**1. Introduction**

Arbuscular mycorrhizal symbiosis (AMS) is a mutualistic interaction formed between more than 80% of terrestrial plants and members of the Glomeromycotina fungi, referred to as the arbuscular mycorrhizal fungi (AMF) [1]. It was proposed that AMS evolved approximately 400−450 million years ago, while root nodule symbiosis (RNS) originated about 60 million years ago [2, 3]. Thus, it was consistent with the generally accepted theory that RNS might be a result of a gradual attenuation of AMS, and both of them might evolve from the ancient plant-pathogens interaction [4, 5]. In the AMS, AMF could help plants to absorb more phosphorus and nitrogen nutrients from environment, and in return, plants provide carbohydrates mainly in the form of lipids for AMF [6–9]. As one of the pivotal nutrients for host plants' growth,

phosphate is known negatively correlate with AMS [10–12]. Meanwhile, AMF could help host plants adapt to stressful environmental conditions [13, 14]. The development of AMS is a highly dynamic process, including presymbiotic communication between both symbionts, colonization of AMF in the plant root cortex and highly branched structures called arbuscules formation [15, 16], vesicle and spore maturation. Before making physical contact, phytohormones strigolactones (SLs) secreted by the roots of plants into the rhizosphere under Pi-deficient conditions [17], promote the branching of mycorrhizal hyphae. Simultaneously, the secreted cutin monomer promotes the colonization of AMF in the host roots. But the molecular basis of SL perception by AMF spores has not yet been elucidated.

Similar to the process of rhizobial symbiosis, branched hyphae of AMF secrete mycorrhizal (Myc) factors, a mixture of short-chain chitooligosaccharides (CO4/ CO5) and lipochitooligosaccharides (LCOs), both of which are similar structures with rhizobial Nod factors [18, 19], to activate the common symbiosis signaling pathway (CSSP), required for both AMS and RNS [2, 20]. Over the past two decades, advances have been made in the areas of symbiotic signals perception, including the identification of LysM receptor-like kinases in non-legumes, especially in rice, and symbiotic signaling transduction in plants. Downstream of LysM receptors, several key common components were identified as shared components for both AMS and RNS, that is, Symbiosis receptor kinase (SymRK), also called Nodulation Receptor Kinase (NORK) or Does not Make Infection 2 (DMI2) in other plant species [21, 22], calcium and calmodulin-dependent kinase (CCaMK) [23], CYCLOPS [24]. GRAS transcription factor (TF) family proteins such as DELLAs [25], Nodulation Signaling Pathway 1 (NSP1), NSP2 [26], Reduced Arbuscular Mycorrhiza 1 (RAM1) [27, 28] are also involved in AMS and/or RNS. In addition, some downstream components, such as TFs like Nodulation Inception (NIN) [29, 30], Phosphate Starvation Responses (PHRs) [31], SYG1/Pho81/XPR1 (SPX) [32], etc., participate in AMS and/or RNS. Overall, the current study has provided a rough linear pathway of AMS/RNS in plants.
