The rhizobium symbiosis in Medicago truncatula induces a priming-like response and enhances drought tolerance

• The M. truncatula -rhizobial symbiotic plant continues to fix nitrogen under drought • Rhizobial symbiosis primes the M. truncatula host and confers drought tolerance • Droughted N-fertilised plants reduce N uptake and induce leaf senescence and growth cessation • Despite the contrasting survival mechanisms, both plants achieve similar yields after long-term drought conditions Legumes can fix nitrogen (N) by symbiotic N-fixation(SNF) through association with rhizobia. However, this comes at a higher carbon cost to the host as compared to direct N fertiliser uptake. We therefore hypothesised that the energy-intensive symbiosis process imposes additional growth and yield penalties to the SNF-dependent host under drought. Medicago truncatula seedlings were either rhizobium-inoculated or N-fertilised, drought stress was applied 20 days post inoculation by withholding water for 12 days, and physiological and transcriptomic responses were measured. Physiological analysis revealed that drought similarly reduced leaf water status in both rhizobium-inoculated and N-fertilised plants. However, only rhizobium-inoculated plants sustained N-acquisition and growth. Transcriptomic analysis revealed that these plants pre-activated jasmonic acid, proline, and trehalose biosynthesis before the onset of drought stress, and induced stress-tolerance hormone pathways in response to the drought. In contrast, N-fertilised plants acquired less N, upregulated ethylene biosynthesis and senescence, and induced strong death responses. Remarkably, despite contrasting molecular strategies, both plants achieved similar dry matter and yield when drought conditions continued throughout the plants’ lifecycle. Contrary to our hypothesis, the results suggested that rhizobium-inoculated plants activated a tolerance mechanism through a priming-like response and sustained N-fixation under drought stress, whereas N-fertilised plants induced a contrasting survival strategy based on growth cessation and leaf senescence. This study advances our understanding of the distinct survival mechanisms of SNF-dependent and N-fertilised legumes under drought, and provides critical insights for advancing sustainable and climate-resilient crops by harnessing rhizobial symbiosis benefits.