Transcriptional Networks Regulating Legume Root Architecture Adaptation to Environmental Conditions

Root developmental plasticity is a major agricultural trait controlling plant adaptation to environmental constraints, such as nutrient deficiency or abiotic stresses. In legumes, root system architecture is determined by the formation of two different organs, lateral roots and symbiotic nitrogen-fixing nodules. Comparative transcriptomic analyses using a genotype of the model legume Medicago truncatula sampled from salty soils, TN1.11, and the A17 reference line, identified several differentially salt-regulated TFs. In addition, certain TFs are targeted by microRNAs identified using deep sequencing of small RNA libraries as differentially regulated in response to a salt stress and/or between root and nodules. We selected several differentially salt-regulated TFs for functional analyses. This includes an HD-Zip TF, MtHB1, controlling lateral root emergence in response to salt likely through the direct modulation of an auxin-dependent LOB-domain TF. In addition, a NAC-type TF, MtNAC969, shows contrasting regulation by salt between roots and nodules. This TF was linked both to root salt stress response and to symbiotic nodule senescence, indicating a dual implication of this NAC-dependent pathway in environmental regulations of roots and nodules. Overall, these studies pinpointed transcriptional and post-transcriptional regulatory networks that are critical for legume root system adaptation to environmental changes.