AM fungi–derived LCO/CO and rhizobia-derived LCO perception. Under low Pi and low nitrogen conditions, AM symbiosis takes place, likely through the action of the transcription factor (TF) PHR, and the chromatin is remodeled, which promotes the expression of nodule symbiosis pathway 2 (NSP2) and other transcriptional regulators (represented as ‘etc’). NSP2 functions with NSP1 to promote the expression of genes involved in the biosynthesis of strigolactones. Then, strigolactones are excreted to the rhizosphere to stimulate the germination of the AM fungal spores and the production of AM fungi–derived LCOs and COs . Downstream signaling allows the expression of LCO receptors, including RLK10 [a homolog of the Nod Factor Receptor 5 (NFR5) in legumes] and RLK2 allowing LCO/CO perception and activation of the common symbiosis pathway, triggering calcium oscillations and symbiotic gene expression such as RAM. Under low nitrogen and optimal Pi, root nodule symbiosis takes place. Low nitrogen triggers the exudation of flavonoids that attract rhizobia, which in turn releases LCOs (also known as NFs) perceived by legumes through plasma membrane receptors such as NFR1 and NFR5 in coordination with SYMRK. 3-hydroxy-3methylglutaryl coenzyme a reductase (HMGR) activates calcium spiking, which acts as a secondary messenger, and activates CCaMK, which in turn activates TF CYCLOPS. This leads to the activation of a common symbiosis pathway for the expression of symbiotic genes such as NIN.