MiR 171 h restricts root symbioses and shows, like its target NSP 2 , a complex transcriptional regulation in Medicago truncatula

Legumes like Medicago truncatula have the unique ability to undergo both, root nodule and arbuscular mycorrhizal symbiosis (AMS). The GRAS transcription factor NSP2 is a key component of the partially shared signaling pathway and is crucial for a successful formation of both types of root endosymbiosis. The miRNA171h has been identified to target NSP2 and it is therefore hypothesized that this Micro RNA (miRNA) is involved in regulating the establishment of arbuscular and root nodule symbiosis (RNS). This chapter highlights that miR171h is functionally expressed from an unusual long primary transcript and that mature miR171h and NSP2 transcript levels show a clear anticorrelation in a variety of tested conditions except for mycorrhizal roots. During arbuscular mycorrhizal symbiosis NSP2 transcript levels are induced despite the fact that increased miR171h abundance can be found. This is supported by a strong correlation between transcript levels of NSP2 and MtPt4, a phosphate transporter specifically expressed in arbuscule containing cells and an indicator for a functional symbiosis. Furthermore, this chapter will provide data that miR171h and NSP2 expression are nutrient responsive and both genes undergo a complex regulation involving the symbiotic status, phosphorus, and nitrogen nutrition. Both genes show a mutually exclusive expression pattern in mycorrhizal roots and a spatial restriction in root nodules. Overexpression of miR171h in M. truncatula roots led to a reduction in fungal colonization and to a reduced nodulation by Sinorhizobium meliloti. Taken together this chapter will show that miR171h has an important function to integrate the nutrient homeostasis in order to safeguard the expression domain of NSP2 during both arbuscular mycorrhizal and root nodule symbiosis.