Nod factors potentiate auxin signaling for transcriptional regulation and lateral root formation in Medicago truncatula.

Nodulation (Nod) factors (NFs) are symbiotic molecules produced by rhizobia that are essential for establishment of the rhizobium-legume endosymbiosis. Purified NFs can stimulate lateral root formation (LRF) in Medicago truncatula, but little is known about the molecular mechanisms involved. Using a combination of reporter constructs, pharmacological and genetic approaches, we show that NFs act on early steps of LRF in M. truncatula, independently of the ethylene signaling pathway and of the cytokinin receptor MtCRE1, but in interaction with auxin. We conducted a whole-genome transcriptomic study upon NF and/or auxin treatments, using a lateral root inducible system adapted for M. truncatula. This revealed a large overlap between NF and auxin signaling and, more interestingly, synergistic interactions between these molecules. Three groups showing interaction effects were defined: group 1 contained more than 1500 genes responding specifically to the combinatorial treatment of NFs and auxin; group 2 comprised auxin-regulated genes whose expression was enhanced or antagonized by NFs; and in group 3 the expression of NF regulated genes was antagonized by auxin. Groups 1 and 2 were enriched in signaling and metabolic functions, which highlights important crosstalk between NF and auxin signaling for both developmental and symbiotic processes.

Abscisic acid promotes pre-emergence stages of lateral root development in Medicago truncatula.

The plant root system is important for plant anchorage and nutrition. Among the different characteristics of the root system, root branching is a major factor of plasticity and adaptation to changing environments. Indeed, many biotic and abiotic stresses, such as drought or symbiotic interactions, influence root branching. Many studies concerning root development and root branching were performed on the model plant Arabidopsis thaliana, but this model plant has a very simplified root structure and is not able to establish any symbiotic interactions. We have recently described 7 stages for lateral root development in the model legume Medicago truncatula and found significant differences in the tissular contribution of root cell layers to the formation of new lateral roots (LR). We have also described 2 transgenic lines expressing the DR5:GUS and DR5:VENUS-N7 reporter genes that are useful to follow LR formation at early developmental stages. Here, we describe the use of these transgenic lines to monitor LR developmental responses of M. truncatula to the phytohormone abscisic acid (ABA) which is a major actor of stress and symbiotic interactions. We show that ABA promotes the formation of new lateral root primordia and their development, mostly at the late, pre-emergence stage.