobium meliloti under nitrogen deficiency

Nitrogen is as a crucial macronutrient necessary for plant development. Legumes form well-known symbiotic relationships with nitrogen-fixing bacteria, but non-leguminous plants such as Arabidopsis thaliana also gain advantages from these associations without developing nodules. This study examines the relationship between A. thaliana and Sinorhizobium meliloti when conditions contain extremely low nitrogen levels. Our results provide functional evidence consistent with biological nitrogen fixation from S. meliloti, associated with enhanced plant growth and root system development. The plant growth response needs two essential regulatory genes, NSP1 and NLP9, which become active exclusively in nitrogen-deficient conditions. Microscopy showed bacterial colonization on the root epidermis, and subsequent analysis identified NSP1 and NLP9 as mediators of plant signaling, which modulate the host program to allow the nitrogenase activity of S. meliloti. NSP1 controls the induction of NLP9, indicating a conserved signaling pathway resembling that found in legumes. The study discovered a non-canonical interaction beyond nodules that regulates bacterial nitrogen fixation functionality and improves A. thaliana survival during nutrient scarcity. The research expands our comprehension of how plants interact with nitrogen-fixing bacteria and indicates conserved molecular systems that allow non-leguminous plants to form advantageous relationships under severe nitrogen scarcity.