Adaptation of Medicago truncatula to nitrogen limitation is modulated via local and systemic nodule developmental responses

Adaptation of Medicago truncatula to local nitrogen (N) limitation was investigated to provide new insights into local and systemic N signaling. The split-root technique allowed a characterization of the local and systemic responses of NO₃⁻ or N₂-fed plants to localized N limitation. ¹⁵N and ¹³C labeling were used to monitor plant nutrition. Plants expressing pMtENOD11-GUS and the sunn-2 hypernodulating mutant were used to unravel mechanisms involved in these responses. Unlike NO₃⁻-fed plants, N₂-fixing plants lacked the ability to compensate rapidly for a localized N limitation by up-regulating the N₂-fixation activity of roots supplied elsewhere with N. However they displayed a long-term response via a growth stimulation of pre-existing nodules, and the generation of new nodules, likely through a decreased abortion rate of early nodulation events. Both these responses involve systemic signaling. The latter response is abolished in the sunn mutant, but the mutation does not prevent the first response. Local but also systemic regulatory mechanisms related to plant N status regulate de novo nodule development in Mt, and SUNN is required for this systemic regulation. By contrast, the stimulation of nodule growth triggered by systemic N signaling does not involve SUNN, indicating SUNN-independent signaling.