Local and systemic N signaling are involved in Medicago truncatula preference for the most efficient Sinorhizobium symbiotic partners

Abstract

Summary• Responses of the Medicago truncatula-Sinorhizobium interaction to variation in N 2 -fixation of the bacterial partner were investigated.• Split-root systems were used to discriminate between local responses, at the site of interaction with bacteria, and systemic responses related to the whole plant N status.• The lack of N acquisition by a half-root system nodulated with a nonfixing rhizobium triggers a compensatory response enabling the other half-root system nodulated with N 2 -fixing partners to compensate the local N limitation. This response is mediated by a stimulation of nodule development (number and size) and involves a systemic signaling mechanism related to the plant N demand. In roots co-infected with poorly and highly efficient strains, partner choice for nodule formation was not modulated by the plant N status. However, the plant N demand induced preferential expansion of nodules formed with the most efficient partners when the symbiotic organs were functional. The response of nodule expansion was associated with the stimulation of symbiotic plant cell multiplication and of bacteroid differentiation.• A general model where local and systemic N signaling mechanisms modulate interactions between Medicago truncatula and its Sinorhizobium partners is proposed.

Rhizobia Isolated from Coal Mining Areas in the Nodulation and Growth of Leguminous Trees

“…Working with the model plant Medicago truncatula, Laguerre et al (2012) observed that even when inoculating mutant Nod+ Fix-strains (capable of stimulating nodulation, but incapable of fixing N), the ability to nodulate the legume species was not affected. Nevertheless, plants showed what could be called "compensation", which led to an increase in the size of nodules for strains capable of fixing N (Fix+).…”

Evolutionary Dynamics of Nitrogen Fixation in the Legume–Rhizobia Symbiosis

“…However, double inoculation experiments have shown that nitrogen-fixing strains proliferate with equal or superior efficacy to ineffective strains colonizing the same plant seedling [4], [11], [15], [33], [47]–[50]. This experimental observation may be attributed to various synergistic effects, but ultimately suggests that the cost function is not a simple decreasing function of nitrogen fixation.…”

Nitrogen modulation of legume root architecture signaling pathways involves phytohormones and small regulatory molecules

“…After Q perception, the shoots produce a shoot-derived inhibitor (SDI) that suppresses nodulation in other parts of the roots ( Figure 1 ; Caetano-Anolles and Gresshoff, 1991). Systemic control is also observed through selective discrimination of rhizobia based on N-fixation and nodulation efficiency (Sargent et al, 1987; Laguerre et al, 2012). Local high nitrate strongly inhibits nodulation locally and initiates a systemic response.…”